In case I am not the last person on the planet to know about these
techniques, let me describe a recent station improvement.
My situation was the use of 75 ohm hardline with 50 ohm antennas. With a
pure 50 ohm antenna, the resultant swr at the station end of the hardline
can be as high as about 2:1, but it can be even higher if the antenna was
not truly 50 ohms resistive.
At HF, it seems common to just live with the additional SWR variations that
are introduced. As long as your transmitter or amplifier can deal with it,
the additional losses due to SWR in the hardline are not very big.
But in my case, I have a couple of extra consideration:
>From my 6M antenna system, I was seeing an SWR at the transmitter as high as
2.7:1 from what I thought was a fairly well matched yagi. The calculated
additional line loss introduced by this was starting to get over 0.5 dB,
which for weak signal VHF and above is getting to be of concern.
An even bigger problem for me was from the fact that when I would switch to
a different 6M antenna, the SWR in the shack from it was close to 1.2:1. If
my amplifier was tuned for antenna 1, it didn?t like antenna 2 too well, and
vice versa. So I really needed to tame the wide SWR (impedance) variations
being caused by the use of the 75 ohm hardline.
A web search showed a number of ways to do this.
1- use a ferrite transformer wound for the ration needed. I chose not to do
this due to concerns with stray coupling paths and properly terminating into
coax connectors, (plus I did not have any suitable cores in the junk box).
2- use a quarter wave matching section employing a coax cable having an
intermediary line impedance. The needed impedance coax was not in the junk
box either so this would have meant building something using proper inner
and outer diameter tubes and pipes to get the impedance right. And at 6M,
this might be rather big.
3- use a twelfth wave matching transformer. This technique uses nearly
quarter wavelength long pieces of 50 ohm and 75 ohm coax cable joined in the
center. Hmmm, I just happen to have a box with some short pieces of 50 ohm
RG213 and 75 ohm RG11, and they just happened to have a PL259 connector
already on at least one end. I could have just cut these to length and added
connectors and joined them with a coax barrel, but I chose to splice them
permanently together and avoid the middle connector joints. I used a ½?
length of brass hobby tubing and soldered ¼? of each center conductor into
it, then packed around it with some pieces of coax dielectric material, one
tape wrap to hold them, and then soldered overlapped shield braids coming
from each side. Initially cut your coax pieces an inch too long, strip back
the outer jacket, fold back the braid, and then cut off the extra inch. This
leaves you with a nice long braid to fold back to the other one. At the
finish, I wrapped the whole joint with tape, laid a long brad nail across
it, and wrapped again. The brad keeps the cable from being kinked at the
relatively fragile center junction.
I made two of these transformers, one for each end of the hardline. How did
they work? Great! Now the different antennas each show an swr close to 1.4:1
and their impedances are within a few ohms of each other. Be aware that
these are single band devices, which in my case was no problem since this
run of hardline is dedicated to the 6M antenna system.
Credit for this approach goes to Darrel Emerson, AA7FV, and you can see his
details on how to do it at
http://ourworld.compuserve.com/homepages/demerson/twelfth.htm. He also shows
how to use the two match phased antennas.
73,
Chet, N8RA
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