[Antennas] 2m antenna for airborne transmitter?

Wed Nov 23 02:41:26 EST 2011

I am putting together plans to launch a high-altitude balloon some
time next year. It may reach 30km altitude, covering 1x ... 10x as
much ground horizontally, for a total flight duration of 2 ... 3
hours. The payload is supposed to gather various data (temperature,
pressure, humidity, etc.) and log it locally to an SD card. GPS data
will be relayed to the ground over APRS on 144.390MHz. The brain of
the payload will be an Arduino. The transmitter will probably be one
of those lightweight small modules that take 5V CMOS input on one pin
and modulate a 50ohm RF output on another pin - something similar to
this:
http://www.radiometrix.com/content/hx1
I intend to relay GPS data to the existing APRS network on the ground,
and track the payload from an Internet-connected portable device. But
the tracking vehicle will also be equipped with an FT-817 and a laptop
and will receive and decode APRS data from the balloon independently,
as a backup system. I still need to fine tune many details related to
APRS itself, since I've never done it before, but that's another topic
for another forum. I live in the San Francisco Bay Area, so at least
for a while the payload will fly above a dense APRS network.
The thing is, I'm not sure what's the best antenna to use on the balloon.
Obviously, it needs to be as lightweight as possible. The balloon
doesn't generate much lift, so any weight savings are welcome.
Large, rigid, sprawling structures are not good either, even when they
are not heavy. There are strong winds at high altitudes, and a large
antenna may spin the payload excessively.
Ideally, I'd like the antenna to more or less survive touchdown after
a long descent on a parachute - this is in case I lose signal on the
last part of the trajectory, I'd like the beacon to keep transmitting
after touchdown, so I can locate it with my transceiver in the woods
or on a farm or wherever it may fall.
Finally, the radiation pattern should be a big wide lobe, facing
downward, extending from horizon to horizon in all directions. That, I
guess, is probably not 100% realistic, but I'd like to stay close to
the ideal.
One thing I could think of is a 1/4 wave ground-plane whip. I could
use thin aluminum tubes to make the elements. Should I keep the
"ground plane" elements parallel to the ground, or should I bend them
45deg upward?
Another idea: the balloon could trail a thin long wire. That design
works okay in HF, but I'm not sure how to make it work in 2m, with a
50ohm output transmitter, lacking a true ground connection.
For the tracking vehicle, again, probably a 1/4 wave ground plane?
Aiming at the balloon with a 2m Yagi from a moving sedan - that
doesn't sound like a sane idea to me. :)
Any suggestions, comments, hints, are appreciated. Thank you.
-- 
Florin Andrei
KG6YHQ
http://florin.myip.org/