[TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?
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
More information about the TowerTalk