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About 30 years ago I worked in the electronic warfare and electronic
intelligence gathering business. We hung antennas of all types,
configurations and sizes on everything (and I do mean everything) that moved
or stood still. We had a sizable antenna range, anechoic chambers, etc., and
some of the best antenna engineers in the world. Computers were not far
advanced back then, so experimentation was critical. It was then (and
remains today) largely a Black Art. Lots of duct tape went into the
prototypes. While a simple mechanical engineer, I did learn a lot from these
guys as I watched them practice their trade.
The earlier discussions on antenna separations had good advice: keep them
apart, preferably at least 2 antenna lengths if they are about the same
elevation. See the top of C-182 and C-210 aircraft for a good minimum
separation standard. And keep in mind that these antennas have good ground
planes (the entire top of the wing).
No, you do not need to be transmitting out both antennas to make them
interfere with one another. A simple piece of rod of appropriate length held
up nearby and parallel to the antenna (same polarization) acts as a
reflector, and its degree of reflectivity depends on its length, orientation
(how parallel to antenna) and the frequency. Such reflectors are commonly
used to shape antenna patterns. Many (most) of the rods on your TV antenna
are passive, and merely used for pattern modification. Only a few are
actually connected to the antenna cable.
Due to the reciprocity rule of antennas it does not matter if you are
transmitting or receiving, The pattern (good or bad) at the same frequency is
the same going both ways. So antennas interfere with each other when
receiving as well as when transmitting. So some directions (where there is a
notch in the pattern) he can't hear you, and to conpensate, you can't hear
him either!
So on an all fiberglass airplane with a transparent fuselage, the antenna is
in fact interacting with pushrods, wires, etc., in the local neighborhood,
primarily with those things that are parallel to the antenna orientation, and
roughly the same size. The best cure for funny patterns under such
circumstances is a good ground plane, suitably sized, and well connected to
the coax shield.
On the Lancair IV, the com antenna in the tail is hurt to some degree by the
two unidirectional carbon fiber spar caps in the vertical stabilizer spar,
generally parallel to the dipole antenna, and nearby. Inclining the antenna
helps some, the fact that the carbon length is quite a bit longer than a
quarter wave helps some, and the Z shaped antennas helps some more. But it
will never be as good as a well designed monopole whip antenna sprouting out
the top of the carbon cabin (ugly, and a high drag location) or out the belly
(not bad looking, and in a low drag location particularly taking the hot,
slow engine cooling air stream into account). I flew with Brent, and I am
convinced the belly antenna is the way to go. Same installation used on
Derek Hine's airplane, same results.
I did the best job I could on the internal antenna using the Z configuration
and even adding ferrites to block reflected power from going down the shield
and affecting the pattern. However, I am counting on the belly bent whip
installed at about the FS 200 location as the COM 1 antenna. I supplemented
the carbon fuselage with an internal ground plane of copper fiber tapes, 8
total, radiating outward like spokes of a wheel bonded inside the fuselage.
The nut plates holding the antenna in place and grounding it to the ground
plane are riveted through the copper foil strips and the carbon of the
fuselage. Someday we shall see how well it works.
Jim Weir's literature on antennas is excellent, and his low buck antenna kits
with copper tape and ferrite rings are cost effective. I am using some of
them for market beacon antennas and such on my LIV, but buying commercial
units for VOR/ILS, COM, and transponder because of the carbon structure. I
think Jim's antennas would be particularly well suited for the LNC2 planes
with all fiberglass construction since there are many mounting location
options. His firm, Radio Systems Technologies (RST) has a web site from
which you can order. Do a search and I am sure you can find it.
Fred Moreno
[Here's the URL to RST... http://www.rst-engr.com/ ]
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