I wonder if anyone has had experience with detuning skirts? (This is where a
section of tower or other object - usually the bottom - sometimes the whole
tower - has a cage of wires connected to the tower at the top and connected to
ground via a reactance at the bottom.) It seems to me that in principle any
wavelength could be synthesised by the use of the proper reactance.
My 160m remote station is based on a 325' former AM broadcast tower. My antenna
has a half wave vertical spaced about 6' from the tower, with some parasitic
elements. I feel (without any real proof) that the tower is acting as a
grounded base 3/4 wave vertical and not helping my radiation pattern,
particularly the back to front. The total tower height is close to 340' when
the owner's vhf array right at the top is included, and there are also a number
of other antennas. The base insulator is bridged, and there are 6 or 7 coax
feeds going up, all of which have the shield grounded at the base and connected
to the tower at their high points.
I tried temporarily removing the bridge on the base insulator and this made no
difference at all - just as well probably because it really has to be there.
I don't necessarily want the skirt to appear like an open on 160 - I want the
whole tower to be as invisible as possible on that band. I can see how it would
be easy to adjust things so that there was maximum rf in the skirt, or minimum
rf outside it, but I can't see how to adjust it for best b/f of my array except
by doing remote fs measurements.
Sorry I have compressed this - I could bore everyone completely with full
details if necessary!
Thoughts would be welcome.
73 Roger
VE3ZI
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