[TowerTalk] 40 meter interaction

[Jay Terleski]

Hi Tom

Good points, I had one private email with another gentleman that I
pointed out the near field will not have the cancellation that occures
in the far field from radials, and raised radials are the worst case.
They usually are unbalanced anyway, as K5IU (ala Moxon) pointed out in
his earlier work with them. If the system was using raised radials they
would really couple closely to that dipole.

As with your commments with decoupling the feedline, we agree totaly
when I pointed out the following in my previous post:

"I would also take great strides to make sure the feedline to the dipole
was not coupling to the verticles. This is probably more of a worry for
you for messing up the verticals patterns.

If you see coupling all is not lost, now you can try detuning the dipole
to take it out of the picture when you not using it.  A capacitor or
inductor across the feedline of the "correct" value could do this.  What
is the correct value? It depends on the feedline length to the dipole,
just an open will do it if its exactly .5 WL. or multiple of .5 WL."


Your point of the dipole affecting the vertical array's pattern is right
on.  I think if the feeline and the array can be detuned the affects
work both ways. In other words, if the vertical doesn't couple to the
dipole also means the dipole doesn't couple to the vertical. 

At any rate we have use dipoles that were parrallel to the transmitting
arrays at several stations I have been at.  Usually having the dipole
situated that it was "pointing" at the transmitting array when the
transmitting array was another yagi stack.  This really works as you so
state.  But is not possible for Tom it seems.

I still think Tom should try it, then we all can learn how well it
worked or how badly it works. Then we can go down the path of decoupling
it, and comparing it to our knowlegde base of what were expecting to
see.  Devising a repeatable test case may be in order, like a nearby ham
in the null of the vertical array making some measurements of F/Rear
pattern.

HNY
Jay


Tom Rauch wrote:
> 
> 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

-- 
Jay Terleski
WX0B - Array Solutions
www.arraysolutions.com

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