OK, I feel like an idiot. I've spent many hours attempting to detune
my shunt fed tower and have met with questionable results. I will
describe my problem and setup in the hope someone can offer comments
that get me moving in a positive direction.
The problem is that I am being bothered by noise that I believe to
be coming from a neighbor's TV. It is about equal strength on all of
these antennas: inverted V, shunt fed tower, reversible Beverage
(both directions).
This year I can have only one Beverage, and it must end near the
road, utility lines and neighbors' houses. It is a 1 wavelength
reversible Beverage using WD-1A wire. Unfortunately it doesn't help
much unless the neighbors are out! Since the Beverage runs within
about 50 feet of the vertical and over its radial field, I thought
detuning the vertical might help. That's where things started
getting confusing.
The vertical is 98 feet of Rohn 25 with a 7 element 6 meter yagi on
a 33 foot boom at the 101 foot level. The gamma arm consists of an
equilateral triangle of 6 AWG wires, spaced 4.5 inches, and the
center of the triangle is 34 inches off one flat face of the tower.
The gamma arm extends to the 60 foot level, with the tap to the
tower at 32 feet.
For the sake of convenience, I have been attempting to detune this
structure by using the gamma arm as part of the detuning loop. I am
disconnecting the bottom of the gamma arm from the series capacitor
which normally goes to the feedline, and inserting a capacitor
between the bottom of the gamma arm and the base of the tower, as
per the diagram in Low Band DXing, 4th ed., page 7-94. I am breaking
the loop near the capacitor and inserting an MFJ-259B at that point.
The book and other references say I should be tuning for minimum
impedance, and that if it is not low, I am probably not effectively
detuning the structure. But what is the definition of "low"? Above 2
MHz, I can get it to R=12, X=0. As I move lower in frequency, things
start to change. at 1825 kHz the best I can get is R=50, X=5. I note
that I end up with about the same capacitance as I use to get a low
SWR on my gamma match (about 600 pF), so it seems all I'm really
doing is matching the tower, not detuning it. However, no amount of
capacitance, high or low, seems to accomplish anything remotely
interesting. Nothing that I have done so far makes any observable
difference in the level of interference received on the Beverage.
I'm not sure what questions to ask at this point. Can anyone
estimate how much capacitance I should be using in this detuning
loop, or see some place where I have gone wrong?
I plan to try some smaller receive antennas since I can no longer
have multiple Beverages, but I suspect I will have even bigger
problems with those if I am unable to effectively detune the tower.
What does noise from a plasma TV sound like on 160? I'm reasonably
certain I only have this problem when a particular neighbor is
watching TV (which unfortunately for me is most of the time). Here
is a link to a recording of what I am hearing, centered on ~1796
kHz, with spurs or sidebands extending to beyond 1900 kHz. There are
no unaffected frequencies in the DX portion of the band.
http://mysite.verizon.net/pnkelley/tvnoise.mp3
73,
Paul N1BUG (perplexed!)
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