GEOFFREY MACKENZIE-KENNEDY wrote:
1C) A full size vertical half-wave
dipole, centre fed. No radials underneath.
<snip>
Having always been suspicious about the effect of feeders
even if 'properly' routed, I changed the feed configuration to the
verticals and the Half Square from centre to bottom, running the coax
to the element centre and using the braid for the lower half of the
element. Coax coiled at the bottom, and a capacitor between the end
sections of the coil braid to form a high R L-C trap.
'Properly routed' in this case means that the feedline would run away
from the antenna perpendicular to the antenna for at least a half
wavelength. I couldn't tell from your message that you had accomplished
that! It's not easy for a 40-meter vertical dipole. This would be one
source of unbalance. The other would be the fact that the bottom part
of the antenna was much closer to the ground than the top, producing
different currents in the top and bottom halves of the antenna. A balun
at the feedpoint would mitigate the second effect, but not the first,
since the feedline would radiate conducted noise that would be picked up
by the antenna.
Your improved feed system in effect embraces the unbalanced condition
but does a much better job of isolating the feedline from the antenna
(the feedline is much farther from the high-current part of the antenna
and is perpendicular to it).
Since your antennas are relatively far from noise sources, one would
expect that noise pickup on the feedline would be very significant in
the overall s/n ratio, since the feedline passes closer to noise sources.
The problem of received noise pickup on feedlines is exactly the mirror
image of feedline radiation when transmitting, and is one reason that
one should pay close attention to eliminating unbalanced feedline
currents (with coax lines, this means current flowing on the outside of
the shield). Your results tend to confirm this.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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