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

[N1BUG]

Thanks everyone for the replies and helpful information!

At first the idea of getting away from so many spliced together
pieces of RG-8 and RG-213 seemed appealing. I would have replaced it
with two runs of 3/4" CATV hardline, one per band. In addition to
having a more reliable and buried feedline, that would have
simplified band switching.

I've built all of my LF/MF gear and designed some of it. I've been
using 75 ohm line at 160m, VHF, and UHF for many years but somehow I
just got myself twisted up trying figure out what would happen in
this somewhat more complicated and demanding system.

Using one of the tools suggested to examine what R + X I would need
to present to the line at the antenna end to see 50 j0 at the
transmitter end I quickly realized this would be opening a huge can
of worms.

Given that I want the scopematch to continue providing reasonably
good (and easily understandable) information about antenna resonance
and my class D/E amplifiers to continue to see 50 ohms, I've decided
using 75 ohm coax on these low bands is not the right solution for
me at this time.

I could of course use transformers at both ends or tune the antenna
to 75 j0 and use a transformer at the transmitter end. But, given
the cost, especially with the power levels involved at 2200m, I will
be better off to wait until I can afford good 50 ohm coax.

And now I am off to move the coax so I can mow... again. :-)

Thanks again everyone for getting me out of a self-dug pit of confusion.

73,
Paul N1BUG



On 09/07/2018 08:23 AM, N1BUG 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