Best way to fold a 70cm 1/4 wave whip

[JordanT]

I know, I know - don't. But if you had to, what is the best way to "shorten" a 1/4 wave antenna for a Big Red Bee beacon - spiral around the tube, looped, zig zag, stuft-it-and-pray?

(I won't get to try it out for a week, but it's never too early to plan!)

 

[CarVac]

I spiraled. You probably should not fold it since that will dramatically change the impedance.

 

[troj]

Why do you need to shorten it?

If possible, angle it such that it can pop back out to full length, once the recovery system deploys.

-Kevin

 

[bigredbee]

I found a cool solution to this problem, thanks to Curt von Delius for pointing this out.

You can purchase braided stainless steel whip antennas from Marshall Radio Telemetry, I bought the 7.8" RT Plus. It has a #2 threaded ferule on one end, and cut the length to 6.25" for 433 Mhz.

Then... get a matching #2 screw, hack off the head, and solder it to your Beeline transmitter antenna connector and screw on the antenna -- seems like a pretty great solution for tight fitting spaces -- the Marshall antenna is very "resilient".

Or you could use something like the Comet sma SMA501, a bit pricey, and I'm sure the performance is less than optimal.

Greg

 

[Worsaer]

Has anyone in the hobby played with conformal antennas? Perhaps too expensive, I just don't know.

For example:
https://www.haigh-farr.com/datasheets/wraparound.pdf

 

[JordanT]

Why do you need to shorten it?
-Kevin

Because the rocket recovery bay and nose cone isn't long enough to leave it straight?

Actually, I wondered about the alternate configurations after seeing CarVac's helix packing in Disappearing Act.

I wonder how hard it is to make a useful antenna like the haigh-farr wraparound. Maybe I'll create one and market it as the "Rocket Reacharound." :lol:

 

[cerving]

Looks like this is designed for in-flight telemetry use, it's got 7dB of gain in the axial directions and only 2dB in the radial direction, and that's at over 2Ghz. At 70cm you'd only get 2dB in all directions. It probably wouldn't be much help for finding your rocket on the ground, unless you happen to be pointing directly at the nose or tail and you're using a 2.4 GHz radio. One of the cheap whip antennae would probably work at least as well.

Has anyone in the hobby played with conformal antennas? Perhaps too expensive, I just don't know.

For example:
https://www.haigh-farr.com/datasheets/wraparound.pdf

 

[CarVac]

ooOOOooo.

It looks like you could even use one of those conformal antennas for GPS reception outside of a carbon-fiber airframe.......

 

[UhClem]

I wonder how hard it is to make a useful antenna like the haigh-farr wraparound.

Making one is very similar to making a printed circuit board.

Materials aren't too difficult. What you need is a nice stable material designed for the job. Something like Rogers RT/Duroid 5880 which can occasionally be found at Nebraska Surplus. I picked up a few square feet several years ago.

Design is both science and art. You can get very close to the final dimensions on paper but then you have to build a prototype and tune it by hand (sharp knife) to the final dimensions. This is complicated by the fact that the radome changes the center frequency but you don't want to bond it on yet since it would prevent trimming the antenna. But you only have to do this once since you can then adjust the artwork used for etching. The high dollar piece of equipment required here is something to measure the VSWR of the antenna as you adjust it.

But then you have to bond a protective radome onto the antenna. The big boys (Haigh-Farr) use adhesive films that require elevated temperatures and pressures to bond correctly along with surface prep using nasty chemicals. The substrate is fiber filled PTFE (Teflon) that requires extreme persuasion to bond to anything.

On the other hand, a patch antenna for 70cm is kind of big and it wouldn't be my first choice. In order to work the patch must be exactly one half wavelength long. This dimension is reduced by the dielectric constant of the substrate which for the Duroid is 2.2. So it would be about 70cm/2/2.2 = 15.9cm or 6.25 inches. The width of the patch determines the impedance with wider patches having lower impedance.

The CSXT rocket had quite a collection of patch antennas although the performance was mixed. That might be related to the missing or eroded radomes.