[TowerTalk] 40 meter interaction

[Tom Rauch]

Hi Jay,

> Vertical and dipoles have about 30db of isolation between them due to the
> polarization being at right angles. So this is good.  If you have raised
> radials it may be worse.

Unless I'm missing something, that only applies to far-field isolation 
in freespace and broadside to the "waves" arrival.

A dipole is actually a fairly good vertically polarized radiation off the 
ends of the antenna at angles above and below the plane of the 
dipole. At all other azimuthal directions it has a "mixed" 
polarization on model which is actually a tilted E-field angle. The 
model only expresses it as two ratios, one purely vertical and one 
purely horizontal, so we "see" it as two distinct polarizations on the 
model when it is really a polarization skew.

Raised radials also couple like crazy to anything around them! 
They only have a clean null at many wavelengths, so they are big 
headaches.

If his dipole aimed the end-axis straight into the effective radiation 
center of the vertical and was far enough away there was no direct 
induction-field coupling, then the dipole would have a good null. It 
would have an even BETTER null if it was aimed broadside at the 
verticals at some distance.

If I had a contest station and wanted to duplex through cross 
polarization, I'd place my distant dipole broadside to my vertical!      
 
> I would build it and then measure the interaction by putting a
> terminated watt meter at the end of the dipoles feedline to see what
> kind of power is coming back down the line. I bet its very small. 

That's a good test for part of the problem....but we have to 
remember two things. A very small amount of re-radiation will 
totally trash the null of a directional array and the re-radiation does 
not necessarily involve differential-mode currents on the feedline 
that will show on a meter. Common mode re-radiation can be a 
problem also, especially since he has to get a feedline down past 
all the verticals! You mentioned that (of course you would).

The best thing to do is measure what you intend to worry about. 
The real test is what it does to pattern.

> You could model it too, and compare the results with your measurements.  

As long as you include feedlines!
 
If I wanted to put a dipole over my four square and it had to be 
close, I'd try to center it over the array. Then I'd make sure I 
detuned the dipole (easy enough doing it just as you said), and I'd 
make darn sure the feedline was choked like crazy at a couple 
points so it had extremely low common-mode current. This would 
be a good application for chokes like Roy Lewallen describes in his 
choke balun article that are "self-resonant" with fairly low Q.

I had a similar mess with a 40 meter EDZ that was centered in my 
160 four square, and beat the problem.

 

73, Tom W8JI
w8ji@contesting.com

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