On May 24, 5:49*pm, Owen Duffy wrote:
mr1956 wrote :

...



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.

Your transmitter is designed for a particular load impedance, almost
certainly 50 ohms... check the specs.

The load presented to the transmitter terminals depends on the feed point
impedance of the antenna, and impedance transformation that may occur on
the coaxial feedline.

In the case you describe, the *antenna system* design was most unsuited
to a 50 ohm transmitter, hence the considerable loss predicted.

Your newer proposal for a quarter wave wire perpedicular to the rocket
body with the coax shield connected to the metallic rocket body is not
exactly 50 ohms, but probably close enough that antenna system efficiency
will be fairly good and the transmitter should deliver most of its rated
power.

Owen

What I have been finding out is developing a matching circuit for the
output of the transmitter is not easy, at least for me and at that
frequency. It could be done if I was a cell phone manufacturer. I
think much will be gained by using the 1/4 wave as opposed to the 1/2
wave on the airframe and a yagi or helix antenna on the ground
station.

I will post a link to some images of the project when I have time.
Information on the flight can be found at www.lokiresearch.com and
look for information about the Proteus 6 flight at the Balls 17 event,
Black Rock, NV September 2008.

I'll also add some thoughts about how I intend to proceed based on
everyone's great input from this forum. One quick question: Am I
correct in assuming that the ground lead of the rocket mounted antenna
should be as short as possible? i.e., the connection between the
shield of the RG-178 and the metal airframe.

Thanks,

Curt