Topband: Open wire line vs. Paralleled coaxials.

[Rudy Bakalov]

The issue here is not about regular coax cable, but about making open wire cable using coax.  Open wire lines do not have to look like the ones everyone is using, but in fact can be made of parallel runs of coax cables.
There are applications in which we need a parallel transmission line that is shielded, that is, that is relatively immune to the unbalancing influences of very nearby conductive materials. For such applications, we can create a series-connected coaxial cable.
The characteristic impedance of a series connected coaxial cable is twice the impedance of a single cable of the same type. However, the power rating will be about the same as for a single cable, since the current- carrying ability of the center conductors has not changed. However, in higher impedance situations--for which series cables are often used--the current level may be naturally lower than when the same cable is used in a well matched situation. On the other hand, the series cable may be used in highly mismatched situations, resulting in high peak currents resulting from the high SWR level.
The losses on a coax are because of dielectric losses. But here you are not using the shield as RF conductor, so the PE of foam dielectric is working in a different manner.  The return goes on the other center conductor and not the shield. Loss figures are between those of coax and those of "real" open wire lines; they are closer to the open wire and not the coax figures.
See Cebik for more details on the topic.  I believe even the ARRL Antenna Book mentions the use of parallel runs of coax to create a "better" open wire line.

Rudy N2WQ
--- On Thu, 11/5/09, Jim Brown <jim at audiosystemsgroup.com> wrote:

From: Jim Brown <jim at audiosystemsgroup.com>
Subject: Re: Topband: Open wire line vs. Paralleled coaxials.
To: "topband at contesting.com" <topband at contesting.com>
Date: Thursday, November 5, 2009, 1:27 PM

On Thu, 5 Nov 2009 06:41:47 -0800 (PST), Dennis OConnor wrote:

>I have runs of coax out to a 1000 feet that on 80 meters and to 
>1500 feet on 160 meters

There is plenty of published data for the loss in coax. In general, 
it increases with wire resistance (skin effect), which is 
aproximately the square root of frequency. While the loss in coax is 
small on 160M, it is not zero, and 1000 ft of Belden 8237 (RG8A) 
will cost you 2.3dB. On 80M it will cost 3.3dB. No big deal on a 
receive antenna, but I sure wouldn't want it on TX! 

Good references are the Belden catalog (online), the ARRL Handbook, 
and the ARRL Antenna Book. 

73,

Jim K9YC


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160 meters is a serious band, it should be treated with respect. - TF4M