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----- Original Message -----
From: "Hal Perkins" <hep@ctv.es>
Subject: Terracom radio problem
> With regard to the radio communications problem, Alan, let me say firstly that
I do not follow why Terracom
> state that the ground plane for the antenna must be 42".
>
> Ground plane type antennas have one part one quarter of a wavelength long to
which the centre of the
co-axial
> cable is connected. The other quarter wavelength part is the "ground plane"
representing 'earth' which can
be
> in the form of radial wires .25 wave long. These can be strip copper or braid
stripped from co-axial cable.
>
> I believe that radio transmissions from towers are vertically polarised? If
I'm wrong, someone please
> straighten me out! If they are VERTICALLY polarised, then so must the
> aircraft
on your aircraft be
likewise;
> i.e. the radiating portion must be in the vertical plane. You will loose
> efficiency
if you use a
HORIZONTALLY
> polarised antenna.
>
> Now, a bit of theory! Let us assume a centre frequency of 123.5 MHz. ( this
will give good performance +/-
> 10% OK. Theoretical wavelength is 300/ freqcy; i.e. 2.429 m.
>
> A quarter wave is 2.429/4; i.e. .6072 m. Due to a shortening factor of about
2%, the true quarter wave is
> 0.595 m.
>
> For our cousins t'other side of the 'pond, this is 1'11 1/2"! If you call it
2', it only changes the
resonant
> frequency to 120Mhz.
>
> So, Terracom, why do you say that the ground plane must be 42".
>
> Would someone care to try 2' sticking out the bottom of the 'fuse'. It would
be vertical and line of sight
to
> the tower too!
>
> Hal
> G-KITS
>
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