[TowerTalk] Modeling question - for the experts!

[Jim Brown]

On Thu,6/2/2016 10:32 AM, StellarCAT wrote:
> But here we have the real world actual situation where someone 
> measures a 40 meter antenna and finds it to have an SWR on 15 of 1.2:1 
> - THAT antenna WILL interact I'm sure 

Why do you come to that conclusion, Gary? It's perfectly normal for a 
half-wave dipole resonant near the low end of 40M to also be resonant on 
15M, which is its third harmonic! That alone does not imply interaction.

Other factors that affect interaction are things like SPACING, and 
orientation relative to each other.

It also matters how the "interfering" antenna is terminated at operating 
frequency of the frequency of the primary antenna. That 40M antenna has 
a feedline, it has some length, and there's some sort of termination at 
the other end. It can be anything from an open to a short AT THE 
OPERATING FREQUENCY OF THE PRIMARY ANTENNA, and the length of the line 
will transform it to some complex Z at the antenna feedpoint. (CAPS 
added for emphasis).

Here's a simple example that took me several months to work out details 
for and use in a productive way. I have a Tee vertical for 160M 
(resonant) with radials on the ground. 200 ft away, I have a 120 ft 
tower with a 10 ft piece of 4-in PVC at the top that holds the top of 
two wires that slope away from the tower in opposite directions, one to 
the east, one to the west. Those wires are resonant on 160M, and I feed 
them from their base against four radials elevated about 20 ft. The 
tower, grounded with radials) acts as a passive reflector for the 
sloping wires, giving them 2-3 dB of gain in the direction of the slope. 
No surprise there. But the Tee vertical, if I short the base, also acts 
as a passive reflector for the sloping wires, moving their main lobes 
north by about 45 degrees (toward EU for the east wire, toward JA for 
the west wire). In addition, shorting the sloping wires at the feedpoint 
makes them also act as passive reflectors for the Tee vertical, giving 
me something like 4 dB of gain (in combination with the tower) to the 
south.

I do that switching in the shack, by leaving the coax to the antennas 
I'm not feeding short circuit, and controlling the length of the feed to 
each of them so that it looks like a short at the feedpoint.

How well does this match the model? I can clearly hear the predicted 
directivity when I switch between the three antennas.

73, Jim K9YC