[600MRG] 50 vs 75 ohm coax, getting confused
Brian, WA1ZMS
wa1zms at att.net
Fri Sep 7 15:27:53 EDT 2018
Correct.
And....
I've used CATV line in the past and with a perfect 50ohm load, the SWR will be 1.5:1. (ie: S11 of -14dB) The mismatch loss is small and I did not worry about it.
-Brian, WA1ZMS
iPhone
> On Sep 7, 2018, at 2:50 PM, david vanhorn <kc6ete at gmail.com> wrote:
>
> 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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