RadioBanter - View Single Post - Piano Wire Antenna for Experimental Rocket

Owen Duffy · May 24th 09, 09:26 AM posted to rec.radio.amateur.antenna

mr1956 wrote in news:943a9bbd-214b-43b2-ac31-
:

I am looking for some help developing a properly tuned piano wire
antenna for an all metal experimental rocket.
This particular vehicle currently uses a Digi International 9Xtream
100 mw transmitter which operates using Frequency Hopping Spread
Spectrum from 910 to 918 MHz.

The first flight used a .062" diameter length of piano wire cut so
that the exposed length was about 1/2 wavelength. The wire antenna was
isolated from the metal airfame with a small nylon penetrator and
connected to the transmitter via about a 12" length of RG-178 coax.
The coax was terminated at the antenna via two small jumpers (soldered
to the center conductor and shield). The shield was grounded on the
metal airframe transforming the entire rocket into an artificial
ground plane (the antenna was also swept back at about a 45 degree
angle to reduce drag.

So you designed for a half wave antenna fed with 12" of RG178.

Lets suppose for a moment that the antenna has a feedpoint Z of say, 2000
+j0 ohms. The line will transform that to 5+j14 at the tx end, and with a
loss of 4.8dB (ie 33% efficiency). The tx is unlikely to develop is rated
output power into such a load, so there will be some further reduction.

Yes, an antenna of half the size (ie a quarter wave fed against the metal
rocket body) might well work ten times as good.

Owen