Topband: Making the bottom of my tower "disappear"
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 structure...by 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