Topband: topband post - waller flag antenna

[Tom Rauch]

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