Easy homemade Outernet antenna
The quest was to build a working solution:
- for less than $1 total cost,
- doable without power tools and soldering,
- in less than 30 minutes.
Tried several designs, ended up with a helical. No surprise, amateurs and government organizations use this type to communicate with satellites for decades.
This picture guide is more of a demonstration that it is possible, rather than a theory dissertation with "assume" frequently popping up. I've built antennas, made mistakes, documented steps and failures.
Overview info here, free online calculator helped a lot, frequency is 1542 MHz, play with it. As a directional antenna, a helical can be scaled up and down for ADS-B or WiFi use, but this guide only details steps for Outernet and possible L-band use.
Chip powered by official battery bank, rtl-sdr.com v.3, Outernet preamp, SNR ratio actually receiving packets:
Plastic bottle perfectly approximates required 6.2 cm diameter, 4 turns, 5cm coil spacing:
No soldering, center conductor wrapped around coil, braid taped to metal. Coax terminates in F-type connector.
Square or round metal plane, as long as edges are straight. 5" / 13 cm dimensions work, no science behind these values, just the observation that a half-liter cut up
There's no way such an ugly/redneck/unscientific/pick one derogatory description works. I must have had signal lock for only a moment.
Update Nov 24, 2016: 17 cm by 17 cm metal plane from a cookie tin, same concept, 7-9 dB SNR ratio:
Improvement could be down to:
- Different location,
- Better line of sight,
- Less electrical noise,
- More attention to construction,
- Larger metal plane,
- All of the above.
Only 3dB or more SNR is required, so 7-9 dB is plenty.
- half litre water bottle with straight sides is just the ideal diameter, height allows 4 complete turns with support,
- mark bottle every 5cm,
- cut coax lengthwise with a sharp knife, always AWAY FROM YOURSELF with no flesh (inquisitive pets!) in the cutting direction,
- on a wooden surface which is NOT the kitchen table as blade can slip off,
- pull out white bit on an easy to sweep surface, cleaning up small wires from carpet takes ages,
- Right Hand Circular Polarisation means that wire rises to the right from the bottom, so it will be on the front of the bottle towards you, not behind it. Unlike with the official Outernet antenna, final orientation does not matter,
- leave white squashy bit on, saves time and effort, easier on your hands and wire will be less prone to kinking,
- wrap wire around smaller diameter object, broom handle works well, leave it on while cutting up metal can,
- do the bottle part first, then do the metal plane,
- bottom of coil should be as close to bottom metal as possible, sources all agree on this and all better looking or pro antennas have a minimal gap.
The official Outernet antenna is much more forgiving, only requires a general direction. In comparison, helicals must be pointed exactly at the satellite, which you obviously can't see, so really small movements are necessary. And I mean feather touch level, but get it right, and you got 5dB+ SNR ratio.
What didn't work
Spent a few frustration riddled evenings reinventing the wheel, sliced bread and hot water at the same time, how do you spell epic failure? Learning curve.
Patch antennas: tried to copy Outernet's antenna and build Adam's design, but simply couldn't. Materials, dimensions, my clumsiness, pick one reason. Helicals are just so much easier.
WiFi Yagi: gave it a go, no joy.
Larger and smaller diameter tubes: some sources indicated that wide bandwidth compensates for construction errors. Probably does, but not when diameter is 4cm or 8cm tubes. It's gotta be 6-ish cm.
Spacing: coils should be evenly spaced. If it doesn't even remotely resembles a helical antenna from Google image results, it won't work.
Cable ties: slip around, tape is easier to use and less expensive.
Top of coil: I use sticks for a reason in images, because top must come out to the same level as coils below, and that requires support. It looks horrid, I know, but I aimed for 3+ dB SNR, not a beauty pageant podium finish.
Power bank, Chip, rtl-sdr.com v.3, and preamp fits into a food saver, throw the antenna into a bag and you're good to go.
Serious gain can be achieved with monstrous (longer helix means double gain) or multiple antennas, so the next step is constructing a quad with long boom lengths. And I mean 20 turns, but that will require PVC pipes. Will see.