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From: (Badwater Bill)
Newsgroups: rec.aviation.homebuilt
Subject: Re: Comm antenna mount position
Date: Tue, 30 Dec 1997 18:02:02 GMT

On 30 Dec 1997 15:50:34 GMT, (JFLEISC) wrote:

>This is a question for you RF experts out there. I have been experiencing an on
>going yet intermittent comm problem. Occasionally my transmissions are heard as
>weak and garbled . I have been through all my wiring (have two separate radios,
>two separate antennas, headset plugs before and after the intercom, and two
>headsets.) This happens air to air or air to ground. (I fly a lot of loose
>formation with my friends) When it happens, swapping radios, headsets, jacks,
>etc. has no effect. A power meter in the coax always shows full power output.
>After landing, ground control often says I'm unreadable.
>Here was the tip off; Recently one of my friends, while about 1000ft away in
>formation, said my transmission was bad (he didn't sound too great either) but
>someone else half a state away said I was loud and clear!

This  happens to me too.  In fact it happened just the other day in my

>Probably needless to say, both my comm antennas are mounted underneath. (About
>1 and 2 ft back from the landing gear pickup on a RV-4).
>Question #1; Is the landing gear absorbing, reflecting, blocking, and or
>distorting the signal at certain angles to the airplane?

Could be.  The gear legs are metal and will resonate at the right
frequencies.  If the spacing between the transmitter antenna is
anything other than one wave length to each gear leg (which can only
happen at one frequency) then you will get parasitic resonance and it
will radiate just like your main antenna but will cancel in various
directions.  Look at a normal TV antenna.  You only have one active
element, all the rest are cut to various lengths related to the
television channel frequencies and become resonant to amplify the
signal so the active element gets a stronger signal.  This happens in
transmission too.  The spacing for a given frequency is critical.  You
aim your TV antenna to eliminate this.  In your RV you are
omni-directional and have no control over it.  I can see a case where
you are Xmitting, both gear legs are resonating and you are
transmitting with many nulls in various directions, not to mention
that your wings are in the way if your buddies are flying above you,
which can cancel everything out because the RV wings are metal.

>Question #2; While landed is the signal largely absorbed by the ground?

No. The ground absorbs nothing.  In fact the ground has an
amplification or cancellation effect because it makes a mirror image
equivalent to your transmitter antenna almost like a mirror does for
light.  You get a "Virtual" antenna underneath the ground plane that
acts just like another antenna.  However, again these waves are about
2 meters long and as you change your spacing to the ground the virtual
antenna acts like an exact opposite dipole antenna and can cancel your
signal in ALL directions for various altitudes.

>Question #3; If #1 is true, why are virtually all transponder antennas mounted
>underneath, even on fixed gear?

Short wave length and much less resonance in the gear legs.  Those
gear legs are about 1/2 wave length for the 2 meter band.  I suppose
they can really get to resonating at the frequencies where we operate.
My transmitter antenna is in the wingtip.  It's pretty much
horizontally polarized and I have a hell of a time transmitting in
various directions too.  In fact, the antenna is a welding rod simply
mounted in the right wingtip.  IF the receiver is behind me at my 5
o'clock,  I can't hear them and they can't hear me.  They're right off
the tip of the welding rod.  If the receiver is at my 9 o'clock they
don't hear me either because my airplane is in the way.  I have to
lift the right wing and punch some right rudder to knife edge her when
I'm trying to Xmit or receive to or from the 9 o'clock position.

The solution is to mount the damn thing on top of the turtle deck and
that's what I'm going to do.  It's vertically polarized as it should
be.  There are no parasitic resonators around it, other than the
vertical stab which will cause some problems but it's a big plane of
metal rather than a narrow tube like the gear.  Now Jim Wier disagrees
with me since he worries about the RF being so close you your head.
Well, as a Certified Health Physicist I have to tell you that there
has been absolutely NO research in the 2 meter range that shows any
biological effects to animals or humans at these frequencies and
wattages.  The energy is too low and there is no ionization from this
radiation. We aren't talking kilowatts here, or continuous operation
either.  We are talking about 10 watts, 2 meters behind your head in
short bursts.  That's where I'm moving my antenna.

Jim told me of a new CFR on this I am unaware of, but that's just
government bull shit.  I can show you tons of stuff in the code of
federal regualtions concerning ionizing radiation that is absolute
crap.  It was written by incompetent lawyers being advised by
incompetent government scientific idiots.  Hell, we all have
handhelds.  If there were a problem that was measurable the government
would have banned them.  The government loves to regulate anything
that can be measured.  Nothing has EVER been measured at these
frequencies and energies as far as a biological effect that caused
harm on a cellular level.  What can be measured is the parasitic
resonance of the human body.  We are good resonators in the 2 meter
band when we stand erect because we are exactly one wavelength.
That's why you may not want a 100,000 watt FM station transmitting
next to where you live. If you stood close enough to it you might get
burned too.  Resonance is easy to measure in the human body, but there
still is no known negative biological effect at low powers since the
frequencies are below the threashold for ionization.  This is NOT
ionizing radiation.  Living next to an FM station is something I
wouldn't want to do only because we don't know all the answers yet and
there might be some long term effect unknown at this time from high
powered, chronic exposure.  Also, I'm not saying the radiation has to
be ionizing to get some bad effects.  Microwaves which are about 5
times higher frequency than where we opperate can cause heating and
specifically problems with the cornea.  But, nothing of any
significance has ever been measured in the 2 meter band where we
operate at the low power densities we are exposed to.
I certainly would NEVER worry about short burst tranmissions a meter
or so behind my head at 10-20 watts.

How this helps you.

Badwater Bill

>                                                   Jim

Newsgroups: sci.aeronautics.airliners
From: (Ed Hahn)
Subject: Re: where is radio antenna
Date: 01 Sep 94 01:14:47

In article <airliners.1994.1550@ohare.Chicago.COM> (John
Oliver) writes:

   Can anyone tell me where the radio antennae are located on jumbo jets? I
   assume they are not inside the metal hull since an electromagnetic field can
   not penetrate a metallic conductor.

   John Oliver
Radio antennas are generally located on the bottom of the
fuselage.  The ~12in long, 2in wide, angled antennas are VHF; the
little 4in by 2in antennas are microwave (transponders, DMEs).  There
are also conformal "patch" antennas for the radio altimeters.

HF, satellite, GPS, and upper TCAS antennas are located in various
other places.  On B747s PRIOR to the -400, they were the big "stinger" on
the right wingtip facing backwards.  On aircraft like the B707 and
B727, they are located on the tip of the tail (the B727 HF antenna is
mostly buried in the fin tip fairing).  SATCOM antennas are generally
located on the top of the fuselage, and look like "humps" about 3 feet
long, 12 in wide, and 8 in high.  The hump is actually a fairing for a
steerable high-gain unit (mechanical or phased-array).  GPS and TCAS
antennas are generally ~1in thick ellipsoidal plates, about 10in by
5in or so, mounted on the top of the fuselage.

Of course, details vary from vendor to vendor, and so do appearances.

BTW, insects have antennae, aircraft have antennas. (At least in the
US.  Your language may vary.)


////////   Ed Hahn | | (703) 883-5988   \\\\\\\\
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.

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