[TowerTalk] Modeling question - for the experts!

[StellarCAT]

Thanks Jim.... re the factors you mention: the antenna is 34' away, about 
2/3 wave at 15 meters so quite close enough and an oft used stacking 
distance, it is in the same plane as the 15 ... and re the feedline - yes - 
I agree with that - but there are three other elements on that antenna 
involved!

What I did to try to see if it would have an effect is this. I made the 
assumption that the linear loading wires would 'breakup' the element as it 
would probably have a complex impedance on 15 meters - thus divorcing the 
rest of the elements - just an assumption but for starters I think a 
reasonable one.

The distance to this connection, unfortunately is almost a 1/4 wave on 15! 
Optibeam uses a much closer spacing to their reactive component (their coil) 
... likewise F12 on their "N" series... but the M2 is set at just under 1/4 
wave.

SO I modeled first a 15 meter dipole .... and placed that above the 15 yagi. 
If this dipole is shifted rearward it has little effect ... but if I do 4 15 
meter dipoles - all equal length (since the distance to the LL is the same 
on all three) and spaced along as they would be on the 40 boom and place 
this at various heights above the 15 yagi It has quite significant 
interaction. I don't remember the specifics at this time - I had written a 
blurb up about it some while ago... but even at 34' there is some 
measureable interaction ... when I modeled it with the sleeve idea I had 
(Greg Ordy said it is probably more properly called a sleeve and not a stub 
as I had earlier referred to it) the interaction completely disappears and 
one can move the antennas as close as 8'.

[I do have an 80/75 rotary dipole on the top of this tower - but it is 
turned 90 degrees]

Gary



-----Original Message----- 
From: Jim Brown
Sent: Thursday, June 2, 2016 6:56 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Modeling question - for the experts!

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