Topband: Center support on K9AY Loop

[Tom Rauch]

> Any suggestions for how to go about decoupling a nearby tower
> from a K9AY loop array, Tom?
> model, I can minimize the coupling by adding a gamma arm to the tower
> and adjusting the series reactance (presumeably this puts the gamma
> current in anti-phase with the tower current).

Sounds reasonable. You probably have severe pattern distortion and 
you can probably detune the tower and reduce it. You simply must add 
out-of-phase currents in the shunt that are exactly equal and 
opposite to the current in the tower. That generally means tuning for 
resonance in the gamma if it is a short gamma and if the tower is 
less than 3/8th wl long electrical length.  

> The real performance of the antenna suggests that something might be
> going on with regard to interaction with the tower. Although the
> arrays shows F/B on 160, it doesn't hear any better than the TX
> antenna. 

S/N is improved by the directivity of the array, assuming you have 
evenly distributed noise. Some typical directivities:

TX vertical	4.2dB
K9AY, Flag, EWE, Pennant	6-7dB
Two vert 1/8th wl spacing (endfire) 8.5dB
RX 4-square (optimized) 10dB
1wl Bev 9-10dB
pair of 1.5wl 350 ft broadside Beverages 14-15dB
 
Most situations lean heavily towards directivity setting the 
improvement. F/B ratio sounds impressive, but really is somewhat 
unimportant unless QRN or QRM is strong and concentrated in the null 
direction.73, Tom W8JI
W8JI@contesting.com