[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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