[TowerTalk] Trimming 75 ohm hardline

[w8ji.tom]

I agree with Dave's suggestion!!

> Rather than using swr, I think it would be more accurate to use
> the impedance method described in the RF-1 manual. Put a temporary
> short on the far end, and adjust the frequency of the RF-1 until
> you see minimum Z.
> 
> Dave Hachadorian, K6LL

If you spend time with a Smith chart and various input impedances you'll
see multiple SWR dips all over the place as you trim a mismatched system.
You might wind up with a pile of two foot long chunks, and never get the
feedline correct!

This is also a problem with measuring antenna impedances in the shack, even
with 1/2 wl long (or multiple) cables. The line is only a 1/2 wl at one
frequency, and if the test frequency is moved even slightly (a few pecent)
impedances can change quickly. Added reactance from the mismatched line
cancels or modifies antenna impedance, making one "think" the antenna is
resonant when it is not!

Having just spent three days looking at a spreadsheet on two antennas in
preparation for software that measures antenna impedance in the house, I'm
amazed at the number of "false SWR zeros" when the line is not perfectly
matched to the load.


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