wrote:
On Mon, 03 Jul 2006 00:22:31 GMT, "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?

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).

3. For maximum gain, what needs to be grounded to the helicopter if
anything: non radiating elements, the receiver, etc.?

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.

5. What are good sites on making the right antenna?

Thanks,

I'll be happy to exchange lots of images of the equipment in action in
exchange.


Simple solution. Lookup a "Half Square" or "Bobtail" as this would
work for VHF and fit you mouning concerns.

WWW.CEBIK.COM Is a good site, read down the tales and technicals page
to the section on VHF antennas.

However, from 4000ft, your line of sight for a simple monopole is
sufficient to get you that far out. Hint: if you can communicate with
VHF aircraft radio (118-136mhz) at that range VHF marine will also
work. For VHF altitude is a valuable assist for range.

Allison

I believe that a helicopter pilot's nose starts bleeding at 1000ft -- at
least that's what my fixed-wing pilot friends tell me.

--

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