"mr1956" wrote
...
On May 25, 5:26 pm, Owen Duffy wrote:
mr1956 wrote
:


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.

The easy solution is the one I suggested.



I will post a link to some images of the project when I have time.
Information on the flight can be found atwww.lokiresearch.comand
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.

Loss on the coax can be much worse when it is mismatched. I gave you
figures for a 2000 ohm load (representing your end fed half wave antenna).

The loss for 0.3m with a 30 ohm load is 0.5dB, it is 90% efficient. If you
shorten it, you will reduce the loss, but I wouldn't obsess over it. The
problem you have right now is due principally to high standing waves on
the
line and unsuitable transmitter load, and you solve much of that using a
quarter wave antenna.

Without a diagram, we are at great risk of misunderstanding. If you cannot
affort the time to invest in making a drawing, you take the risk of
misinterpretation of your words. The connection from the shield to the
metal air frame and presumably metal skin should be short. Regard that the
effective length of the antenna in that case is the distance from the
shield to the tip of the whip.

Owen

OK, I think I know generally how to proceed from here. FYI, some
images of the previous design can be found at the following URLs:
http://i647.photobucket.com/albums/u...9/Curtjeff.jpg
http://i647.photobucket.com/albums/u...09/liftoff.jpg
http://i647.photobucket.com/albums/u.../Coupler02.jpg
http://i647.photobucket.com/albums/u...009/Elec05.jpg
http://i647.photobucket.com/albums/u...ery04small.jpg

This one is an image of the type of gear I normally work with:
http://i647.photobucket.com/albums/u.../Guam02004.jpg

So the key points are as follows:

1. Fabricate a 1/4 wave antenna and mount that is totally exterior to
the metal airfame and terminate the coax as close as possible to the
antenna.
2. I imagine that I could start off with a longer than needed whip
antenna, then trim it back while taking measurements with the RF
millivolt meter until it looks like I've gotten maximum signal
strength.
3. Use either a helix or yagi receiving antenna on the ground station
to boost the signal strength on the receiving end.

I accept that words alone are not always sufficient to describe such
things but I think the important issues have been well described by
everyone here. I can only make the RG-178 whip connecting the
transmitter to the antenna so short, as I need sufficient length to
remove the electronics from inside of the avionics compartment to
change batteries,etc. But I think the critical issues are having a
direct and solid electrical connection between the antenna and coax
and using a higher db antenna on the receiving end; the ground based
antenna may make the most difference in the end.

I'll have to go back and look at the posts to make sure I've gotten
all of the information right

You miss the most important. Helmut wrote "Do not use too thin wires, the
thicker the better."

and if anyone has any more questions
after reviewing the images please let me know and I will respond as
needed. Also, there should be an article coming out in Smithsonian Air
& Space magazine in the "Above and Beyond" section sometime in the
future describing this particular flight and more information is
online at www.lokiresearch.com (the Proteus 6 flight).
S*