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Date: Sun, 30 Oct 2005 21:52:51 -0500
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From: "Robert R Pastusek" <rpastusek@htii.com>
Subject: RE: [LML] Re: TCAD Antenna's on a Legacy
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Barry Knotts/Craig Bergland wrote:

  I'm building a IV-P and I'm starting to install stuff in the lower half of
the fuselage.  That brings up antenna placement and I'm pretty ignorant of
what I need to plan for.  I'm not planning on TCAD or a stormscope now, so
that will simplify things.  I like the idea of hidden antennas, but I like
good communications even better.  I've been discouraged about some of the
internal antennas...but mostly I've been confused about conflicting posts on
LML.  Here's my current take of my planned antenna needs:

  ELT - under glass portion of turtle deck
  TRANSPONDER - external antenna on belly (with ground plane)
  GS1 - external strip antenna at left wing root
  GS2 - external strip antenna at right wing root
  COM1 - belly whip (with ground plane)
  COM2 - belly whip (with ground plane)
  NAV1 - vertical stab
  NAV2 - ?
  GPS & satellite radio antennas - under the glass portion of the turtle
deck.

Barry/Craig,

I'm not an authority, but my study gave the following additional
information:

The com radio receivers on the ground are vertically polarized, so the
antennnae in the aircraft should likewise be vertically polarized to yield
best comms. I have a fiberglass vertical stab, and installed a simple
aluminum strip antenna inside. I've heard mixed reviews on it's
effectiveness; these are pretty efficient designs; I expect performance is
highly affected by installation and wiring? In addition to this one, I put a
whip antenna between the gear legs with a large AL ground plane inside the
carbon fiber belly for my second comm antenna. Main argument is that it's
serviceable/replaceable...

The ground nav systems are horizontally polarized, so the nav antennae in
your airplane need to match. The bottoms of my winglets are made of
fiberglass (rest is carbon fiber) to accomodate the nav antennae. I
originally put one in each winglet, but later took one out to install a
retractable landing light. Rationale was that I expect to primarily use GPS,
and thought one Nav antenna sufficient... In any case, I wouldn't recommend
using the vertical stab for a nav antenna. Should be a good place for a comm
antenna however. If you're not installing winglets, consider a second whip
antenna on the belly. Keep all the antennae on the belly as far as possible
apart and you'll do fine. Comm forward as you can install the largest ground
plane up there; transponder antenna in the back as it only needs about an 8"
ground plane, and curving it to the fuselage will only minimally affect
performance to the sides of the airplane.

GPS and satellite radios need minimal obstruction to work properly. I hate
to blaspheme in this distinguished forum, but I'm putting my GPS antenna on
top of the cabin where the connection and mounting can be accessed through
the overhead panel. Rationale is that my BlueMountain uses the GPS to
stabilize it's other functions, and maintenance of a good/continuous signals
(there are typically several comming from different satellites
simultaneously) is important to me. Any antenna you put under the glare
shield will be blanked to part of the sky; if it's important to have
continuous coverage, move it to the top of the airplane. I considered
putting the GPS antenna in the top forward part of my vertical--they are
typically small enough--but closed and painted that area long ago. That
might be an option for you?

Hope this helps.

Bob Pastusek