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[TowerTalk] 50:75 ohm transformation

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Subject: [TowerTalk] 50:75 ohm transformation
From: n4zr@contesting.com (Pete Smith)
Date: Wed, 10 Feb 1999 11:13:59 -0500
At 03:27 PM 2/10/1999 +0000, Mike Lamb wrote:
>Hello Pete,
>
>Yes, you are quite right if you are operating only on one band.  I use my
coax
>for feeding a triband stack and it needs to be flat across that portion of
the
>spectrum.  Parenthetically, I had a devil of a time getting a respectable SWR
>with my stack last summer after 3 years of no maintenance.  I finally had to
>just make everything flat to 50 ohms  with UNUNs, etc. and it finally started
>behaving.  Before, I just went direct from 50 to 75 ohms and vice versa.
That
>introduced all kinds of strange impedances back in the shack at different
>frequencies.

Well, a 6-high tribander stack isn't THAT common ;^}

There is no reason I know of why a stack shouldn't behave just fine on the
end of a half-wave multiple of 75-ohm.  It would be interesting to work
through the transformations that actually happen when the terminating
impedance is reactive to the extent that a stack might be.  I just know
that in my case, I'm feeding everything through a length of 75-ohm
carefully pruned to be a half-wave multiple at 28100 kHz -- including an
80-meter array, a 2-element 40, and a C-3.  The SWR curves are essentially
the same in my shack as they are at the antenna switch at the bottom of the
tower, where the 50-ohm feedlines start.

73,  Pete N4ZR
Loud is good


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