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

[JamesDuffey]

< http://fermi.la.asu.edu/w9cf/tran/  >

James Duffey KK6MC
Cedar Crest NM

> On Sep 7, 2018, at 06:24, Ben Gelb <ben.gelb at gmail.com> wrote:
> 
> http://tools.rfdude.com/RFdude_Smith_Chart_Program/RFdude_smith_chart_program.html
> 
>> On Fri, Sep 7, 2018, 5:23 AM N1BUG <paul at n1bug.com> wrote:
>> 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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