On Thu, 29 May 1997, Chuck Dietz 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.
Hate to disagree, but changing the length of coax by a short amount should
not change the SWR, although it will change the resistance and reactance
presented to the meter. However, they still calculate to the same
reflection coefficient and hence the same SWR.
The exception occurs if there is significant antenna current on the
outside of the coax, which is, of course, a difficulty to be cured.
> 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.
This may be the more likely cadidate, assuming neglible current on the
outside of the shield. It is also possible that there is enough coupling
remaining to some portion of the circuitry that it is acting as a "stub"
there by introducing R or jX to the remaining line.
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