You Can’t Fix What You Can’t Measure

Last year, as my Corona Hobby™, I took up RC plane flying. I started out with discus-launched gliders, and honestly that’s still my main love, but there’s only so much room for hackery in planes that are designed to be absolutely minimum weight and maximum performance; these are the kind of planes that notice an extra half gram in the tail. So I’ve also built a few crude workhorse planes — the kind of things that you could slap a 60 g decade-old GoPro on and it won’t even really notice. Some have ended their lives in trees, but most have been disassembled and reincarnated — the electronics live on in the next body.

The journey has been really fun. I’ve learned about aerodynamics, gotten an excuse to put together a 4-axis hot-wire CNC styrofoam cutter, and covered everything in sight with carbon fiber tow, which is cheaper than you might think but makes the plane space-age. My current workhorse has bolted on an IMU, GPS, and a minimal Ardupilot setup, though I have yet to really put it through its paces. What’s holding me back is the video link — it just won’t work reliably further than a few hundred meters, and I certainly don’t trust it to get out of line-of-sight.

My suspicion is that the crappy antennas I have are holding me back, which of course is an encouragement to DIY, but measuring antennas in the 5.8 GHz band is tricky. I’d love to just be able to buy one of the cheap vector analyzers that we’ve covered in the past — anyone can make an antenna when they can see what they’re doing — but they top out at 2.4 GHz or lower. No dice. I’m blind in 5.8 GHz.

Of course, I do have one way in, and that’s tapping into the received signal strength indicator (RSSI) of a dedicated 5.8 GHz receiver, and just testing antennas out in practice, but that only gives a sort of loose better-worse indication. More capacitance or more inductance? Plates closer together or further apart? Try it out and see, I guess, but it’s time-consuming.

Moral of the story: don’t take measurement equipment for granted. Imagine trying to build an analog circuit without a voltmeter, or to debug something digital without a logic probe. Sometimes the most important tool is the one that lets you see the problem in the first place.

13 thoughts on “You Can’t Fix What You Can’t Measure

  1. You could do range testing the archaic way: keep moving the object further and further apart. The aircraft FPV forums are full of practical advise for extreme long range.

  2. I read with anticipation but article ended too early. This could easily be a multiple part article. Love to see the plane and tools you used. Over all ardupilot setup. Write up more.

  3. It’s good that you don’t trust it out of line-of-sight. RC aircraft must be kept line of sight, recent FAA rules disallow flying beyond line of sight without special premission.

  4. If the 5.8 GHz video link complies with EU regulations, “a few hundred meters” is not bad at all, considering the 25mW EIRP limit, omnidirectional antennas and a bandwitdh of several MHz.

    1. Yup. That’s actually where I am, both in the project and geospatially. A 25 mW Eachine TX06 camera/sender, in Germany.

      But I’d like to get some directionality on the ground station / receiver, so that I can push the range out a little bit. Where I fly I have great line of sight, but the thing turns into a tiny little speck. If I can point a gain antenna at it, I can probably get some more range?

      We’ll see.

  5. I could suggest

    A HackRF because it has two RF mixers, an IF mixer (MAX2837) from 2.3GHz to 2.7GHz and a LO mixer (RFFC5072) from 85MHz to 4200MHz. So on paper it could, with 20MHz of bandwidth, be tuned using the two mixers up to 6.86 GHz ((2700MHz-20MHz) + (4200MHz-20MHz) )

    And if you look in the source code it could go a bit further 0Hz to 7,250,000,000 Hz

    And if you dig deeper still
    It could probably be coaxed to tune a bit higher because they can tune the IF (2150 MHz 2750MHz) and LO (84.375 MHz to 5400 MHz) to go beyond what is specified in their datasheets (8110 MHz might be possible).

    But the circuit was never designed to handle frequencies that high so the expectation would be that everything would be heavily distorted and majorly attenuated and for it to even work that high up would require external highpass filtering and amplification, and even then it may not work due to a million and one reasons. But it is still interesting, at least to me anyhow.

  6. Won’t work beyond line of sight. Correct. Always maintain elevation for a radio link. 400 ft gets you about 20 miles LOS (rule of thumb) depending on intervening terrain. Keep in mind the ‘beyond visual control’ FAA regs if in the U.S.
    You might try using a directional antenna at the monitoring end, pointed to the aircraft. If you have a 6 dBi gain (typical) antenna on the aircraft and a 23 dBi gain antenna at the “base station” you will get an additional gain of 50 times the signal strength.
    Keep in mind it is routine to communicate 23,500 miles on 6 GHz with a directional antenna and about 3 watts TPO. (the wifi is typically 0.2 to 1 watt transmitter power output). You won’t have the gain of the VSAT but it isn’t the limitation of the LOS radio link losses, it is terrain obstruction, reflective attenuation losses and localized carrier to noise and interrference ratio from other co-channel users and thermal noise. Antenna gain improves the path C/(N+I) ratio (or really Eb/No) by using a focused beam which increases the desired carrier signal strength and attenuates undesired azimuths and associated interference from elsewhere.
    I would imagine you could get much farther than a few hundred meters with additional antenna gain. Unless you are very sophisticated you won’t be able to put a high gain antenna on the aircraft because of the azimuth directionality and the requirement to point it back to the source.
    It is very common in WISPs and point to point comms to meet multiple mile LOS links with directional antennas.

    1. Erm, WiFi is limited to 0.1W at 2.4GHz, and 0.2W at 5GHz. There are *licensed* 5GHz bands that allow 1W and 4W respectively, but not for general WiFi use. Also there are variations per country, but the above is the general ETSI standard.

  7. Seems like you could still get some useful data from the cheap vector analyzers at 2.4 ghz. Start with a 2.4 ghz transmitter and antenna on your RF model, tune based on measurements, and use what you learn at the larger scale to tweak your setup for 5.8 ghz. Many engineering disciplines use scale model testing to validate plans for full scale systems. RF at 5.8 ghz is certainly black magic to me, but not that much crazier than fluid dynamics. Even NASA doesn’t have the budget to build full scale wind tunnels, but the engineering must continue, so scale models in smaller wind tunnels have always gotten the job done.

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