[TowerTalk] Twin-coax balanced line and Ant Sys

[Chris BONDE]

I think that I remember something from long time ago.  Twin lead was inexpensive, 
coax was expensive, dipoles multi dipoles and non-resonant dipole were fed with 
twin lead or home-brewed twin lines.

Now, I think that this was terminated by an open air coil with multi taps outside of 
the house.  The tapes were moved for the best connexion via two pieces of short 
length coax to the tuner or pie net work next to the Tx.  Is this correct?

I have been thinking of doing something similar and all this talk has brought this 
back to mind.  What comments

Date sent:      	Tue, 5 Aug 2003 17:03:57 -0400
To:             	towertalk at contesting.com
From:           	Chuck Counselman <ccc at space.mit.edu>
Subject:        	[TowerTalk] Twin-coax balanced line

> 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
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