I read some of the links, and you have to be careful about
some of these articles!!!
It is *NOT* correct to detune the tower by adjusting the
tuned circuit for minimum skit current, or minimum skirt
voltage. Let's look at the reason why....
You have a tower that is a problem, then you add a skirt.
You tune the skirt so it has minimum current in the skirt.
If you are measuring near the bottom of the skirt the tuning
condition that results in minimal current is in effect whan
the skirt is open. Now you don't have a skirt, you just have
the tower with an open circuited wire hanging down the side.
You really have no idea at all what this did to currents in
the tower.
The way a broadcaster detunes a tower is to tune a field
strength meter to the frequency of the transmitter (while
the transmitter is on) and adjust for minimal FS near the
tower being detuned. This is entirely **different** than
measuring voltage in the wire or current through the wire.
As a matter of fact if I sectionalize a small area of a
tower with a drop wire, maximum isolation over that area
always occurs when the detuning wire has MAXIMUM current!
Not minimum current. This is the condition of parallel
resonance in the drop wire, where that area of the tower
looks like a very high impedance. (A trap antenna works the
same way, when isolation of the trap is maximum voltage
across the trap and circulating currents through the
inductor and capacitor are maximum. This is why peak power
loss in a trap always occurs at or very near exact trap
resonance.)
Somehow, I think probably because Hams do not have loop
coupled FS meters, Hams have fixated on the idea that
minimum current in the skirt is minimum radiation. That is
totally wrong.
Now in your case you have a drop wire all the way down the
tower (like one of the articles did) and you have "stuff" on
the tower. This complicates the picture from the
sectionalized situation I described above, but the end goal
is the same. You want the NET field from the tower and skirt
wire to equal zero. This happens when the current in the
skirt exactly equals the current in the tower BUT they are
180 out-of- phase. Now you might be a very lucky fellow and
it just might happen when current at the drop wire
measurement point is zero, the current further up the drop
wire equals the tower current but is 180 out-of -phase. But
statistically the odds of that are low.
With a skirt wire all the way up the tower the only sure way
to determine if you have really detuned the tower is to use
an open loop style field strength meter and measure the
combined radiation of the tower and the drop wire, and tune
that for minimum. We can't tell a thing by looking at the
skirt wire current (or voltage) in this case.
I have probably detuned about 500 different utility poles
and towers, and they all work this way. The article on NCJ
web is wrong.
73 Tom
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