Re:More antenna problems.

[Geoffrey Mackenzie-Kennedy-2]

Greetings to all.
Over the past 50 plus years of operating from various parts of the planet, I had come up with various unproven hunches concerning noise vs antenna type and the contribution of feeders to local noise pick-up vs type and deployment. After moving here, which is on a farm, I ran a series of controlled tests to get some more useful information. All tests were made on 7.020 MHz, with all antennas tuned to resonance and matching networks tweaked to give the main feeder a 50 +j0 load (almost).

The basic set-up here is approx 80 ft of RG-213 from shack to a remote 5-way co-ax switch, then on to the antennas. When the feeder becomes airborne, I have switched to RG-58 to cut down weight as all antennas are supported by a long Kevlar rope up about 80 ft. The rope is also supported here and there by trees to prevent too much sag. It runs E -W.

My closest man-made noise sources are a)In Our cottage - about 60 ft South of the antenna support rope b) Neighbours - about 200 ft South, c) A pole pig about 200 yards South, d) Vehicles on an E-W road about 550 yards South, e) A small but busy tourist centre on the other side of that road - approx 550 yards South and f) a small village a mile or so to the South-west. All mains supply lines come in underground. The pole pig is up about 12 feet, and its mains lines surface and go back undergound at the bottom of its pole.

Antennas (first runs ) The physical 'top' of each antenna was 80 ft plus-minus. Major lobes N-S ( or South if directional). Antennas were well separated, kicked off resonance also when not under test. Double checked lack of coupling by running down 'inactive' antennas to ground.
 
1A) A full size equilateral Delta Loop, apex up, fed a quarter-wave down from the apex.
1B) A Half Square with legs pointing down, fed at top corner. Really up too high as high angle lobe is becoming a problem. No radials under legs.
1C) A full size vertical half-wave dipole, centre fed. No radials underneath.
1D) Two full size vertical half-wave dipoles both centre fed, spaced 45 degrees 135 degrees out of phase. No radials underneath.
1E) A horizontal half-wave dipole, centre fed by half wavelength of cross-connected open wire line to balanced L network and 1:1 balun to 50 ohm coax.

The winner by a small margin was the horizontal dipole, then the phased verticals, the Half Square, the Delta Loop and last the single vertical. 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. Antenna positions and heights unchanged.              
A complete change in results !! The winner now by a large margin was the pair of phased verticals, next the Half Square, then neck and neck the Horizontal Dipole and Vertical Dipole. The unchanged Delta Loop is a little better than the last two.

Finally, I always bury coax feeder down about a foot, and always use 4 wire cross-connected open wire line for phasing (also for long feeder runs at VHF). Sometimes the noise reduction is quite amazing.

73  from Geoff    GM4ESD      K2/100  3255
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[Vic K2VCO]

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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