I'm surpised no one has asked how much transmit power is available, and how
high the rocket is expected to fly.

Regardless, unless a micropower transmitter is being used, it seems to me
that a very simple quarter wave nose tip made if 1/8" rod or similar
material would only protrude a few inches above the nose and would probably
do the job. At this frequency, even a tiny metal mass inside the nose would
probably be a sufficiently good counterpoise for feeding it.

This seems like a simple problem (or 'no problem'). I'm surprised at the
complex solutions.

Joe
W3JDR


wrote in message
...
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.
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...


Paul (Kl7JG)