Van,
Transmission line theory says the impedance seen at the input of a line will
be equal to the load impedance every half wavelength, when the line
impedance is different than the load impedance. When the line is an odd
multiple of 1/4 wavelength, the impedance seen looking into the line will be
equal to the line impedance squared divided by the load impedance. In your
case the line is 75 ohms and the load is 50 ohms, so the impedance seen
looking into the line is about 112 ohms at the frequencies where the line is
an odd multiple of 1/4 wavelength. At other frequencies the impedance seen
looking into the line will fall between 50 ohms and 112 ohms, so the SWR
will vary from about 1 to 1 to about 2.2 to 1. So we see the SWR varies in
a regular and predictable way between these values.
If you change the termination you are using in your test from 50 ohms to 75
ohms, the 75 ohm line will be matched and the impedance seen looking into
the line will be 75 ohms at all frequencies. Then the SWR will be 1.5 to 1
at all frequencies.
You are using your SWR meter correctly. If you put your 50 ohm meter in the
middle of the 75 ohm line, the reading would not be correct. If you had a
75 ohm SWR meter and put it in the middle of the 75 ohm line, you would see
the SWR in the 75 ohm line is always 1.5 to 1, even when the SWR seen
looking into the line is varying from 1 to 1 to 2.2 to 1!
75 ohm coax can conveniently be matched to 50 ohms on a single band using a
"twelfth wave transformer". See
http://ourworld.compuserve.com/homepages/demerson/twelfth.htm for a great
description by AA7FV. For a multiple band set-up, there are broad-band
transmission line transformers, but they are probably not worth the trouble.
If you are using an antenna tuner, you can have a 50 ohm load for your rig
and ignore the standing waves on your line - the extra loss in the hardline
will be small.
Mike Rauh, NV7X
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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