Calling all hackerspaces

This is a shout out to all hackerspaces and engineering schools within easy reach of Philadelphia. I’ve got a nice little design-and-build project that would do the world some good, but I don’t have the skills or facilities to do it myself.

The problem: build a ruggedized special-purpose test enclosure to be mounted on a roof or utility pole and host a bunch of GPS sensors. The tricky part is that it needs to be outside and not under top cover (for good skyview) and thus weatherproof, but also transparent to the GPS radio frequencies. Another part of the design problem is getting data and power cabling back to my development computer.

UPDATE: I’m now pursuing a different path – trying to figure out how to build a GPS repeater on the cheap so I can effectively pipe the RF from a roof antenna to be retransmitted in my office. This has the obvious advantage that the GPS test rack will be able to live inside, near my desk, rather than outside in an enclosure that can only be reached with a ladder. So now I’m looking for a hackerspace frequented by radio hams.

I need this because I’m the lead programmer of GPSD, responsible for the correctness and robustness of software used in hundreds of thousands of deployments in GIS software worldwide. GPSD is used for navigation, fleet tracking, scientific telemetry, autonomous-vehicle guidance systems, and disaster response. Lives may depend on our quality.

Most GPSes can be simulated with canned datafiles run through a test framework. A few cannot be, because the handshaking between host software and device is too complex to be reliably simulated. These problem children need frequent live testing to avoid regressions, and the problems with doing that have made for a troubling weak spot in our test regime. I want to be able to push a button and get test results every time I perform a potentially sensitive commit.

The test enclosure needs to be able to host a minimum of four RS232 devices and four USB devices; more, up to a maximum of 16 each, would be better. Cooling isn’t an issue as they’re all milliwatt devices. Swapping in new test devices needs to be easy. There are a bunch of practical issues about where to site the test enclosure for both good sky-view and serviceability, how to power it, and so forth.

I’m looking for student and hobbyist EEs that would enjoy developing such a design and the real-world experience of building and deploying it. The deliverables for the project would (1) be a set of blueprints, to be published under Creative Commons, (2) two working copies of the test enclosure (one for me, one for my test guy on the West Coast), and (3) an installation expedition to mount one of the copies out here where I live.

This would make a good practical engineering project for a class in power and signals engineering. Anybody interested?

122 thoughts on “Calling all hackerspaces

  1. I used to work for a company that did this sort of thing professionally. I didn’t build any of the enclosures, and I didn’t go out in the field to install them, but I still learned a couple of things about the art and practice of putting computers and sensors outside.

    * For starters, keep in mind that sensors rarely need to be co-located with their computers. I’d recommend putting a fixed number of GPS antennas (antennae?) outside in a weather-proof enclosure with good visibility, and then running a bundle of cables from the antenna pod back to your computers / GPS units / whatever, which can then reside inside.

    * While you want to keep anything outside from exposure to the weather, you’ll still want some ventilation, and you’ll definitely want drainage holes — water is the most persistent solvent known to man.

    * It’s always a good idea, when running cable, to throw the odd bight in the cable, so that any condensation that accumulates on the cable falls to the bottom of the bight, instead of following the cable into housings (or your house).

    * Think seriously about setting up a wireless connection to your external equipment, if possible. XBee radios are relatively cheap and very effective, and much less likely to get hacked than a WiFi connection.

    Useful advice can be found here:

    * http://www.dulley.com/diy/dw107.htm
    * http://www.acehardware.com/info/index.jsp?categoryId=1284747
    * http://www.ambientweather.com/eaofin.html
    * http://www.weathershack.com/education/weather-station-set-up.html
    * http://www.ehow.com/how_2251706_install-outdoor-electric-wiring.html

  2. A tupperware box, silicon sealant & rs232/usb->Ethernet converter? or am i missing something?

  3. My immediate thought was the same as “alankat”s — tupperware. This is the solution a bunch of community wireless people in New York picked for their exterior enclosures, too. It is simple and largely foolproof.

  4. >For starters, keep in mind that sensors rarely need to be co-located with their computers. I’d recommend putting a fixed number of GPS antennas (antennae?) outside in a weather-proof enclosure with good visibility, and then running a bundle of cables from the antenna pod back to your computers / GPS units / whatever, which can then reside inside.

    The class of devices we’re interested in testing is mainly consumer-grade GPS mice like this typical example. Unlike the more expensive GPSes found in (for example) marine navigation systems, these don’t have separate antennas and can’t be retrofitted with an antenna lead. Actually, extension antennas have been falling out of use even on those high-end systems – today’s receivers aren’t those of 20 years ago, they have vastly improved sensitivity and signal discrimination to the point that an antenna is not usually a cost-effective improvement.

    So the sensors are going to have to live in the enclosure. My assumption is that a cable bundle will run from the enclosure back to my office. Exactly how it will get from the outside of the house to my office is another interesting question.

  5. Craig Trader’s advice is the way to go. When the TV network I worked at chose to use GPS to set our master clocks, we installed two commercial GPS antennas on the roof. The antennas were completely weatherproof, and contained active electronics to amplify the signals prior to feeding them through about 100 feet of coax cable into the enclosure that already contained our emergency generators. From there, the signals were fed down about seven floors to the actual receiver/decoders. There’s no need to mount a lot of equipment on a pole or a roof.

  6. @esr: Sorry, your reply to Craig Trader came up while I was composing the last comment, so I didn’t see it. I will note, though, that if you are going to want to test out a GPS mouse, you should be able to do all your testing indoors – that’s the way the device is going to be used, after all.

    “Exactly how it will get from the outside of the house to my office is another interesting question.”

    What kind of windows does your house have?

  7. >My immediate thought was the same as “alankat”s — tupperware.

    Well, maybe. But a stock tupperware tub would have problems; no venting, and no interior supports to hold the sensors off the bottom where moisture would accumulate. Also I’d be a little concerned about the plastic going brittle under long-term UV exposure, but maybe the New York Wireless people can put that to rest from experience.

    Tupperware with vents cut in it and some kid of 3D-printed internal plastic rack might be a good way to go if UV degradation is not a problem. The rack could provide a hardpoint for a mounting bracket so we wouldn’t have to rely on the strength or stiffness of the Tupperware shell.

  8. “…don’t have separate antennas and can’t be retrofitted with an antenna lead.”

    If you do install an outdoor antenna, then just make up a little box with a blocking capacitor so you can phantom power it from inside. The RF from the other side of the capacitor goes to a small piece of coax with a small loop of stiff wire at the end. Put the loop near the GPS device. Should work….

  9. >that’s the way the device is going to be used, after all.

    Er, not normally. GPS mice don’t work reliably indoors.

  10. >make up a little box with a blocking capacitor so you can phantom power it from inside. The RF from the other side of the capacitor goes to a small piece of coax with a small loop of stiff wire at the end. Put the loop near the GPS device.

    Is this, in effect, an all-frequency RF conduit from outside to in? If so, it might be a good solution. It would certainly make the test devices easily accessible if I could just site them in my office.

    I don’t understand “phantom power”, though.

  11. >What kind of windows does your house have?

    Metal casement windows of early 1960s vintage.

  12. >Is this, in effect, an all-frequency RF conduit from outside to in? If so, it might be a good solution. It would certainly make the test devices easily accessible if I could just site them in my office.

    I think he’s talking about something like those old curly cellular antennas that were mounted on cars. The antenna was outside, and the signal ran to an inch square piece of metal that was stuck to the glass. Then another square of metal was glued to the other side of the glass, which was connected to a line that went to the cellular phone mounted in your trunk. The two squares of metal and the glass act like a capacitor and pass the high frequency radio waves. Thus the signal goes from trunk to ether (and the reverse) without drilling any holes in the car.

    Regardless of how you get the signal inside, I think he’s suggesting a high gain antenna omni-directional outside the house, connected to coax, connected to an antenna inside near the devices under test. The search-fu on this would be “passive repeater” and it’s a legitimate way to pipe RF.

    You might get lucky with a local Ham club. Depends on if they’re “appliance operators” or the RF experimenter type.

  13. Go to Home Depot or Lowe’s and buy a big waterproof plastic box (designed for outside use). Get a USB hub with enough ports for all the GPS receivers you want to connect (sounds like a 14-port hub to me). Buy a Chumby. Configure it to connect to your home wifi network. Buy some USB serial devices for the serial GPS receivers. Plug everything into the appropriate holes. Compile your code on the chumby and test away.

  14. >The search-fu on this would be “passive repeater” and it’s a legitimate way to pipe RF.

    Thanks, that’s really interesting. That might be a much more simple and effective tactic than an outdoor test enclosure. I will google on “passive repeater” and see what I can learn.

  15. My thoughts run toward a fiberglass NEMA 4X enclosure – it’s designed to house electronic components in washdown areas.

  16. >My thoughts run toward a fiberglass NEMA 4X enclosure – it’s designed to house electronic components in washdown areas.

    You’re the expert. I wanted to call you to brainstorm this but don’t seem to have a current cell number for you.

    I’m beginning to think a passive repeater antenna for GPS frequencies may be a better solution. Mount the head antenna outside, sit the GPSes near a wire loop in my office.

  17. >I’m not certain how transparent they are to the electromagnetic spectrum

    A microwave oven (at 2.45 Ghz) is probably close enough (to 1.2-1.5 Ghz for GPS). Put a sample of the plastic and a separate glass of water in the oven and see if the plastic gets warm. I’m fairly sure you will be safe with any normal consumer plastic item besides mylar.

    How about a 5 gallon plastic bucket? Mount the lid upside down to the roof or pole. drill a few holes through the lid for drainage. Attach a platform on top of that made of plexiglass, using 2 inch long pieces of 3/8 nominal pvc pipe as standoffs. Cable tie equipment onto platform. Then cover with the standard, unmodified bucket (which could either take a coat of paint to protect against UV, or just replaced every six months.

    Serial lines should be fine if you use shielded twisted pair and you use real -/+ voltage instead of the recent TTL 5v to zero signaling levels. For USB, hopefully someone makes a ethernet USB extender that does PoE. If that’s the case, you only need to run Cat5e or Cat6 for both signal and power.

  18. Also, at work we have an “active repeater” to pipe cellular signals into the basement. They work with nearly every make of phone and do not need to be programmed for a specific phone like the personal femtocell-to-broadband things need.

    A quick check, and yes, they exist for GPS too (first link found, YMMV, etc).

  19. > A quick check, and yes, they exist for GPS too (first link found, YMMV, etc).

    I’ll note that Roger-GPS doesn’t have a US distributor; presumably their product hasn’t passed the FCC.

  20. >I have a sneaking suspicion that legally using a GPS repeater won’t be feasible.

    It appears the regulations only control active (powered) repeaters, and can be met by obtaining a Part 5 Exprimental License.

  21. > I have a sneaking suspicion that legally using a GPS repeater won’t be feasible.

    A case could conceivably be made for actual experimental use here, but I fear that it could take ages and copious amounts of cash to obtain. That’s assuming it isn’t just flat out denied. Kind of ironic, as GPSD is used by the military and other government organizations.

    ESR, could you perhaps sketch and scan the placement of the pole relative to your office? Or at least visualize the options you’re considering? I used to work for the construction end of American Tower, I’ve put some pretty ‘experimental’ things at strange heights. I’m pretty tempted to suggest modifying a standard TV antenna mount to handle this as long as local zoning doesn’t prohibit doing so.

    If you have an attic, or even an overhead crawl space, all of the Ethernet (and the bulk of the condensation sensitive connections) could be indoors. This would also reduce the weight of the final loaded enclosure, which would allow a better choice in mounts (you have to consider a pound of snow and ice on it with 50 mph winds). A 1″ ‘seal tight’ flexible conduit could protect the cables to the point of entry in the house. Perhaps an attic vent?

    If you can get this so that the only thing going in the enclosure are the units and connecting cables, it gets a bit simpler.

  22. Additionally, you might be able to obtain (and gut) a couple of these:

    http://www.data-connect.com/Motorola_Canopy_5'2Ghz.htm

    They should be just big enough to hold a couple of units snug. If you can find a place wiling to just sell you one or two, you might have a good solution. Some of them come with a standard PoE injector, which might also come in quite handy. I can’t find the exact RF specs for the housing, but I’m pretty sure they’d be transparent for your use.

  23. >A case could conceivably be made for actual experimental use here, but I fear that it could take ages and copious amounts of cash to obtain.

    Maybe not. The FCC has an online form for the purpose, and the appurtenant text suggests that Part 5 licenses aren’t particularly uncommon. In any case, the first thing to try is probably an unpowered or passive repeater, which isn’t covered by the regs.

    The option I’m currently considering is an antenna mount at the roof peak on one end of the house, coax running to the same louvered and unglazed attic vent that admits our phone line, from there over the attic floor to where it can drop into one of the walls adjacent to my office. The total cable run would probly be about 25 feet horizontal, 8 feet vertical.

    GPS frequencies are 1575.42 MHz and 1227.60 MHz. It looks like this is off the high end of the UHF TV band and I have no clue how to estimate an antenna’s frequency response. Would a normal TV antenna have useful sensitivity in the GPS band? If so, your suggestion of a TV antenna mount sounds good – I’m certain I could find a contractor to install one relatively inexpensively, and zoning regs won’t be a problem.

  24. > Would a normal TV antenna have useful sensitivity in the GPS band?

    The utility I saw in using one was the mast and mounting kit, not really the antenna itself (though, you could install it as well just to have an extra antenna on the roof for some other purpose). I don’t think it would have useful sensitivity in the GPS band but ICBW.

    The kits usually come with everything you need, including some steel guy wire to secure it in case the mount itself fails, and plenty of adjustable brackets and things to get a good fit.

    If you are able to go the repeater route, it would be a great mounting option. Don’t forget to ground it, you’ll want around a #2 AWG grounding conductor which can attach to the nearest available cold water pipe.

  25. >I don’t think it would have useful sensitivity in the GPS band but ICBW.

    Hm. I found a description of a helix antenna optimized for GPS use here.

  26. > Don’t forget to ground it, you’ll want around a #2 AWG grounding conductor which can attach to the nearest available cold water pipe.

    2 AWG would probably be overkill (and a lot more expensive). Solid conductor 10 AWG or 8 AWG (8 being my own preference) would be more than sufficient for most applications. Telephone and cable companies use 10 AWG to ground their protectors for residential applications.

    Cold water lines also aren’t sufficient these days. Many homes have PVC supply lines, and even older homes which originally had galvanized supply lines have been refitted with PVC from the utility lines or sections of PVC inside the home as the older supply pipes have deteriorated.

    The proper way to ground an antenna mast is with a dedicated grounding electrode (commonly a 1/2″x8ft copper plated rod) which then also needs to be bonded to the building’s main grounding system (this last step is critically important). A length of solid 4 AWG or 6 AWG bare copper wire (buried) is generally sufficient to bond the grounding systems together.

  27. How much device do you need to run GPSD?

    You could put an embedded linux board (raspberry pi or similar) inside the enclosure as well, so you can terminate all RS-232 and USB right inside the enclosure. Then you could ssh into the dev machine over wifi. No cables through your walls.

  28. Since you have casement windows, you’ll probably need to drill through the house walls for cable access. See your wired telephone/cable TV installation if you have one – there might be room for another cable there.

    Phantom power: If you are going to pipe signals at those frequencies through coax cable, there’s quite a bit of signal loss. This greatly degrades the signal. To avoid that, ‘active’ antennas have a preamp mounted directly at the antenna end. This preamp needs to be powered. The small amount of DC needed is sent down the cable, and is often supplied by the receiver. The antenna itself might come with a tiny power supply that will do the job, while picking off the RF out to a connector for you. (Plug-in microphones for PCs use the same sort of arrangement.)

  29. Come to think, there are a boatload of wifi routers that already run linux and have USB ports. Getting an RS-232 to USB converter running on one of those might be a PITA, but I’ll be that some of them already have RS-232 pinouts on the board, if not actually populated.

  30. I confess, I didn’t read all of the comments before making this one and apologize if this has already been suggested.

    Have you skimmed any used boat supply places?
    I’d be shocked if you couldn’t find something like this fairly cheap in any sizable chandalry.
    Boat GPS and radar systems are shrinking every year (both physically and price wise) so the older and larger ones are always being traded in.

    The problem with tupperware, rubbermaid, and just about every other plastic solution that wasn’t specifically designed to be out in the sun is that it breaks down fast under constant UV light. If it was made to be on a boat, someone already spent the money and effort to insure that it will withstand constant sunlight.

  31. > Hm. I found a description of a helix antenna optimized for GPS use here.

    I’d say that’e experiment worthy, so long as you don’t have to factor it into the tests that you ultimately intend to run. Remember, this is for _tests_ that could ultimately cause you to back out working code.

  32. I have spoken with a sales engineer at GPSSource and learned that passive repeaters won’t work with the length of coax I’d need to run from my roof.

    On the other hand, he says the FCC has long since given up on enforcing the ban on active repeaters. There are according to him, about 70K to 80K such setups in civilian use and trying to actually enforce licenses for them all would swamp the Part 5 program. He says the Part 5 disclaimers on his company’s and other sites are basically historical relics.

    On the gripping hand, while his company will cheerfully sell me an active repeater he says the minimum toll is about $750 and the competition isn’t any cheaper.

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  35. Garmin have been making passive (remote) antennas for their fixed mount marine units for some years. This was the first site that came up when I googled:

    http://www.gpscentral.ca/accessories/ga30.html

    For the horrifying price of $55.00 plus $35 for 10m of cable, it avoids a lot of problems of a do-it-yourself setup. 10m of cable might be just enough for what you are doing. Solder a small stiff loop of wire to the end of the coax cable and leave it hanging out the wall near the ceiling as a source for whatever you are testing on the desktop. If it is a couple of feet from the units you needn’t worry too much about placement differences affecting anything. You need only add some sort of bracket to hold the antenna securely outside the vent, preferably, out and up at ridge level for a good sky view.

    You could consider adding a new plastic airvent through the roof into the attic space, mounting the antenna under the new vent. Might be subject to snow cover, but the antenna would be ‘indoors’.

    For more dollars but a vast collateral benefit, treat yourself to a sun tunnel leading down into the kitchen, bathroom or the hall (about $400 total). I put one in about 5 years ago in the bathroom and another a couple of years ago in the hall. It was actually quite easy to do. Requires some confidence in your completion capabilities to make holes in the roof and the drywall ceiling !

    The antenna would sit quite nicely, supported on a cross setup, under the plastic dome. And due to the diffusion of the tunnel and bottom diffuser, you would not know that the antenna was even there when looking up. Despite lots of snow, I have never seen the tunnel dome fully covered. You might also gain some flexibilitiy but being able to position the antenna closer to where you want the end of the cable.

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  37. >For the horrifying price of $55.00 plus $35 for 10m of cable, it avoids a lot of problems of a do-it-yourself setup.

    Some of them. The GPSSource guy says pure passive won’t work for a repeater setup with that much cable run, because you get 25-30dB of dissipation from the retransmission on the inside end. He says if you want to go pure passive you can only get away with a foot or two of cable.

    On the other hand, mating this setup to a small RF amplifier feeding a retransmission loop could work. You are right that 10M is likely as much cable length as I need – I estimated a bit less than 33 feet,

  38. Hi Eric,

    I have a number of weather-tight cases that have been salvaged form industrial control equipment.

    Give me an idea if dimensions and I’ll give you some photos of the most suitable specimens.

    You can probably pick them up at Hive76 on Wednesday if we can winnow the selection down a bit …

  39. >I have a number of weather-tight cases that have been salvaged form industrial control equipment.

    Right now I’m trying to make the GPS repeater path work. I’ll get back to you if it doesn’t.

  40. we can do it at HIve76 with our 3D printers. Water-tight enclosures that are transparent to GPS, 3D printable FTW.

  41. >we can do it at HIve76 with our 3D printers. Water-tight enclosures that are transparent to GPS, 3D printable FTW.

    Got any ham-radio tinkerers in residence? I think the build objective has changed from “outdoor test enclosure” to “low-cost active GPS repeater”, probably mating a cheap marine GPS antenna to a length of coax, an RF amplifier, and a retransmission loop on the inside end. That way there wouldn’t be any servicing issues – the sensors would live inside in my office, near the retransmission loop.

  42. >The GPSSource guy says pure passive won’t work for a repeater setup with that much cable run, because you get 25-30dB of dissipation from the retransmission on the inside end.

    This seems high to me. 3dB gain is like double signal strength. I guessed +3dB gain on the antenna, and -3-4 dB loss for the 35 feet of coax + connectors + impedance mismatch. 25-30dB to squeeze the signal out the other antenna seems like he forgot a decimal point. (or he wants to sell you a $750 solution). Good coax at 1.5 Ghz should only lose 6-7dB per 100 feet. (He said it would work if the coax was only 1 foot?)

    I’ve done the passive repeater thing before with a TV antenna, normal TV RG6 coax and an improvised loop wrapped right around a radio scanner’s little ducky antenna and the improvement was considerable. Knocking heads with an experienced Ham or radio engineer might be prudent.

    OTOH if the freight seems reasonable, throwing money at the problem should at least get you a solution that is returnable if it’s not an adequate solution.

    If you homebrew the antenna, one of those offered cases might be a bit easier on the eye then an upside-down bucket.

  43. >This was the first site that came up when I googled: http://www.gpscentral.ca/accessories/ga30.html

    What we have here looks like a “passive” antenna (i.e not “active”; without an amplifier), a length of coax, and mounting brackets. It really looks as though the other end of the coax is meant to go to a “pigtail”, and then electrically attached to your GPS. In other words, this is a “passive antenna”, not a “passive repeater”.

    Of course, with an antenna at both ends, it would work as a passive repeater, but this does not represent an off-the-shelf tested, warrantied and shrunk-wrapped solution by itself.

  44. >If you homebrew the antenna, one of those offered cases might be a bit easier on the eye then an upside-down bucket.

    It’s been pointed out that a marine GPS antenna can be had for $55. At that price I don’t see a lot of gain in homebrewing.

  45. This one looks like it might work:

    http://www.gilsson.com/navman_gps/antennas/sma-marine.htm

    It can be pole-mounted, and has an SMA connector, which is workable, though not easy. You’ll have to feed it 5 volts at about 30 ma. If the 30 foot cable is long enough, terminate it this way:

    -
    -
    - SMA|
    - ___| wire loop
    - |__ |————–
    - | |
    - | ___||___
    - |_______| |______ L-shaped chassis (a small piece of copper or aluminum, bent)
    - || |_______|
    - || || Feed-through capacitor
    - – 5 VDC +
    -
    The wire loop dimensions should not be critical.

    Possible problems:
    1. The 30′ cable might be a little too short.
    2. They just might have designed for the cable center conductor to be -, instead of +, but I doubt it. Anyone know for sure?

  46. > Would a normal TV antenna have useful sensitivity in the GPS band?

    the literature is full of references to television providing an interference source for gps.

  47. @LS: I thought of a marine antenna as well — feeding it 5V @30 ma is drop dead easy since an extra USB port will supply 5V at up to 500 ma. (I’m also betting that esr has no fear of soldering irons) As for the length of the cable, that depends on how far esr needs to go from his office window to get a good skyview. I’ll bet not very far, as I think I remember reading somewhere that his office is in an upstairs bedroom.

  48. >(I’m also betting that esr has no fear of soldering irons)

    I kind of hate to admit this, but you’d win only on a technicality. I don’t fear soldering irons, but don’t have the skill to use them either.

    >I’ll bet not very far, as I think I remember reading somewhere that his office is in an upstairs bedroom.

    That was the old place on Warren Avenue. The new place is a ranch pattern with a finished basement – attic is directly above the main level where my office is. Cabling from an attic vent to my office should be easily doable in ten meters.

  49. I haven’t read through all of the comments, but we used to do this for freighter ships. We just used a very simple computer at the sensor for running the sensor and Power over Ethernet to get a very simple, very long cable from the computer/sensor housing back to the main computer in the ship. We could do updates to the computer in the remote housing, uploading a new kernel and/or other software and rebooting. Very cost effective, very simple, and very reliable. For a housing, we had to withstand 100 mile/hour winds, temperatures from -40 C to +60 C, etc. You could probably get away with an inverted Tupperware box.

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  51. I don’t understand why you don’t put a pole-mounted router up in your hard, with some fairly straight-forward power. It doesn’t need to be that high off the ground, it just needs CLOS to an interesting fraction of the sky.

    A Gumstix with a Stagecoach board (https://www.gumstix.com/store/product_info.php?products_id=247) would allow up to 7 USB connections, with an easy to understand IP address per USB connection. It’s trivial to get linux (or android) running on these.

    and you won’t need a soldering iron, much.

  52. “feeding it 5V @30 ma is drop dead easy since an extra USB port will supply 5V at up to 500 ma.”

    Yes, but…the computer or hub will put up the power momentarily, try to initialize things, and shut the power off when it doesn’t get a response. (I’m not really sure, but I think that’s the way USB powering works….)

  53. “I’ll try again:”

    ” | SMA”
    “_|__ wire loop”
    “|__ |————–———–\”
    ” | |”
    ” | |”
    ” | ___||___”
    ” |______________| |______ L-shaped chassis (a small piece of copper or aluminum, bent)”
    ” || |_______|”
    ” || || Feed-through capacitor”
    ” – 5 VDC +”

    The wire loop dimensions should not be critical.

  54. I think D. Curmudgeon has it right, except for this part:

    “Solder a small stiff loop of wire to the end of the coax cable and leave it hanging out the wall near the ceiling as a source for whatever you are testing on the desktop.”

    For the price, I’d buy two antennae and connect them on both ends of the cable. Remember what recieves frequency X also radiates frequency X. Plus, you’ll get alot more gain on the transmission than with a “stiff piece of wire”. The only consideration is you’ll need to place you test equipment at the proper point of the radiative patter, which should be a tear-drop shaped lobe direct out the “top” of the antenna if I’m not mistaken. Check page 10 on this link for a sample pattern study.

    So, if you drop the inside antenna through the ceiling directly above your test bench, you should be good.

  55. I kind of hate to admit this, but you’d win only on a technicality. I don’t fear soldering irons, but don’t have the skill to use them either.

    Now that is surprising.

    Then again my skills are probably all obsolete. When I was a kid, men were men and solder was solder. You warmed up the pad, stuck the pin through the hole, then pooled just enough solder on to cover the pad, secure the wire and ensure good contact. Simple enough that a 9-year-old could do it. These days, it’s all surface-mount; the components are tinier, the amount of solder used correspondingly more minuscule and the act itself quite a bit more tricky.

  56. Jeff Reed (Jan. 17 at 11:30 am) said:

    Then again my skills are probably all obsolete. …

    Maybe. I grew up with the sweet stink of rosin in my nose, soldering my Heathkits and various home-brew gizmos. I still sometimes repair things that, at the circuit board level, are all surface mount microdots — but the wires off the edge of the board are still just like the leads of capacitors and resistors in the days of yore. The old-school techniques still work fine.

    Just my $0.02 worth.

    P.S. – For a challenge, try sweating copper plumbing – that, my friends, is soldering writ large.

  57. > P.S. – For a challenge, try sweating copper plumbing – that, my friends, is soldering writ large.

    Absolutely trivial, unless it’s a) large diameter (> 1.5″), b) vertical, or c) full of water.

    (‘c’ is near impossible, but there are ways…)

  58. > and the act itself quite a bit more tricky.

    to say nothing of the lead-free ROHS requirements, and the complexity that brings.

  59. Have you considered posting this request to an appropriate forum on eham.net? Despite all the political wrangling on that site, it’s probably one of the larger concentrations of hams on the Net.

  60. >For the price, I’d buy two antennae and connect them on both ends of the cable.

    That’s…brilliant. How would you power such a setup?

  61. ESR: I will pass this on to Valli Hoski (I think you remember her from GT). She and her husband are hams, and have just relocated to the Harrisburg/Gettysburg area. They don’t tinker much, I think, but they should be plugged into the local ham community by now.

  62. esr> How would you power such a setup?

    No power, just passive. All I’m suggesting is use a (somewhat) directional antenna on both ends of the connection. By directing the radiation with a higher gain antenna indoors, you get the advantage of having lower loss in the total system. Curmudgeon’s suggestion of a “stiff wire” is great if you’re looking to be mobile indoors, but you’re not, so take advantage of that.

    It’s going to produce a lower SNR ratio, and with a higher directional antenna on the inside, you’re limiting the area in which you can place you test rig, which is why I suggest placing it directly above the test bench.

    Basically it’s the same idea as capturing daylight with a reflector, transmitting it through a fiber, then focusing that beam on the inside with a lens to get a “spot light”, only longer radiation waves.

    If I were doing this, I’d probably go with a pair these (here’s the rest of that catalog page so you browse) because they are high gain (meaning they have a narrow beam) and they have TNC connectors, so you can use LMR-400 low-loss cable (I think it will work in that frequency range ok… I’ve seen it in UHF, and I”ve used in 2.4 with ~5db/100′ loss). I think that would be the lowest loss system you could probably build.

    Also, remember inverse square. If you’re getting too much loss indoors, put the receiver close to the antenna (not sure what the limit is there, I think 1/2 wave length, but I may be off on that, any HAMs in the house?).

    If I’ve done this right, you’ll only have about an 18-20 DB loss inside your house, and the gain from the two antennae (one ‘up’ toward the satellites, and the other ‘down’ toward your workbench) should more than compensate for the losses (20db freespace, and about 5 in the system).

    WARNING: My experience is limited in this area (particularly outside 2.4, 5.2, and 5.4 frequencies)! However, you’re looking at less than $200 worth of parts and a Saturday afternoon, and you can set it all up outside and run the cable through a window, and use a hand-held GPS to test it, so if you order from a place with a good restocking policy, you’ve got minimal risk. I’m sure Cathy will enjoy standing outside the window holding the antenna in the air while you test the signal strength inside ;^)

    One more thing, don’t know how much experience you have with this stuff, but make darned sure you seal that antenna connection outside! You’ll want some “pooky” (mastic putty) and then at least 2 overlapping (by 2″ minimum) layers of electrical tape over the connectors. Check with a local ham or radio shop and they should have what you need and be able to tell you how to do it.

  63. “When I was a kid, men were men and solder was solder. You warmed up the pad, stuck the pin through the hole, then pooled just enough solder on to cover the pad, secure the wire and ensure good contact. Simple enough that a 9-year-old could do it. These days, it’s all surface-mount; the components are tinier, the amount of solder used correspondingly more minuscule and the act itself quite a bit more tricky.”

    The recommend soldering approach for SMT is quite different than for through-hole. It’s probably best to use solder paste and a hot air gun for SMT. The key idea is that you are hitting every pin of a component at once, not doing the leads one at a time. The biggest pain for hobbyists is that the paste isn’t shelf-stable like solder. It needs to be refrigerated and has a short shelf-life.

    I’ve done very little SMT construction, and it’s all been with the old-fashoined iron-and-roll-of-solder approach.

  64. January 17th, 2012 at 9:41 am
    “I’ll try yet again:” (copy the text into an editor and replace ‘.’ with a space.)

    ”….| SMA.connector”
    “.._|__….”
    “..|___|————–———\”
    ”….|…………wire loop..|”
    ”….|………………………|”
    ”….|…………………___||___”
    ”….|==||======|…………|===== L-shaped chassis (a small piece of copper or aluminum, bent)”
    ”……….||………….|_______|”
    ”……….||……………….|| Feed-through capacitor”
    ”………..–….5 VDC……+”

    The wire loop dimensions should not be critical.

  65. Now, it’s become an obsession:

    “I’ll try yet again:” (Copy the text into an editor and replace ‘x’ with a space. You’ll still have to tweak it. The variable width font is giving me fits.)

    ”xxx|xSMA.connector”
    “xx_|__”
    “xx|___|————–———\”
    ”xxx|xxxxxx wire loopx|”
    ”xxx|xxxxxxxxxxxxxxx|”
    ”xxx|xxxxxxxxxxxx___||___”
    ”xxx|==||======|xxxxxxx|===== L-shaped chassis (a small piece of copper or aluminum, bent)”
    ”xxxxxx||xxxxxxxx|_______|”
    ”xxxxxx||xxxxxxxxxxx|| Feed-through capacitor”
    ”xxxxxx–xxx5 VDCxxx+”

    The wire loop dimensions should not be critical.

  66. What about combining the two solutions? Have a passive repeater to bring the signal in from outside into your attic with a fairly short cable (to reduce loss), then stick your test devices in a box in the attic, with serial and USB connections running to your test machine downstairs. I don’t know how serviceable that will be for your purposes, but it’s probably easier to get at the devices in your attic than on top of a pole outside. Piping the signal all the way to your office would be ideal, of course.

  67. >No power, just passive. All I’m suggesting is use a (somewhat) directional antenna on both ends of the connection.

    Sweet Goddess, what an elegant hack this is! A week ago I wouldn’t have understood how elegant, but I’ve been reading about antenna physics and the symmetry theorem you are exploiting caught my attention.

    I checked with my friend Eric Baskin, who sometimes comments here as NotGump. He’s a power and signals engineer who’s done a bit of antenna design. He thinks this is likely to work, and in the worst case all I will have to do is add a small RF amplifier between the coax and the inside antenna.

    This sounds like a plan. Now to execute it.

  68. >Have a passive repeater to bring the signal in from outside into your attic with a fairly short cable (to reduce loss),

    I’m no longer very worried about cabling losses. My current plan is to mount the antenna at one end of the roof and bring coax in through the attic vent to my office. That’s going to be 30 feet max; Don says LMR400 has losses of ~5db per 100ft suggesting that I shouldn’t see more than about ~1.5db loss. The antenna gain ought to swamp that easily.

  69. “I checked with my friend Eric Baskin, who sometimes comments here as NotGump. He’s a power and signals engineer who’s done a bit of antenna design. He thinks this is likely to work, and in the worst case all I will have to do is add a small RF amplifier between the coax and the inside antenna.”

    Ask him if he thinks you might be better off with an active antenna to start with. There doesn’t seem to be much difference in price, and I’ve had a hard time finding an inexpensive amplifier for 1500 MHz that you could add if you needed it. (Maybe you’ve found one, yourself.)

    Also ask him if he thinks that putting a loop at the end of the cable might allow the GPS receivers to get a better signal by coupling to the loop’s near fields. (Maybe worth a try.)

  70. >Ask him if he thinks you might be better off with an active antenna to start with.

    If I understand correctly, to tun this active all I’d need to do is feed 5V DC to the center connector.

  71. > Don says LMR400 has losses of ~5db per 100ft

    Times Microwave LMR400 has a Loss per 100ft spec closer to 6dB

    For a 30 ft run, you’ll have an attenuation of 1.8dB

    http://www.timesmicrowave.com/cgi-bin/calculate.pl

    Then you have to account for the insertion loss of the connectors at both ends, and the front-to-back ratio of the antenna(s). Figure 0.5dB each, and you’re darn close to a 3dB loss, which, in RF-terms means you have 1/2 the power.

    No antenna is perfectly efficient at all frequencies. All have some ‘reflected power’. If you’re very fortunate, the reflected standing wave won’t act to cancel (or maximally interfere) with the forward power.

    Remember that an amp will raise signal and noise at the same time, so while you may get a higher signal level, the all-important SNR (or Eb/No, if you’re a real RF type) will not increase. The AMP connectors will also supply further insertion loss.

    “passive repeaters” do work (subject to the laws of physics). Their best application is in cases when you can summarise the link as: “We would have HEAPS of signal on this path, if only that HUGE obstacle wasn’t in the way”. You don’t have that here.

  72. > The biggest pain for hobbyists is that the paste isn’t shelf-stable like solder. It needs to be refrigerated and has a short shelf-life.

    You may not have worked with SMT paste in a while. This isn’t necessarily true these days. For example: http://www.zeph.com/zephpaste.htm
    (this is my paste, there are many like it, but this one is mine.)

  73. > My current plan is to mount the antenna at one end of the roof and bring coax in through the attic vent to my office.

    I have to ask, “How do you plan to keep the rain out?”

    I understand that you plan to use an existing penetration. Most of these are either sealed to the plumbing (they’re vent stacks for your home’s plumbing), and therefore, can handle water, or they are something like an attic fan, which has design features to keep the rain out. You’re going to penetrate those, with a run of coax. What are you planning to do to keep the water from dripping down the coax? (And does Cathy know?)

  74. On thing: If you get an amp, get an outdoor remote amp and put it outside. That way you’re not amplifying the noise you pick up in the cable, just the signal (and noise) from the receiving antenna. That will optimize your SNR.

    Let me know how it goes!

  75. anonymouse: That would work about the same, but it would be far less convenient for testing the NEXT reciever (climb into the attack, open the box, replace the receiver, etc.). This way he goes in the attick once (hopefully) in January or February (not June or July, in Texas, we appreciate that distinction ;^).

  76. One more thing. If you do try this, you might think carefully about not using the (less expensive) PVC-jacketed cables. When PVC burns (should you have a house fire), it releases some deadly gasses.

    PVC produces HCl upon combustion. Hydrogen chloride forms corrosive hydrochloric acid on contact with water found in body tissue. Inhalation of the fumes can cause coughing, choking, inflammation of the nose, throat, and upper respiratory tract, and in severe cases, pulmonary edema, circulatory system failure, and death. Skin contact can cause redness, pain, and severe skin burns. Hydrogen chloride may cause severe burns to the eye and permanent eye damage.

    You’ll also have dioxins formed in the condensed solid phase by the reaction of inorganic chlorides with graphitic structures in char-containing ash particles.

    Copper (which you will find as the center conductor of your LMR-400) acts as a catalyst for these reactions.

    Just sayin’ “be safe”.

  77. >What are you planning to do to keep the water from dripping down the coax?

    A hanging bight just outside the attic vent should do the trick. It’s exactly how our existing fiber line is set up. The vent itself is a louvered unglazed window – it’ll let in some water, but only in high-wind conditions where rain is flying hiroizontal.

  78. “If I understand correctly, to turn this active all I’d need to do is feed 5V DC to the center connector.”

    I don’t think so. Active antennas have an amplifier built into them. Without power there would be a heavy loss through the amp. (It is possible to design such an antenna as you describe with clever use of hybrids, but I doubt that this is the case here.)

    Not to nit-pick Larry Yelnick, but in a passive antenna, any cable loss adds directly to the noise figure of the receiver. If the antenna has a low noise preamp right at the antenna, it will dominate the noise figure of the system, and much reduce degradation in the cable.

    (A good way to see that is to note that the noise generated by the first stage of a receiver generally dominates the overall noise figure. If the radio has ten transistors, the noise generated by the first stage is amplified by nine transistors, the noise generated by the second stage is amplified by only eight transistors, and so on.)

  79. Larry:

    You keep rain out with a drip loop. You let the cable hang down in a “U” and make your penitration into the house (he’s using a gable vent) in an upward direction. Water sheds down the outside of the cable and Bob’s your Uncle.

    The PVC thing doesn’t really matter here because the wire’s in the attic and (since there’s a gable vent), it’s not being used as plenum space.

  80. >Active antennas have an amplifier built into them.

    Yes. The marine antennas I was going to use as per Don’s suggestion are actually designed to run active, they have an LNA inside them.

    But now it looks like the best solution is a $100 unit designed for this job, complete with the 10m of cable.

  81. esr: Oh, I thought you were using gable vents!

    Considering your setup with that window, might I suggest you grab a strip of plexiglass and cut it to fit (with notches for the cables of course). Water penetration is a deadly serious business (for your house, that is). Even a scrap piece of vinyl siding cur to fit with a little silicone could do the trick in about 10 minutes. Beats replacing rotted wood in that window fram in 10 years (when you’ll be old enough to appreciate NOT doing it ;^).

  82. And disregard my comment on the PVC thing. Looks like that DOES matter (maybe).

  83. >esr: Oh, I thought you were using gable vents!

    Turns out I am. I didn’t know that term.

  84. “Looks like I can get 10m of cable as an option. So all I need is the installation – a mounting bracket near my roof peak, a cable run to my office, and another mount for the inside antenna.”

    You’ll also need a 12 volt power supply. I notice that the device is for L1 only. Will that be enough?

  85. >I notice that the device is for L1 only. Will that be enough?

    Yes. L1 only is normal for the class of consumer GPSes I test.

  86. > and I’ve had a hard time finding an inexpensive amplifier for 1500 MHz that you could add if you needed it. (Maybe you’ve found one, yourself.)

    Maybe a 2 GHz rated CATV/satellite TV amp would do the trick?

  87. > One more thing. If you do try this, you might think carefully about not using the (less expensive) PVC-jacketed cables. When PVC burns (should you have a house fire), it releases some deadly gasses.

    Standard Romex building wire is PVC jacketed. FEP plenum rated wire should also never be used outdoors. It isn’t UV resistant and it mold seems to love it.

  88. > But now it looks like the best solution is a $100 unit designed for this job, complete with the 10m of cable.

    Thus do we see the ultimate power of the bazaar.

  89. > in a passive antenna, any cable loss adds directly to the noise figure of the receiver. If the antenna has a low noise preamp right at the antenna, it will dominate the noise figure of the system, and much reduce degradation in the cable.

    I’m always amused by people who think they understand these things.

    First, noise figure (NF) is a measure of degradation of the signal-to-noise ratio (SNR), caused by components in a radio frequency (RF) signal chain.

    Loss along a conductor (coax) decreases the signal level, and, in most cases, will slightly raise the noise level, due to thermal noise. I think everyone here will understand that
    the cable will have some resistance. The formula to find the RMS thermal noise voltage of a resistor is: Vn= 4kTRB, where
    k = Boltzman constant (1.38*10 Joules/Kelvin)
    T = Temperature in degrees Kelvin (K= +273 Celsius)
    R = Resistance in ohms
    B = Bandwidth in Hz in which the noise is observed

    So, simply, and unavoidably, the signal level has decreased, and the noise level has, unavoidably, increased however slightly.

    SNR has degraded along the coax.

    Your “low-noise amplifier”, no matter how sophisticated, and pricy, can only raise (amplify) both signal and noise level. It can not increase SNR. In fact, even though it is “low noise” it has some noise, so it will, unavoidably, further degrade the SNR.

    If the newly-amplified signal level is high enough for the receiver to detect, and the SNR (or Eb/No) is also high enough for the receiver to decode, then you’ll be able to decode the signal. If, however, the SNR is now too low to decode, then no amount of signal level is going to make your receiver perform black magic.

    You may also wish to note that the device that Eric plans to use is completely illegal in the USA, and no longer marketed by the company that invented it.
    http://transition.fcc.gov/eb/Orders/2006/FCC-06-30A1.html

    > Standard Romex building wire is PVC jacketed. FEP plenum rated wire should also never be used outdoors. It isn’t UV resistant and it mold seems to love it.

    Yes, I understand, but I doubt you’re half as smart as you think you are here. First, LMR-400 (at least the stuff from Times Microwave) is jacketed in black polyethylene. Eric is no longer planning on using a run of LMR-400 (I think, but perhaps now that the legality of the device he was planning to use has been pointed out, he’s planning on changing back). If he does switch back, to be code compliant, he would need to source LMR-400-FR or LMR-400-PVA.

    Most jurisdictions have an electrical code based on the National Electrical Code, published by the National Fire Protection Association. The NEC is quite a large document, running over six hundred pages in length. It details the code requirements for a large variety of electrical installations. Most of the NEC is devoted to electrical power wiring, but installations of other wiring are covered as well.

    The NEC provides particular requirements for certain cable locations because of their special potential to facilitate the spread of fire or fumes. A “plenum,” under Article 100 of NEC, is “a compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.” The most common “plenum” space one sees in a/v installations is a dropped ceiling space in a commercial building, being used as a ventilation return. Most residences do not have any significant plenum spaces, as they normally use duct work or the interior of the house for a return. Plentum areas are rarely a consideration in a residential installation. Plenum cables are required to have jackets and dielectrics which don’t easily give off toxic fumes when burned–the reason being that a fire in one part of the building can, through the ventilation system, feed toxic fumes to the entire building.

    A “riser” is a term never specifically defined in NEC, but basically, one must consult the “riser” requirements whenever cable will penetrate from one floor of a building to another. In Eric’s case, he’s penetrating from the attic into his lab or office.

    NEC Article 820, “Community Antenna Television and Radio Distribution Systems.” is what you need to understand here. Article 820 is a bit of an odd fit, in that the drafters of NEC obviously had in mind the distribution of externally-generated cable TV signals around a home rather than the distribution of internally-generated RF or baseband video signals; however, of all of the NEC Articles, it’s the closest fit and is therefore the Article upon which your local inspector is most likely to rely in evaluating Eric’s code compliance with regard to any cabling he installs for his GPS retransmission device.

    Riser requirements are governed by NEC 820.53(B). In one or two family dwellings, CATV or CATVX may be used for cable TV (or any of these higher-rated substitutions: CM, CMG, CMR, CMP, CATVR, CATVP). For “communications”, however, Eric will probably need to use cable rated CL3X, CL2X, or CATV (the higher-rated CMP, CMR, CM, CMG cables will also satisfy.)

    Are we done yet?

  90. >You may also wish to note that the device that Eric plans to use is completely illegal in the USA

    First, it wasn’t “completely illegal”. It was against FCC regs to for a civilian to operate an active repeater without a Part 5 Experimental license, with an exemption for military contractors. It’s good that you dispel misinformation about radio electronics and building codes; you shouldn’t spoil that by broadcasting misinformation about FCC regulations.

    Second, a sales engineer at one of the outfits I talked to tells me the FCC has stopped enforcing that regulation. He says that (a) active repeaters do not in fact cause GPS issues for emergency services, and (b) there are 70-80K active repeaters in civilian use in the U.S. – if they had succeeded in enforcing the Part 5 program would have collapsed under a wave of applications. (This report is corroborated by my discovery that the web forms on fcc.gov required for a Part 5 license application are broken.)

    The engineer told me that nowadays the Part 5 licensing disclaimers on various vendor sites are historical relics, and the only way the repeater ban will ever be invoked is if some regional FCC manager determines that a specific repeater is causing an actual interference problem.

  91. “No discussion of satellite receiving systems would be complete without mentioning preamplifiers and their location. The mast mounting of sensitive electronic equipment has been a fact of life for the serious VHF/UHF operator for years, although it may seem to be a strange or difficult technology for HF operators.

    While a preamplifier can be added at the receiver in the station, it will do no good there. Vastly better results will be obtained if the preamp is mounted directly to the antenna. To get the most out of your
    VHF/UHF satellite station, you’ll need to mount a low-noise preamplifier or receive converter on the tower or mast near the antenna, see Fig 23.37, so that feed-line losses do not degrade low-noise performance.
    Feed-line losses ahead of the preamplifier or converter add directly to receiver noise figure.”

    – The 2008 Radio Amateur’s Handbook, page 23.16

    @Larry Yelnick: These guys know what they are talking about. You don’t.

  92. I believe that the FCC only regulates “Radiators”, e.g. devices which generate radio-frequency radiation. Connecting two antennae together with wire is NOT a “Radiator” because it does not generate radiation, only directs and focuses it.

    Using amplification is, of course, a completely different matter.

    Otherwise, the FCC could literally regulate ANY metallic device as all metallic devices are antennae and reflectors.

    Don’t give them any ideas, guys ;^).

  93. > I believe that the FCC only regulates “Radiators”, e.g. devices which generate radio-frequency radiation. Connecting two antennae together with wire is NOT a “Radiator” because it does not generate radiation, only directs and focuses it.

    Except as provided elsewhere in this section, no person shall sell or
    lease, or offer for sale or lease (including advertising for sale or
    lease), or import, ship, or distribute for the purpose of selling or
    leasing or offering for sale or lease, any radio frequency device unless:
    (1) In the case of a device subject to certification, such device has been
    authorized by the Commission in accordance with the rules in this chapter
    and is properly identified and labelled as required by Sec. 2.925 and
    other relevant sections in this chapter.

    Under Section 15.201 of the Rules, intentional radiators must ordinarily
    be authorized in accordance with the certification procedure prior to
    marketing. However, under Section 2.803(g) of the Rules, intentional
    radiators and other radio frequency devices that could not be authorized
    or legally operated under the current rules – for example, intentional
    radiators, such as GPS re-radiators, which operate in the restricted
    frequency bands listed in Section 15.205 of the Rules — may not be
    “operated, advertised, displayed, offered for sale or lease, sold or
    leased, or otherwise marketed absent a license issued under part 5 of this
    chapter or a special temporary authorization issued by the Commission.”

    During the course of the Division’s investigations, retail companies
    found to be marketing the subject GPS re-radiator kits in the United
    States, identified San Jose as the manufacturer of such kits. The
    Division staff subsequently found that San Jose was marketing four
    models of GPS re-radiator kits, Models RA-45, RA-46, RK-104 and
    RK-304, on its website. These models consist of a receive antenna, an
    amplifier to boost the signal level, and a radiating antenna. A GPS
    re-radiator does not internally generate a radio frequency signal.
    Rather, a GPS re-radiator is designed and configured to take radio
    frequency signals from an outside source, the global positioning
    satellites, amplify those signals, and radiate those signals through
    its antenna. The Commission has determined that this type of
    configuration constitutes intentional radiating devices under Section
    15.3(o) of the Rules.

    As explained above, the
    Models simply cannot comply with Part 15 technical requirements
    because the equipment operates within the restricted frequency bands
    and thus cannot be authorized.

  94. > These guys know what they are talking about. You don’t.

    Interesting.

    What you said was:

    but in a passive antenna, any cable loss adds directly to the noise figure of the receiver. If the antenna has a low noise preamp right at the antenna, it will dominate the noise figure of the system, and much reduce degradation in the cable.

    Which is not correct. inserting a low-noise amp at the antenna will merely boost the signal level, and, along with it, the noise level of the received signal. The LNA will add its own noise, of course, so the SNR coming out of it will be lower.

    As I stated, if the SNR is still high enough for a successful decode, but the signal level would not have been, then adding the LNA at the antenna is a fine thing. But a high enough signal level in the absence of sufficient SNR isn’t going to perform magic.

  95. @Larry Yelnick (and others who may be interested):

    1. At 1500 Mhz (GPS L1), background noise is very low, and the noise you are working against is thermal noise generated in the radiation resistance of your receiving antenna. Antennas are generally designed to have an equivalent resistance of 50 ohms, and will be feeding a coaxial cable with a characteristic impedance of 50 ohms to match it.

    2. At the receiver, looking back through the cable at the antenna, you will also see a resistance of 50 ohms. You will get THE SAME amount of thermal noise coming out of the cable as went into it. (The cable attenuates the noise from the antenna, but adds its own thermal noise to compensate.)

    3. At the same time, the cable does attenuate the signal, so with less signal, and the same amount of thermal noise, signal/noise is LOWERED.

    The above is why the authors of the Radio Amateur’s Handbook said what they did. It’s also why satellite receivers have an LNA (low noise amplifier) right at the dish, and not inside the building. It’s not magic, just straightforward radio engineering.

  96. >The Commission has determined that this type of configuration constitutes intentional radiating devices under Section 15.3(o) of the Rules.

    I read the FCC finding the same way Larry Yelnick does. Under the letter of the regulations the device I just bought is banned. On the other hand, there are two independent lines of evidence that the FCC has ceased enforcement of this regulation. (The report of a vendor engineer, and the fact that relevant web forms on the FCC website have been allowed to bitrot.)

    The proper thing for the FCC to have done, of course, would be to take this regulation off the books.

  97. > Yes, I understand, but I doubt you’re half as smart as you think you are here.

    @Larry Yelnick:

    Wow… Now I understand how flamefests get started. I sold off my Andrew Heliax tools about 6 years back, but having installed my fair share of LDF5-50 and LDF6-50, I’d rather hope I know a thing or two about coax…

    I also specifically said FEP (fluorinated ethylene propylene), not “polyethylene”.

    That said, you might also want to Google “polyethylene” and “hygroscopic”. A perfect example here is type SE aluminum service entrance cable (which has a polyethylene jacket) which has been in service for many years. If you strip its jacket back when cutting over to new service equipment, you’ll find the aluminum under the polyethylene jacket has a nice white powdery oxidized finish (which has to be wire-brushed clean before making up a new splice).

    > Most jurisdictions have an electrical code based on the National Electrical Code, published by the National Fire Protection Association.

    I’m not really sure why you’ve even brought up the NEC in this instance. A passive (or even an active) repeater _of the type being discussed here_ isn’t even going to have 40VA available on its coax cable (although /technically/ the cabling could still be considered class 2 low-voltage). Article 820 only marginally applies here (mainly grounding and bonding) and then only if Eric had an antenna on a mast (and not a small plastic encased mouse or puck antenna with a non-detachable cable mounted to an exterior wall).

    > The NEC is quite a large document, running over six hundred pages in length. It details the code requirements for a large variety of electrical installations. Most of the NEC is devoted to electrical power wiring, but installations of other wiring are covered as well.

    Curious… None of my various copies of the National Electrical Code are that long. Which year of the NEC book are you looking at? Granted the hardback NEC Handbook /is/ pretty large (for those who’ve never seen one, think large phonebook), but the majority of the Handbook is dedicated to explaining the code with examples and color illustrations (which are /explicitly/ not part of the NEC itself). Now, perhaps you meant article numbers… Article numbers do indeed go well past 600 (well past 800 in fact).

    I suppose we /can/ agree that FEP plenum rated cabling is not /required/ in a typical residential attic since the attic isn’t a plenum air-handling space. However, CM/CMG [general purpose] rated cabling (vs CMR [riser]) /is/ perfectly acceptable in a residential installation. CMR is generally only required where cabling has to pass certain requirements for flame spread tests in /riser/ applications, and Eric specifically stated above “[...] attic is directly above the main level where my office is” which means the argument for riser rated cabling is moot. That said, certain types of CMR cabling (think twisted pair network cabling) are sometimes more readily available so you’ll sometimes see them used in place of CM/CMG.

    Now, what I was pointing out is FEP (fluorinated ethylene propylene) jacketed cabling aka “plenum rated” is generally /not/ UV stabilized for outdoor use (FEP jacketed twisted pair cabling especially) and also how FEP tends to attract mold growth when exposed to the elements.

    I wasn’t talking about “polyethylene” jacketed cable, which /is/ readily available in UV stabilized versions, and while it doesn’t have the same problems with mold growth as FEP, it /can/ have issues with it being hygroscopic.

    > Are we done yet?

    Unless you want to have a discussion about making parallel concentric bends in metal conduit?

  98. “Under the letter of the regulations the device I just bought is banned.”

    Yeah…maybe the Iranians bought one of these things and used it to bring our drone down…..nah….

    Just in case, turn the power off when you’re not actively testing.

  99. > Yeah…maybe the Iranians bought one of these things and used it to bring our drone down…..nah….

    I thought they used a spark gap transmitter?

  100. >Yeah…maybe the Iranians bought one of these things and used it to bring our drone down…..nah….

    You’re joking, but on the off sense someone might miss that and take this idea seriously I will now, er, shoot it down.

    Drones use GPSes for target location, not for obstacle avoidance or measuring altitude, for the excellent reason that the system’s intrinsic uncertaity is good enough for the former but not the latter. For the latter you want active sensors – radar and (I’m guessing) lidar, and certainly a plain old pressure altimeter. Thus, the best you could do with a GPS re-radiator is misdirect the drone in flight, not fool it into crashing into the ground or an obstacle.

  101. > I’m not really sure why you’ve even brought up the NEC in this instance.

    Because it is what building inspectors in Eric’s locale are likely to rely upon. “Code is not prescription” and all that.

    > The proper thing for the FCC to have done, of course, would be to take this regulation off the books.

    Some would say the proper thing for the FCC to have done would be to commit seppuku retroactive to June 1934. I don’t agree with the FCC here, but that doesn’t mean they won’t get a wild hair and go all Albert Knighten on you.

  102. 1) Damn, you can learn a lot just reading the comments here!

    2) Re: “..the unit I have bought…”
    Now you have to promise to report back. In a new post methinks. Having, I think, almost inadvertently pointed the research in a useful direction, leading to a hopefully successful result, *I want to know how it all works out*.
    ‘And I want it yesterday. And more tomorrow’ to quote Billy Connolly.

  103. >Because it is what building inspectors in Eric’s locale are likely to rely upon.

    I agree. I’m in a good situation, though – my wife is the Vice President of the Malvern borough council and the local cops and other politicos like me; they think of me as an upright citizen, which is reasonable because in my own flinty libertarian way I actually am. While I don’t suppose this would protect me if I committed a malum in se (and a bad thing if it did), I could expect to be cut a lot of slack on a minor local malum prohibitum like a building-code violation.

    >Some would say the proper thing for the FCC to have done would be to commit seppuku retroactive to June 1934.

    Agreed.

    >I don’t agree with the FCC here, but that doesn’t mean they won’t get a wild hair and go all Albert Knighten on you.

    Not worried. The FCC’s in-house geeks probably know who I am and would tell their bosses that persecuting me would be a really bad idea. Selective enforcement aimed at a geek who’s famous for doing worthy public-service stuff and has the press’s ear? Stooooopid. Recipe for bad publicity. Not the kind of move a bureaucrat aiming for a smooth cruise to retirement makes.

  104. If you want to dress up the wall penetration, you might look into something like an Arlington CER1. There are also some other options such as the CED1 and CED130 but I’m not sure if you would be able to pass the preassembled cable through either of those two.

    Pair one of these up with a bracket such as an Arlington LV1, Erico Caddy MP1, Leviton C0224, Carlon SC100RR, etc and I seriously doubt anyone will ever question the coax cable. Around here you constantly see CATV coax cables hanging out of holes in walls which were installed that way by the cable company.

    Actually… If the GPS signals work reliably through plastic, you might consider mounting a Keptel CableGuard CG-1000 or CG-1500 box outdoors and placing the antenna inside that. It would protect the antenna from the elements and make it easy to mount outdoors.

  105. Interesting news. Before I bought the $100 active repeater that the FCC got all pissy about back in 2005, I had sent an inquiry to a Finnish company called Rogers-GPS. They got back to me.

    It seems the guy who designed their $1100 high-end active repeater, Mikko Syrjälahti, is something of a fan of mine (he mentioned being an Emacs user, which suggests he’s probably a serious hacker who’s been following my work for some time). He went to the company CEO, got permission to send me an unit, and wrote me a nice email explaining that he’d used my software and wants to help. So, supposedly on its way to me is exterior antenna and re-radiator, 12 meters of cable, and a U.S. power supply. All he wants in return is a tip of the hat on the GPSD site.

    Larry Yelnick: Mikko says the cable is RG58, the old-style thin-Ethernet 50-ohm coax. A little Googling seems to identify that as a class CL2P/CMP cable, fully code-compliant for a residential installation. Do you know any differently?

    Now what am I going to do with the San Jose unit? Anybody wanna buy one cheap?

  106. >If you want to dress up the wall penetration,

    No wall penetration. My plan is for the outside antenna to be mounted on a TV mast attached just below the roof peak at one gable end of the house. Cable will run a couple feet down to the gable vent with a small hanging bight to shed rain, through and down the attic wall, then under the half-flooring to where it can drop into one of my office walls. 12m of cable should be plenty; finish with a BNC cover plate and season to taste.

  107. These have been in use for years in the aviation and emergency services industry to improve acquisition times for firetrucks, police vehicles, airplanes, etc when leaving garages, stations and hangars.

    Generally you still need a Part 5 license from the FCC to operate one if you’re not the government.

    In direct answer to your question, Eric. It’s going to depend on which RG58 you get. A big of googling shows that most are rated NEC CM/CL2/CEC/CMP, which should be fine. The rating will be on the jacket, of course.

  108. > Not worried. The FCC’s in-house geeks probably know who I am and would tell their bosses that persecuting me would be a really bad idea. Selective enforcement aimed at a geek who’s famous for doing worthy public-service stuff and has the press’s ear? Stooooopid. Recipe for bad publicity. Not the kind of move a bureaucrat aiming for a smooth cruise to retirement makes.

    OK. Believe what you like, but this FCC is currently back in front of the SCOTUS in FCC v. Fox.

    http://en.wikipedia.org/wiki/Federal_Communications_Commission_v._Fox_Television_Stations_(2012)

    Fox has a much larger megaphone than you, Eric. Might want to ask that lawyer wife of yours about it.

  109. @Larry Yelnick:

    Please reread that post you quoted. I SPECIFICALLY said that if you amplified the signal, then it was a different matter.

  110. Fox and ABC are claiming “selective enforcement” in the SCOTUS case. An argument that the FCC efficiently rebuts:

    http://www.americanbar.org/content/dam/aba/publications/supreme_court_preview/briefs/10-1293_petitionersreply.authcheckdam.pdf

    Second, a party whose conduct is clearly covered by a challenged law cannot evade the rule reaffirmed in HLP simply by packaging its claim as one of potential discriminatory enforcement. The Court in HLP made clear that the rule regarding as-applied vagueness challenges applies to both:

    “A conviction fails to comport with due process if the statute under which it is obtained fails to provide a person of ordinary intelligence fair notice of what is prohibited, or is so standardless that it authorizes or encourages seriously discriminatory enforcement.” We consider whether a statute is vague as applied to the particular facts at issue, for “[a] plaintiff who engages in some conduct that is clearly proscribed cannot complain of the vagueness of the law as applied to the conduct of others.”

    This blog thread being somewhat prima facie evidence that you both are aware of the law, and choose to ignore it. (Perhaps you should delete it?)

  111. > No wall penetration. My plan is for the outside antenna to be mounted on a TV mast attached just below the roof peak at one gable end of the house.

    By wall penetration I meant the interior drywall of your office wall, not the outdoor part. A BNC cover plate would look even nicer though ;)

    With the current plan for an active antenna system with a real antenna (not the prefab plastic encased one from earlier), I’d feel a lot better about the installation if you had a ground block on the coax cable run before you took it inside the vent into attic. (See “Earth Grounding and Bonding of Communications Systems” and figures 5-7, 6-3, and 6-4 in this document for some examples.) Having seen wifi antennas mounted on homes that had been hit by lightning, this is something you really shouldn’t overlook.

  112. On second thought, with that short of a run between the antenna and where the coax enters the attic, as long as the mast/antenna are properly grounded, you probably wouldn’t even need a ground block.

  113. D. Curmudgeon> 1) Damn, you can learn a lot just reading the comments here!

    Yep, that’s sometimes I just skim Eric’s articles and go straight to the comments ;^).

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