[TowerTalk] SB-220 affecting SWR curves

[Dean Fredriksen]

Pete,
If the feedline shield is properly decoupled from the antenna, SWR does not
vary with line length, except to be gradually reduced by the line loss.  If
the line is part of the antenna (e.g., you should have used a balun, but
didn't), then varying the feedline length is actually varying a part of the
antenna itself.  When the feedline shield is properly decoupled from the
antenna and you see different SWR readings with varying lengths of
(relatively short in electrical wavelength) cable, they are most likely due
to an impedance mismatch between the source (the transmitter output
impedance), the SWR meter's directional coupler and the line.  This can
cause the MEASURED SWR to be differently, but it's an artifact of imperfect
measurement equipment and techniques.

 You CAN however, take advantage of the fact that you are rotating the phase
angle of the load presented to your transmitter.  Some TXs won't load a
mismatch at a given angle, but will load it just fine when it's rotated by
adding some line.  This, combined with poorly decoupled feedlines, is how
the misconception of "tuning out the SWR" by changing the length comes about.

It's a symptom with multiple causes.  It's interesting, but if it doesn't
cause you any problems, it's not worth worrying about.

73, Dean K4KR (ex-W8ZF)

At 03:11 PM 5/29/97 -0500, you wrote:
>Pete Smith wrote:
>
>> I have an SB-220.  This morning, I measured the SWR curves of my Force 12
>> C-3 through the SB-220 (with the amp off) and then with a barrel connector
>> replacing the amp.  To my surprise, there was a huge difference! 
>
>If you have some SWR on your feedline from a mismatch between the
>antenna and line, it will change when you vary the line length.  Try
>adding a couple of feet of line in place of the amp to verify this.
>
>If you have some resistance in the amp internal line, probably due to
>dirty or pitted relay contacts, there would be DC resistance from the
>input center conductor to the output center conductor.
>
>The problem is probably due to the first circumstance.
>
>Chuck, KE5FI
>
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