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[TowerTalk] Adjustable Phase Shift networks

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Subject: [TowerTalk] Adjustable Phase Shift networks
From: (W0UN--Signal Hill Ranch)
Date: Wed, 25 Jul 2001 16:04:16 -0600
At 02:56 PM 2001-07-25, you wrote:

>When building a multi element array, I have had to put in odd phase shifts
>to properly achieve the pattern I wanted.  Using different lengths of feed
>line works, but is obviously difficult to adjust.  Anyone have a good
>reference for designing adjustable feedline phase shift networks?  I vaguely
>remember an article (many moons ago) on doing so, but could not drag it out.
>Mel Farrer
>Director of Engineering
>Tality Corporation
>555 River Oaks Parkway
>San Jose, CA  95134
>408 456-8254
>408 944-8000 FAX

Mel, one way to do what you want is to build a 90 degree hybrid
coupler.  Terminate two of the ports with a parallel resonant circuit
and use the other two as in and out.  By selecting the proper
values of L/C you can get 0-180 degrees phase shift at a constant
impedance.  With the right choices you can get (almost)
0-180 degrees with a ganged variable cap, keeping the L fixed.

If you need 360 degrees you can either add a 180 degree line
(or phase reversing toroidal transformer) that is switched in
and out depending on what quadrant you want.  Or you can
cascade two of the hybrid networks and gang their variable
caps to get 0-360 degrees.

Some have done this with relay switched binary lengths of
coax.  Enough relays can give you some impedance bumps unless
you are pretty careful.

Think this might have been address in an old issue in Ham Radio
magazine--at least in passing.  And may be casually mentioned
in the old Unitrode PIN diode handbook.  But I don't have any
references at hand and I don't want to do ALL of the work for
you! ;-)

John  W0UN

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