Mid-airframe tracking antenna solutions/pictures

Planning a 3" diameter scratch build where I want to use the nosecone volume for the main chute in a dual deploy set-up to keep the payload section as short as possible. I want to be able to use a GPS tracker mounted in the av-bay/electronics bay aft of the main chute. I have not purchased a GPS tracker yet. Leaning towards BRB900, EggFinder, or possibly waiting for MissleWorks GPS solution to hit the market. I did a search and read a bunch of threads on trackers and antenna mounting/position noting the avoidance of positioning the antenna in the same space as the threaded rods. I saw a lot of pictures of nosecone mounted trackers and antennas but very few of how trackers mounted in av-bays with threaded rods get the antenna into relatively interference free space. Any help/picutres would be appreciated. Thanks.

I'm a fan of the BRB RF beacons, and spring for the SMA antenna version. The first one I had, I mounted in a pill bottle. Rather than using the supplied whip, I bought a flexible rubber duck antenna, and mounted the beacon to the lid by putting the SMA jack through a hole in the lid, and using the rubber duck as a nut to hold the works in place. The lid also had an eyebolt mounted to it. This arrangement didn't work very well, as the enclosure broke upon ejection, and the beacon was lost. I did recover the rocket, however. It was rather embarrassing. in retrospect, it would likely have worked better if the eyebolt had gone through the bottle, as well as the lid. I recently got a second beacon, and my current plan is to use a 2.1" av bay, and rather than the stock whip, have four aluminum strips epoxied to the outside of the av bay in a cruciform pattern, and fed as a pair of dipoles, in phase. I haven't modeled that antenna, but it's on my to do list.

See my Frenzy build. Mid av-bay with threaded rod, BRB900, and Perfectflite MAWD. No problems.

Buckeye said:

See my Frenzy build. Mid av-bay with threaded rod, BRB900, and Perfectflite MAWD. No problems.

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Nice Frenzy! So, is the thought in your layout that you have the antenna off to the side, not between the threaded rods? Did you do any testing on range with different antenna placement(s)?

kjkcolorado said:

Nice Frenzy! So, is the thought in your layout that you have the antenna off to the side, not between the threaded rods? Did you do any testing on range with different antenna placement(s)?

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Nope. I am not that sophisticated. Altimeter on one side, BRB900 on the other just so that the mounting screws did not clash.

Granted, I have not had to do any hardcore tracking over great distances with this setup. So, I am not sure if there is troublesome attenuation of the signal.

So, I played around with an av bay in EZNEC, and ended up with a dipole of sorts, each leg being just under seven inches long. the feedpoint is located at the center of an end bulkhead, and the legs of the dipole are routed across the bulkhead, and down the side of the av bay. The threaded rods are oriented perpendicular to the dipole elements. They didn't seem to have much effect on the radiation pattern, it was almost omnidirectional. I tried to upload the file, but this forum didn't like the format...

I don't think you need anything special as far as mounting goes. I flew a homemade tracker with the the same 250mW Xbee transmitter that the BRB 900 uses to over 7 miles a couple weeks ago, The transmitter was mounted between two pieces of allthread and I had no problems tracking it. Only significant difference was I used a longer (about 18 inches) 5dbi rubber duck antenna on the receive side from L-com. Pics of av-bay are here: https://rocketrycenter.com/showthread.php?tid=86&pid=1556#pid1556.

Kendall,

Why not use the all thread as the antenna and avoid signal attenuation. If you look at: https://www.hamuniverse.com/jpole.html , you'll see a simple algorithm for constructing an omni-directional simple J pole antenna. Section A of the J pole can be the all thread. Three caveats pertain: 1. better results would occur with a readily available copper (rather than steel) all thread; 2. The size of the AV bay would be somewhat dependent on the antenna length; and 3. ground testing is recommended to rule out potential interference with GPS unit function by the altimeter and possibly the reverse.

Regards,
Fred (KG4YGP)
Level 2, TRA 15606
ROSCO member

I would think that the threads on the rod would make it a less than ideal antenna element, and the presence of other electronics and batteries in the AV bay wouldn't help the signal going out either.

FMarvinS said:

Kendall,

Why not use the all thread as the antenna and avoid signal attenuation. If you look at: https://www.hamuniverse.com/jpole.html , you'll see a simple algorithm for constructing an omni-directional simple J pole antenna. Section A of the J pole can be the all thread. Three caveats pertain: 1. better results would occur with a readily available copper (rather than steel) all thread; 2. The size of the AV bay would be somewhat dependent on the antenna length; and 3. ground testing is recommended to rule out potential interference with GPS unit function by the altimeter and possibly the reverse.

Regards,
Fred (KG4YGP)
Level 2, TRA 15606
ROSCO member

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That's worth a try, though I'd lean more towards a dipole rather than a J pole, as a 70cm J pole would be about 20in long (3/4L), whereas the all thread in a typical avbay is pretty close to 1/4L at 70cm. It would be possible to make a J pole by attaching a wire whip to one of the rods, and having the other form the tuning stub, but I'd be wary about having a wire whip dangling loose outside the av bay, getting tangled with the recovery harness. In any event, I wouldn't be terribly worried about the threading affecting the signal, not at UHF anyway.

Chris,

In principle-I agree for the 2 meter or 70 cm bands. However, my presumption was that Kendall is unlicensed and using the 33 cm (900 MHZ) band, then the larger "arm" of the J pole is only 9.36 inches while the shorter section is only 3.12 inches (based on a transmitting frequency of 900 MHZ). So, for a 3 inch diameter rocket, an approximately 9 to 10 inch AV bay is feasible. Alternatively, your thought of a dipole or possibly of an inverted 1/4 wave length ground plane (in order to maximize "downward reflected transmission") would work. The inverted ground plane would have a vertical element of only 8.25 cm (1/4 wave length) and ground plane radials of 4.12 cm (1/8 wavelength) all of which would take up less than 4 inches in length.

Regards,
Fred (KG4YGP)
Level 2, TRA 15606
ROSCO member

FMarvinS said:

Chris,

In principle-I agree for the 2 meter or 70 cm bands. However, my presumption was that Kendall is unlicensed and using the 33 cm (900 MHZ) band, then the larger "arm" of the J pole is only 9.36 inches while the shorter section is only 3.12 inches (based on a transmitting frequency of 900 MHZ). So, for a 3 inch diameter rocket, an approximately 9 to 10 inch AV bay is feasible. Alternatively, your thought of a dipole or possibly of an inverted 1/4 wave length ground plane (in order to maximize "downward reflected transmission") would work. The inverted ground plane would have a vertical element of only 8.25 cm (1/4 wave length) and ground plane radials of 4.12 cm (1/8 wavelength) all of which would take up less than 4 inches in length.

Regards,
Fred (KG4YGP)
Level 2, TRA 15606
ROSCO member

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I could see a ground plane antenna working well during flight, but once the AV bay's on the ground, it would most likely be on it's side. I would think that a ground plane antenna would become quite directional if the ground plane was perpendicular to the ground.

Fred, you are correct that I do not have a HAM license and my tracker choices noted in my original post reflect that. To clarify for anyone else who has a suggestion, I am looking for antenna placement advise/pictures either in, or outside of a mid-frame av-bay. I just want the best performance from the tracker and am concerned about an antenna inside an av-bay with the threaded rods. Here's a little more info that might help. The av-bay will probably be the kit from MAC Performance - 8" long and aluminum threaded rod. I believe I read somewhere that alluminum is a little less of a problem when it comes to signal disruption. Thanks for everyone's input so far. Please keep it coming.

kjkcolorado said:

Fred, you are correct that I do not have a HAM license and my tracker choices noted in my original post reflect that. To clarify for anyone else who has a suggestion, I am looking for antenna placement advise/pictures either in, or outside of a mid-frame av-bay. I just want the best performance from the tracker and am concerned about an antenna inside an av-bay with the threaded rods. Here's a little more info that might help. The av-bay will probably be the kit from MAC Performance - 8" long and aluminum threaded rod. I believe I read somewhere that alluminum is a little less of a problem when it comes to signal disruption. Thanks for everyone's input so far. Please keep it coming.

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I'd tend to think that aluminum would be worse than steel in terms of signal disruption, as it's a better conductor. Another thought I have would be to use a non metallic all thread, and make a J pole out of copper strips epoxied to the side of the AV bay. Mcmaster Carr has several types of non metallic all thread, ranging from nylon to fiberglass.

I'm not familiar with the relative strengths of steel, aluminum, nylon and fiberglass all thread. For a 3" rocket, totally guessing on a final 'to the pad' weight between 5 and 8 lbs., would nylon or fiberglass all thread provide enough strength during a less that optimal seperation?

Just took a quick look at McMaster-Carr. Nylon and fiberglass threaded rods are not rated for tensile strength or hardness. The following is for your reading enjoyment. From McMaster-Carr's website, assuming 1/4" threaded rod:

High-strength steel - minimum tensile strength 150,000 psi; Rockwell hardness C33
Medium-strength steel - 125,000 psi, C35
Stainless-steel - 70,000 psi, B70
Low-strength steel - 53,000 psi, B61
Nickel - 80,000 psi, no hardness rating
Aluminum - 42,000 psi, B40

kjkcolorado said:

Just took a quick look at McMaster-Carr. Nylon and fiberglass threaded rods are not rated for tensile strength or hardness. The following is for your reading enjoyment. From McMaster-Carr's website, assuming 1/4" threaded rod:

High-strength steel - minimum tensile strength 150,000 psi; Rockwell hardness C33
Medium-strength steel - 125,000 psi, C35
Stainless-steel - 70,000 psi, B70
Low-strength steel - 53,000 psi, B61
Nickel - 80,000 psi, no hardness rating
Aluminum - 42,000 psi, B40

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If you look under raw materials, they give information on the mechanical properties of the plastics they sell. I'd look there for a ballpark estimate. I would assume that when they say the plastic all thread isn't rated for tensile strength, it means that there hasn't been any proof testing done on it.