Topband: Center support on K9AY Loop

[Tom Rauch]

> FWIW, I have been modeling a set of K9AY loops with EZNEC.
> My modeling results (real high-accuracy ground - average conductivity)
> indicate that their is only a slight difference (about 2dB F/B) at 1.8
> MHz between a grounded metal support, and floating metal support, and
> no support (or non-conductive support). The loops I modeled follow
> K9AY's suggested dimensions (85' total loop circumference). Modeling
> results at 3.8 MHz are similar (minor changes due to mast
> configuration).

2dB null depth variation is the extreme with the mast near the wire. 
It would be even less if the mast were insulated at the antenna by a 
few feet of PVC, and less still if the mast were floated at ground. 

What many of us miss is how unimportant the peak null depth is in the 
ability of an antenna to hear weak signals. What we really want is 
directivity, unless we happen to have a very strong problematic 
unwanted signal sitting right in the minima of the pattern null. I'd 
wager that almost never happens.

The common rumor or idea that a metal mast will noticeably hurt 
performance is unfounded, unless you nearly wrap the antenna wire 
around the mast. What will destroy the antenna is a poor feed system 
or interaction with other structures like towers or power lines. I'd 
spend my time worrying about that, and less on things that are 
unnoticeable.73, Tom W8JI
W8JI@contesting.com