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Re: [TowerTalk] 75 ohm - v - 50 ohm coax

To: towertalk@contesting.com
Subject: Re: [TowerTalk] 75 ohm - v - 50 ohm coax
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Mon, 18 Feb 2013 09:54:06 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 2/18/2013 5:47 AM, Stan Stockton wrote:
Cut the coax to a common multiple of 1/2 wave length for as many bands as you 
can.  You can easily get 20-10 with the length you have without losing too much 
coax and maybe 40-10.  It won't help the loss but will make your readings look 
the same on both ends without any further matching.

Yes. I use a lot of low loss 75 ohm coax in my station. Much of it is carefully cut lengths of 1/2-inch CATV hard line that are some multiple of a half wavelength -- 3 wavelengths feeding a 15M monobander, 2 wavelengths on a 20M monobander, three wavelengths on a 10M monobander. I'm also using Belden 8213 to feed high fan dipoles (up about 110 ft) for 80 and 40M.

In an earlier post, I noted that if you make an NEC model of a simple half-wave dipole, you can export the impedance curve to SimSmith and add various lengths of feedline to it and study the result. And as I noted in that earlier post, these resonant lengths of line can (and usually do) have the effect of broadening the SWR bandwidth of the load presented to the transmitter.

Also, as Stan has observed, you can feed a tri-bander with a length of 75 ohm coax that is 1 or two wavelengths on 20M. The 1-wavelength line will be 1 1/2 wave on 15M and 2 wavelengths on 10M. The 2-wavelength line will be 3 wavelengths on 15M and 4 wavelengths on 10M. If your feedline needs to be longer, simply add 50 ohm coax to it on either end. If the resonant lengths of 75 ohm coax are a bit long, coil up the excess.

I've taken this a major step further by using a Vector Network Analyzer to measure the complex antenna impedance over the band, exporting that data to SimSmith, and designing matching stubs to improve the matching. Adding a suitable stub at the antenna end of the cable can match the load presented to the 75 ohm line, reducing the loss in that line, AND presenting a matched load to the transmitter. It's usually not practical to use this technique for antennas that cover more than one band, but it's both easy and effective for mono-band antennas.

SimSmith is free and very easy to use, but you must understand transmission line concepts to be able to use it. Google to find it. The author has written some excellent tutorials, and there's a rather different tutorial on my website that goes through what I've described here in a lot more detail. In addition to allowing us to design matching networks, SimSmith also computes and displays all the losses between the transmitter and the antenna, so we can study the usefulness of our designs.

http://audiosystemsgroup.com/publish.htm

Another point with respect to loss in that coax. Because its diameter is quite large, loss is MUCH lower than the RG8-size coax we usually use. It is also lower because the loss in ANY coax at HF is I-squared R loss in the copper, and the current in 75 ohm coax is proportionally less for the same power level. Further, the ADDED loss due to this relatively small mismatch is quite small, even for rather long cables. So, since the matched loss is much lower to begin with, the total loss with the mismatch is usually less than with even the best RG8 and RG213-size coax.

And yet another observation. The loss in coax depends on the impedance on the ANTENNA end of the coax, NOT the match at the transmitter. High dipoles are a closer match to 75 ohms than to 50 ohms, while low dipoles are a closer match to 50 ohms. The reason is the mutual impedance into the antenna of the reflection from the earth. You can see families of curves showing this for various antenna heights in ON4UN's book, and in the ARRL Antenna Book. I suspect that there rae similar graphs in an RSGB publication. That's why I'm feeding my high 80/40 fan dipoles with 75 ohm coax -- I get a closer match between the antenna and the feedline over a greater bandwidth (this matters on 80/75M more in the US, because the band is 500 kHz wide).

73, Jim K9YC
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