[TowerTalk] Detuning Skirt

[Roger Parsons]

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