Yes, Chuck, I ran similar calculations and found that I needed to
go with at least 7/8" heliax in order to be competitive with open
wire line when trying to feed antennas with very high input
impedances. If you have a sturdy tower and can find the line
used for a good price, it does offer than advantage of shielding
which alleviates the problem associated with routing open
wire line. The other approach is try to find a quasi broadband
antenna that offers a better than ~6:1 VSWR (ref 50 ohms)
over the frequencies of interest. This also makes the tradeoff
a little better.
73 de Mike, W4EF...........................................
----- Original Message -----
From: "Chuck Counselman" <ccc@space.mit.edu>
To: <towertalk@contesting.com>
Sent: Tuesday, August 05, 2003 2:03 PM
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
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "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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