"mr1956" wrote
...
On May 24, 4:26 am, Owen Duffy wrote:
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

I have been reading the ARRL antenna book and while there is much
information, a lot of it is over my head. But one point I think a
chapter makes is that a 1/4 wave monopole will work better than a 1/2
wave using an artificial ground plane because of the way the voltage
peaks at the end of the antenna; or that is how it seems.

You are right. Antenna radiate from the end. So it should be thick as
possible.
How the length should be I do not know.
See: http://www.arrl.org/news/features/2007/09/28/03/?nc=1
"
The antennas are described as matched pairs, one 2.4 meters and the other
2.9 meters in length. The available literature gives no information on the
electrical characteristics of the antenna system but it is probably safe to
assume that they were some variation of a center-fed dipole with the longer
pair radiating the 20 MHz (14 meter) signal and the shorter pair set up for
40 MHz or 7.5 meters. Figure 6 shows technicians attaching two of these
elements to their angled connectors during assembly. Based on our assumption
about the antenna configuration, we can use modern modeling software to
analyze a dipole with of 5.8 meters in total length, angled to 70 degrees at
the center where it is fed by a single 20 MHz source."

Is possible to determine the length of the antennas in wave fractions?

S*