> Tom, thanks for your reply. In all honesty when I read
the article, it
> made sense to me because if one were trying to minimize
the interaction
> between the delta and the tower, minimizing the RF current
flow on the
> tower seemed logical. I went back and rechecked the
article to make
> sure I hadn't gotten it backwards, and I hadn't. I am
going to have to
> reconsider the whole issue in light of your explanation.
I have something on my webpage at:
http://www.w8ji.com/detuning_towers.htm
Basically the drop forms a parallel tuned circuit that
isolates the section. When you measure the cirrect in the
drop wire, which is what I believe the book suggests, if you
set the shunt wire current to zero the shunt wire has NO
effect!!!
Going from the top of a structure to ground, or if the tower
is long in terms of 1/4 wl sections, the results are
different. In that case the only reliable way to detune is
to measure the overall effect of the structure on pattern or
re-radiation with a FS meter.
Anyone who has worked with resonant systems knows the
circulating current is maximum when the system is resonant.
For example a trap isolating two sections of an antenna has
maximum current circulating in the trap when the loop
through the inductor and capacitor forming the trap has
highest current!
Intuitively, as another example, people think moving a balun
to the input of a floating network makes balance better and
reduces balun heating. That isn't correct, yet everyone
believes it. This is why we have to take everything we read
with a bit of caution, and not go by first instinct.
> BTW, I did mention that the deltas were for 30 and 40M and
I had planned
> on vertically polarizing them and feeding them with
balanced line. The
> reasons for using the loops are that I am looking for some
gain and
> directionality on 30 and 40M with a low takeoff angle,
simplicity and I
> have a lot of experience with quad loops.
Must have missed that. Loops reduce height requirements, and
that's a good thing. Just be careful with the feedline. The
feedline can be more of a worry to decouple than the tower
supporting the loop.
I'd install it first, and worry about problems later if they
show up.
73 Tom
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|