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I'm a CS college student working on the flight computer of my Rocketry Club's rocket. My flight computer has a 915MHz radio transceiver, and it needs to transmit data to my computer with another radio receiver on the ground. The rocket at the maximum height will be at least 10,000 feet (3048 meters) away from the ground radio, and probably no more than +2,000 feet of that. (Specifically, I'm using an Arduino paired with an Adafruit RFM69HCW @915MHz radio breakout board for both the flight and ground computers)

Both the ground and rocket radios have SMA connectors. I have not yet figured out how to deal with the antenna situation.

I don't know too much about amateur radios and whatnot, so I was wondering if anyone had a suggestion for an antenna that could handle the needed range, and if not, suggestions on what/how to build an antenna for the radios to handle my required range.

Thanks so much in advance!

chungmcl
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    Hello and welcome to ham.stackexchange.com! – rclocher3 Oct 20 '20 at 14:28
  • For the rocket end, try searching for "conformal antenna missile". They're often focused on patch arrays for (side-looking) radar, but there are low frequency designs of dipoles and other things. Plenty of testing required to get it to work. – tomnexus Oct 21 '20 at 12:16

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Your project may demand some engineering, I'm afraid. I see that Adafruit quote a range of 500 m with simple wire antennas for those modules, and that range is probably with optimal orientation of the antennas; in other words, the antennas oriented vertically with the antennas separated horizontally. You probably wouldn't be able to put a gain antenna on the rocket, and your antenna orientation may not be so favorable, so you may not even get 500 m.

You might try buying a couple modules, attaching simple wire antennas like in the picture on the Adafruit site, putting a module and antenna in a rocket body, and seeing what kind of range you actually get. Try all sorts of antenna orientations that you might encounter in a real flight. (Hint: you might be better off with the ground antenna a ways away from the launch pad, so you can trade range for a more favorable antenna orientation.) Maybe Adafruit are being very conservative.

If you don't have enough range, then you need to start thinking about your link budget. In other words, how do you get enough signal strength for a reliable connection. The answer might be more transmitter power, or a directional antenna on the ground. Maybe it would be easier to just store the data, and then process it after you recover the rocket.

If the answer is a better receiving antenna, one could be built very cheaply with solid copper wire on a handheld wooden boom aimed at the rocket. What isn't so cheap is the expertise and test equipment you would need to design and test such an antenna. Perhaps you could join forces with an amateur radio club.

rclocher3
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The rocket will have significant physical and aerodynamic constraints, so the best you can hope there is a whip. I'd design the rocket's antenna so that the rocket flies right and worry less about the RF. 3000m isn't all that far for light-of-sight radio, except that you're using low power, and moving really fast. That implies a receiving antenna with some gain (a Yagi, a quad or stacked Yagis or quads) that you can point toward where the rocket will be. It doesn't have to be fancy, just something you can move with your hands may work ok. Try it out on the ground first of course.

Duston
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  • I saw a presentation once about a model rocket logging circuit. The acceleration would rip a TO-92 package off the board if it wasn't glued down. So while a whip has the right radiation pattern it's not likely to survive. – tomnexus Oct 21 '20 at 12:13
  • First of all, if the rocket is only going 3000m, it's not going to be going that fast. Second of all, if the rocket body is non-metallic then the whip can be made part of the body. My brother built payloads for the "Super Loki" rocket, and you're right, the G forces were pretty amazing (0 to Mach 5 in 2.2 seconds, plus 0 to 1000rpm in less than a second, and that's all before the ejection charge detonates!) He literally would test his device by throwing it as hard as he could at the floor. But then his payload was going to 180,000 feet and not 10,000. – Duston Oct 21 '20 at 12:41
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While someone flagged this question as off-topic for this site, I think as a “technology of radio” question, it still stands. And I would say that it should be amateur radio-related, as I would recommend using ham bands for this application. Simply because a licensed amateur can use more power than is possible on an unlicensed band. With a good yagi-uda antenna on the ground pointed at the rocket, the rocket itself would not need much of an antenna at all (or much power), in order to be received from the ground. And if the ground station wanted to send signals to the rocket, a few watts of power into a yagi-uda antenna would be plenty.

Anyone wanting to use radio applications in a hobby environment should at least consider getting even the most basic amateur radio licence, just for the avenues it opens up for future experimentation. For UHF, you could use the 430MHz or 1296MHz bands, and have some very good antennas in a very small space.

Scott Earle
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