[TowerTalk] Twin-coax balanced line

[Chuck Counselman]

Twin coax is _not_ always a good, or even a fair, substitute for 
open-wire line.  Here's an illustration of how bad it could be.

The driving-point impedance of my quasi-G5RV doublet for f = 10.1 MHz 
is equal to ( 2020 - j 3126 ) ohms, according to a NEC-4 simulation 
using all the exact dimensions, realistic soil parameters, etc. 
Divide this value by two to get what one of the two coaxial cables 
would see: ( 1010 - j 1563) ohms.  Put this halved value into W9CF's 
transmission-line calculator <http://fermi.la.asu.edu/w9cf/tran/> 
with just 15 meters of RG-8A or RG-213 and you find that the loss in 
this relatively short length of coax would be 5.6 dB.

In other words, nearly three-quarters of my transmitted power would 
be dissipated in the coax.  Ouch!

OTOH, with 600-ohm ladder line the loss is 0.16 dB -- trivial.

I use 600-ohm ladder line.  Needless to say, I'm not about to switch 
to twin RG-8A.  Yes, I know that 30 m is the worst band for this 
antenna, but this is my antenna; I do operate on 30 m (as well as 80, 
40, 20, 17, 15, 12, and 10 m) with it; and it works fine.

73 de Chuck, W1HIS