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## [TowerTalk] Twin-coax balanced line

 To: [TowerTalk] Twin-coax balanced line W4EF@dellroy.com (Michael Tope) Tue Aug 5 21:17:49 2003
 ```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" To: 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 > 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. > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > ```
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