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

[david vanhorn]

The loss in matching components may exceed the loss from the mismatch.

On Fri, Sep 7, 2018, 12:42 PM patrick hamel <pehamel at cableone.net> wrote:

> 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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