Topband: Making the bottom of my tower "disappear"

Tom Rauch
Mon, 7 Oct 2002 19:55:47 -0400

> I had a 1000 pf variable cap.  I saw a big peak in the current between
> the CAP and tower around 300 pf and then it just kind of goes down as
> I keep increasing the capacitance.  I tried some parallel capacitance,
> but other than a general decrease in current as I increased the
> capacitance, I don't seem to have a sharp null of any kind.  

I have not read the article, but why would you see a null in current 

When we tuned such systems for BC stations, we would see a *peak* in 
the loop current through the detuning system at nearly the same point 
where we saw a null in current beyond that connection point in the 
structure we were detuning.

I would expect to see a current maximum, because the PHASE of the 
current though the capacitor has to be exactly opposite the current 
in the tower at that point and the amplitude has to be the same. 
Then, when they are added together, the net current is zero.

If your clamp-on meter was very very large and fit around the tower 
and the detuning wire, you would see zero. In other words if you 
installed an open sampling loop a few feet off the face of the tower 
opposite the detuning wire, you would also see it read zero or near 
zero while the current through the detuning system was maximum.

Phase shift is what makes current in the structure "vanish", and the 
shunt is always opposite phase and equal to the tower current when it 
is detuning the definition. Just like a parallel tuned 
trap, which also has maximum current in each element when resonant. 
(Which is why traps always have less loss when tuned slightly off the 
actual operating frequency, but in you case it does not matter so 


73, Tom W8JI