Hello Dick / Group.
I have read through Dick's analysis, of the counterpoise / earth comparison
and have a little trouble trying to realise the antennas described. It's
quite late and I'll probably see the obvious tomorrow.......but after I've
sent this mail.
In essence, my problem is "when is a counterpoise not a radial"
1) The antenna has a loading coil to resonate it......with a length of wire
running under the main span of the antenna called a "counterpoise". If the
"counterpoise" is not resonant, should it not simply be considered an earth
radial which works due to its capacitance / proximity to ground? Or I guess
that for the purposes of analysis the two coils could be combined and fed at
a tap.
2) Mike has (this weekend's report) experimented with a resonant wire
instead of his earth system without success, but I understand the idea here,
as a series resonant circuit ( or one quarter wavelength wire) will present
a low impedance at its point of connection.......Or, if you like, a signal is
returned after travelling a total of 0.5 wavelengths and therefore arrives
back at the feedpoint 180 degrees out of phase producing a cancelling effect.
Unfortunately, the losses associated with a coil are usually high and using
this technique to resonate a short counterpoise would probably introduce
substantial losses.
Similarly, if a full 0.25 wavelength wire were used, I imagine its proximity
to ground would also introduce losses. How high above ground are the popular
160m "elevated radials"?
One other possibility to achieve a low impedance "earth" is to use a drum of
transmission line cut for resonance. ( a quarterwave stub) .....OK, it's
about 300m for normal coax with a 0.667 V.F. but the losses at 136k would
be low.
3) Finally. If a short vertical is resonated with a coil and then, with the
system isolated from ground, a counterpoise wire is added - and resonated.
Isn't this just a loaded dipole with one leg close to the ground and the
other vertical? If so, then the highest radiation resistance of this
configuration should be achieved when the two sections are in line i.e. an
improvement of about 50%. It seems that everyone is looking at antennas
worked against ground and not exploring self-resonance within the antenna.
(loop owners excepted..)
I wonder where the "equilibrium" is in the comparison of a short vertical
operated against a poor earth and a loaded "dipole type" antenna operated
close to the ground?
No wonder there are so many antenna books around
and then there's the CFA.......Whoops Sri.
Goodnight es 73
David
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