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[TowerTalk] Stacking quandry

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Subject: [TowerTalk] Stacking quandry
From: (Tom Rauch)
Date: Fri, 19 Oct 2001 19:52:07 -0400
> So I need to ask the audience, I guess: What's your take on a new C3
> or C3S or something instead of a C3XL at the 40' level, using a
> Stackmatch and a swinging gate? Is this more hassle than it's worth?
> Is it a phasing nightmare? I'm very reluctant to give up this great
> single antenna just to have two alike on the tower.

Jerry and all,

The warnings are correct. The phase shift in dissimilar antennas (or 
dissimilar tuned antennas) is a big problem. Any prediction you will 
never get phase "correct" is not necessarily true. Any claim that by 
using equal length lines phase will be correct is also not true, 
because shift in each antenna will be different.

The most simple way to get phase correct is to build a variable 
phase delay section, with a switch selectable 180-degree flip. 
What you do is listen to stations in your target area, and adjust 
phase and level ratios for a NULL in signal level.  Then you simply 
flip phase 180, and you have maximum gain for that combination at 
that particular target!

If I were doing it, I'd probably just use any old length of cable from 
each antenna to the shack and a  "T" network in the shack for 
phase shift, with a "in-out" switch for a broadband phase-inverting 
transformer. Try the T network in either feedline, and combine the 
antennas the normal way (stepping impedance back up to 50 ohms 

This is similar to what I do on receiving antennas, and it will work 
the same when transmitting. The drawback is you need a T 
network like used in an antenna tuner, and two transformers. 

It should also be possible to build a BCD-type of delay line, where 
line sections of planned lengths are switched in series to make a 
stepped delay. Only five delay lines of  3, 6, 12, 24, and 48 
degrees will give you 0 to just over 90 degrees of shift in 3 degree 
steps, and you can move that to either leg of the antennas to get 
180 degree rotation. With the additional 180 degree flip, you cover 
360 degrees.

Find the null, flip it 180, and you have the peak!

This may be more complex than what you want to do, but it has 
the advantage that stack phase will be spot-on for any wave angle 
you want to peak!
73, Tom W8JI 

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