Yet another way of attacking this problem, or if you have a bit more SWR
than you would like, is to build a "line flattener" box. I made one with
three lengths of 50 ohm coax - 2,4 and 8 feet long, with relays so I could
insert 0-14 feet of coax in two foot increments. Works great for 10-20
meters, but it does take a bunch of relays and some cable - both coax and
power. And, it can also be used to vary the phase delay between two driven
antennas that are already pretty well matched. Obvoiusly, the number of
cables, impedance, and lengths can be tailored to your needs.
Gene / W2LU
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <towertalk@contesting.com>
Sent: Monday, February 18, 2013 12:54 PM
Subject: Re: [TowerTalk] 75 ohm - v - 50 ohm coax
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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