Message-ID: <3937A5C9.FECF0A61@regandesigns.com>
Date: Fri, 02 Jun 2000 07:17:13 -0500
From: Brent Regan <brent@regandesigns.com>
To: Lancair List <lancair.list@olsusa.com>
Subject: RE: ELT antenna
Mime-Version: 1.0

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Scott writes:

"I was just mounting my ELT and noticed that it needs an antenna that
appears
to be identical to my com antenna. has anyone split the com antenna and used

it for the ELT as well? also, do the "whip" antennas that come with ELT's
need a ground plane to operate in a composite aircraft?"

Splitting the COM and ELT is a bad idea. When the ELT is tested it could
damage your COM receiver. Not to mention that the performance of both will
decrease ~30%. Also, the COM antenna should be vertical to radiate to the
sides and the ELT should be horizontal to radiate up to the search planes.
Whip antennas radiate to the sides and not out the tip. Two winters ago a
Money pilot perished in the Sierras because the search helicopter lost the
ELT signal as they passed directly overhead. They searched a ring around the
plane where the signal was strongest but it took an extra day to find the
pilot, who had expired from injuries and exposure by then.

If memory serves me (a risky proposition) the wavelength of 122.5 MHz is
about 8 feet. A quarter wave antenna would be about 2 feet long (measure the
whip that came with the ELT). Electromagnetic waves travel slower in metal
than air so subtract a little, say 10%. Put a piece of copper tape about 22
inches in the tail cone. Put a piece tape about two inches below the first
(ground plane), at the end you are going to connect to, and connect it to
ground. Solder a coax whip to your antenna and connect it to the ELT. Test
it at the top of the hour. If you can borrow a VSWR meter you can play with
different lengths and configurations to get the best coupling.

Once you have it the way you like, then bond everything in so it will
survive a crash.

You could mount the whip horizontally in the tail to a grounded aluminum
plate, where is the fun in that?

Regards
Brent

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