[Elecraft] Baluns and 450 ohm line

[k6se@juno.com]

John, KC4KGU wrote:

"I currently feed my 75m doublet with twin pieces of RG8X.  This presents
a line impedence of about 100 ohms.  Prior to feeding with twin pieces of
coax, I cut the doublet for resonance at 3.875Mhz.  It showed a 1:1 match
at 50 Ohms according to the MFJ-269 analyzer."

Now, since the MFJ-269 is designed to give readings based on 50Ohms, what
should I be looking for when I check one of my antennas that is being fed
with twin coax with a 100 Ohm feedline impedence?"
==========
I'm curious to know why you would want to feed an antenna that has a
perfect match at 50 ohms with a 100-ohm feedline.

If the antenna input Z is truly 50 ohms, then if the feedline is a
multiple of 1/2-wave long electrically, the shack end will "see" 50 ohms
Z.  Otherwise, an Autek Research VA1 RX Vector Analyst may be used
instead of the MFJ-269 becaise the VA1 is not limited to 50-ohm lines.
You can select SWR readout for over 10 common feedline impedances from 25
to 450 ohms with the VA1.

I'm not associated with Autek, but I own a VA1 and find it more useful
and accurate than my MFJ-259.  The Web site for the VA1 is
http://www.autekresearch.com/va1.htm .

73, de Earl, K6SE
67 countries with K2 #2622 and counting.....

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