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@contesting.com
From: Chuck Counselman <ccc@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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> "Wireless Weather Stations", and lot's more. Call Toll Free,
> 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>
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