Robert Haston wrote:
I am a pilot in the 301st Rescue Squadron at Patrick AFB, south of Cape
Canaveral. We use helicopters to clear boats out of the Space Shuttle and
Rocket danger zones off the coast.
I've bought some marine band 161-162 MHz (87 & 88 marine band) Automated
Identification System (AIS) receivers to receive data transmissions from
larger vessels. To maximize the reception of targets up to 60 miles out, I
need a somewhat directional antenna, say a 2 or 3 element yagi.
The belly of our Blackhawk variant Pavehawk helicopters offers a mount and a
7 foot wide ground plane via the cargo hook on its belly. I can make the
mount sideways and reversible so we can track big boats coming at us from
the poles while we work all the little sport fishermen that infest the
narrow east west azimuth out to 50 miles.
What I would like to do is use half of an antenna and use the aircraft as a
ground plane to give us more ground clearance and less drag.
Here are my questions:
1. Can you use a yagi with elements only on one side when you have a ground
plane just like a quarter wave dipole?
In theory, with a perfectly flat perfectly conducting infinite plane --
yes. In practice -- probably. You could spend a bunch of time messing
with EzNEC or other antenna modeling software, or you could just give
this a try.
2. Would the 17" square hole that the antenna sticks out in the belly of
make much of a difference in the ground plane? (this could be remedied).
Oh yes. If possible you should make the antenna's ground plane
electrically contiguous with the skin of the heli, which should itself
be electrically contiguous -- I'm not sure if you can count on that
being the case.
3. For maximum gain, what needs to be grounded to the helicopter if
anything: non radiating elements, the receiver, etc.?
All of above. For this to work in theory you need to have the center of
each element well grounded to your ground plane.
4. What effect does diameter or material of elements have. I figure I can
chop down car antennas (my present dipole) which are very cheap and sturdy
enough to take 170 MPH winds.
In general elements need to get shorter as they get fatter, and the
bandwidth goes up.
5. What are good sites on making the right antenna?
Dunno, Google on "Yagi". If it were me I'd get a copy of the ARRL
Handbook or antenna book, copy a compact 2m yagi from that and scale it
as appropriate for the new wavelength.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See
http://cfaj.freeshell.org/google/
"Applied Control Theory for Embedded Systems" came out in April.
See details at
http://www.wescottdesign.com/actfes/actfes.html