[TowerTalk] use of RG11 coax - 75 ohm

[Jim Brown]

On Sun, 27 Mar 2005 10:01:23 -0500, Tom Rauch wrote:

>but for longer feedlines 75 is a good option.

Yes. I'm still a fan of 75 ohm cable for dipoles that are relatively 
high (as a fraction of a wavelength) and otherwise in the clear, and 
50 ohm cable for those that are relatively low. The reason -- if you 
study models in the various texts (ON4UN, and others) for the 
impedance of dipoles, they tend to vary around 50 ohms +/- 33% for 
low dipoles, and around 70 ohms +/- 33% for higher ones. In other 
words, the presence of ground affects the feedpoint impedance, but 
tends to approach 70 ohms as the antenna gets further from ground. 

Jim Brown  K9YC