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> From: Michael Lamb <n7ml@imt.net>
> To: 'Randy Lake' <rlake@monad.net>; 'towertalk@contesting.com'
> Subject: RE: [TowerTalk] re: CATV question
To: <towertalk@contesting.com>
> Date: Friday, October 09, 1998 6:07 PM
> I have been using .75 inch CATV cable for many years and never paid
attention to the lengths. I just thought I would accept the raise in SWR
because it should only be about 1.5 : 1 increase.
The SWR on the 75 ohm line is only 1.5 : 1 if the load is 50 ohms. That's
because the SWR is referenced to the 75 ohm line.
The problem is the 50 ohm SWR meter and the 50 ohm radio "sees" a new
impedance caused by the transformation of impedance in the 75 ohm line.
While the SWR in the 75 ohm system connected to a 50 ohm load is only 1.5
to one, the SWR in a second 50 ohm system connected to a 50 ohm load
**after** passing through a 75 ohm line can vary from 1:1 to 2.25 to one
depending on the exact length of the 75 ohm line.
That's the problem with mixing alternating impedances in loads,
transmission lines, and sources in a system. SWR can increase by the square
of the initial mismatch if the line length is right. If you think that is
bad, mix a bunch of alternating line impedances and watch what happens on
some frequencies!
73 Tom
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