REFLECTOR: ground plane

[Bob Jackson (Jax Tech)]

I'm not a fields or RF type of EE, but I do know that the new ELTs are
transmitting UP to a satellite receiver, not DOWN to ground receivers, so in
general it's preferable to have the ELT antenna on top of the aircraft, not
on the bottom.  And I think the ELT transmitters are a lot higher power than
GPS antennas that are also transmitting up to satellites and where you
almost must have the antennas facing up, so maybe you can get away with
having the antenna on the bottom for the ELT.

 

The ELT antenna itself is better on top of the aircraft, and the ELT box and
crash sensor on the bottom and in a place that is most likely to survive
crash damage.

 

We put our antenna on the top, center of the fuselage, behind the full wing
doors.  It's the only external antenna we have on our plane.  We made a
'sparse' 4 ft diameter ground plane on cabin ceiling using 1/2" copper tape
glassed on.  I believe the center 12" or so is solid copper, but the
remainder of the 4ft ground plane consists of about (8) radial 'spokes' made
with the 1/2" copper tape that extend out about 2 feet from the center.  My
understanding is that the spokes are adequate to create a full, circular RF
field that provides the effect of a 4 ft diameter ground plane.

 

Obviously, being on the fuselage roof our ground plane is not flat, but is
curved to follow the shape of the ceiling.

 

It works great, we know, since I set off the ELT once by accident inside our
hangar when the door was closed.  Our hangar does have six skylights high up
on the ceiling, so obviously enough power was radiated up through the small
skylights to get to the satellite.  When I got home that night, my wife said
the Air Force had called to let her know that our plane was 'done' and had
crashed in a location that oddly coincided with where we park it in the
hangar!  She thought it was a prank call, but promised to send out a search
party pronto!  That little mistake also cost us close to $100 to replace the
one-time-use ELT battery!

 

Bob Jackson

N2XF

 

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From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Lou Stedman
Sent: Wednesday, April 20, 2011 2:24 PM
To: Canard Aviators; Velocity Reflector
Subject: REFLECTOR: ground plane

 

I too just received my new ELT, Ameri-King AK451-S. My old elt was mounted
under my rear seat which is where I plan to mount this one. My instructions
say that 24" legs on the ground plane would be the best but at least the
same length as the antenna I will be using. That antenna is 14". I think I
have room on the exterior side of the belly to install the foil which they
suggest. I will have to install something into the floor to accept the
antenna but not sure what that should be. Any suggestions as to what
material to use for the foil ground plane and the hard point would be
appreciated. Thanks

 

Lou Stedman
Velocity SEFG
N7044Q
Olean, NY

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