[TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?
Jim Brown
jim at audiosystemsgroup.com
Sat Dec 14 02:04:10 EST 2013
Another important point. Many antennas, especially dipoles that are
either high or over lousy ground, are a MUCH closer match to 75 ohms
than to 50 ohms. I'm using 75 ohm coax (Belden 8213) on my high
dipoles, which are at 120 ft over LOUSY ground. Measured Z at resonance
is about 85 ohms.
This fall, I modeled (in NEC) and built dipoles for 80M and 40M for a
CQP county expedition. Our QTH is rocky/sandy, really lousy ground, and
we managed to get the the dipoles up 40 ft. NEC predicted a feedpoint Z
of about 76 ohms, and once they were up in the air, we measured 1:1 at
the end of a short length of 75 ohm cable.
Low dipoles over GOOD ground tend to be closer to 50 ohms. We don't have
much of that in the mountains.
73, Jim K9YC
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