On Thu, 05 May 2005 21:43:43 -0700, wrote:

I've voulenteered to help the SDSU mechanical engineering studens get
telemetry from their rocket see:
http://www-rohan.sdsu.edu/~sharring/sdsurocket.html.

I have all the electronics working, I'm using a commercial 910Mhz
telemetry radio, I have every thing working except the antenna.

For the last launch I burred a dipole in the plywood fin, alas
the rocket did not launch it caught fire and burned up the fins.
(It did not burn as far as the electronics.)

The new fins are carbon fiber composite so no antenna there...

The rocket will get to mach 2 so small wires sticking out will
probably break or burn up.


I have enough power and ground side gain that I need no gain
from the rocket, an isotropic radiator with 3db of loss would be fine.


Any suggestions?


My ideas and thoughts:

1)Simple 1/4 wave vertical sticking out the bottom plate of the rocket
near the engine.

Pros:
simple.
Cons:
lots of metal to block the signal and mess up the pattern.
Not clear if the ionized exhaust will block the signal.

Phoenix, Standard and other missiles use rear data link antennas
buried behind the rocket plume without trouble. Of course these are
at X-band, not 900 MHz, and receive only with *really* high powered
transmitters.

Antenna pattern is almost exactly wrong.

(Telemetry really needed for recovery tracking so ionization fading is
not a deal killer)


2)Horizontal dipole at the bottom plate of engine.
All the problems of #1 except pattern.




3)Put Fiberglass windows in the electronics bay near the nose of the
rocket. One window on each side, Driving two hosrizontal dipoles with
a power splitter, one dipole on each side.

Pros: Easy to do.
Cons:
I don't know what the pattern would be like, or exactly how I shoudl
phase the two antennas on opposite sides. (Some metal between then so
not a clean situation.)

Resources:
It have a minicircuits SMA 2 way power splitter, and can make precise
metal parts (0.002" or better).
I do not have any antenna testing equipment that is any good at
900Mhz.
so any suggestions...

Well, the fact that you don't have any test equipment is a real
downer.

The best suggestion, although I think time is an issue for you, would
be to go to a commercial vendor and beg for a "contribution". My
former employer (Hughes) gave money, time and materials to various
universities all of the time.

The elegant solution would be a conformal patch but I imagine this is
beyond your resources.

http://www.uaf.edu/asgp/asrp/srp4/sr...chantennas.htm

It doesn't look like there is any roll stabilization so you might need
some pseudo "omni" pattern, during flight. But if you are only
looking for TM after burnout and during the return to Earth (I assume
dangling on a parachute) then you will know the attitude (at least
"up" and "down"). Personally, I think that flight dynamics data
during the powered phase would be more interesting to engineering
students. [g].

Fiberglass window(s) with dipole(s) behind them would probably work
okay. Two with equal power split would be fine, however, one might do
okay too.

Regardless, it sounds like a fun project. Have fun.