[600MRG] 50 vs 75 ohm coax, getting confused

[patrick hamel]

Paul,
I don't know of a commercial 630 meter antenna on the market.
There is only one magic thing about 50 ohms, that is as a standard to build commercial radios, antennas, and accessories and make them match.
You have a custom antenna and it can be matched to 75 ohm cable (if you have the cable) just as easily as to 50 ohms - then all you need to do is make the transmitter match the feedline.
This should be fairly simple if you are otherwise willing and capable to learn a smith chart or wade through antenna handbooks feedline and matching chapters and remember how to do the math. But the transmitter might already have an adjustment that will do the job.
73,
Pat W5THT & WD2XSH/6

----- Original Message -----
From: "N1BUG" <paul at n1bug.com>
To: "Discussion of the Lowfer (US, European, & UK) and MedFer bands" <lowfer at mailman.qth.net>, 600MRG at mailman.qth.net
Sent: Friday, September 7, 2018 7:23:03 AM
Subject: [600MRG] 50 vs 75 ohm coax, getting confused

Is there any free and easy to use software which can be used to
investigate the load R + X required at the end of a known length 75
ohm coax to get 50 j0 at the source end?

I'm getting myself confused. Nothing new there of course.

Right now I have about 20 short pieces of 50 ohm coax spliced
together going to the 2200m/630m transmitting antenna. This is a
failure waiting to happen, especially since it is tightly stretched
across the lawn to reach the antenna. Not to mention I am unhappy
about moving it every time I have to mow the lawn. Naturally when it
does fail it will be when there is four feet of snow over it.

I had an idea to use 75 ohm coax to feed the LF/MF transmitting
antenna. I have a lot of this stuff and it could be direct buried...
out of harm's way and out of my way.

I understand 50:75 ohm transformers are easy to make but there is
the cost. Zero cost was the reason for wanting to use 75 ohm cable.

Next I wondered if transformers are really necessary. It seemed I
should be able to adjust the load R + X to get 50 j0 at the source.

Then I realized this implies a non-zero X component at the antenna
to get 50 j0 at the load. It could be significant at 630m where the
line length is about 45 degrees. Running the antenna non-resonant to
get this X component can't be a good idea. If I'm not mistaken that
means lowered antenna efficiency, which is no good!

So I'm trying to figure out whether using 75 ohm line is reasonable
or just another of my really bad ideas which would lead to a whole
new set of problems.

At 2200m it might not be so bad, as the line length would be about
13 degrees. But even then, I'm not sure.

If anyone can help me sort this out I would appreciate it. The more
I think about it, the more confused I become.

73,
Paul N1BUG
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