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Using 75 ohm CCTV coax for ham radio

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Mike Oxlong
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Using 75 ohm CCTV coax for ham radio

Post by Mike Oxlong » Sat Mar 27, 2021 9:35 am

There's been an on going off and on discussion with a few ham friends about the following:

One of them has a hefty spool of high quality 75 ohm CATV outdoor coax left at their house by Comcast. I guess the stuff cost a bit per foot from what he says. Anyway, he wants to use it for ham radio. Some say it will work, some say it won't.

Let's say I got a 50 ohm coax and a 50 ohm radio. The match is off. So that means the impedance that the radio is seeing is not 50 ohms. So what do I do? Adjust the length of the antenna so that the match is 1:1 and the impedance the radio sees is 50 ohms. I'm sure we are all in agreement.

So let's say I use the 75 ohm coax with an antenna and a 50 ohm radio. And of course the match is off.
Do you see any reason why, using the 75 ohm, the length of the antenna can't be adjusted so the radio sees a 1:1 match?

In a nutshell, that's the discussion. Some say it should work. Some say it won't.
What sayeth you?


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bmw
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Re: Using 75 ohm CCTV coax for ham radio

Post by bmw » Sat Mar 27, 2021 10:01 am

Outside of having to live with a 1.5:1 SWR, it'll work fine from what I've read.



Mike Oxlong
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Re: Using 75 ohm CCTV coax for ham radio

Post by Mike Oxlong » Sat Mar 27, 2021 4:50 pm

bmw wrote:
Sat Mar 27, 2021 10:01 am
Outside of having to live with a 1.5:1 SWR, it'll work fine from what I've read.
I would be happy with 1.5:1 too. I'm thinking about just trying it. We've been talking about it since he got the spool last year. I'm just gonna put an antenna on a mast, bungee it to the fence, and see what the MFJ analyzer thinks about it. I got some F to BNC adapters around here somewhere.


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SolarMax
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Re: Using 75 ohm CCTV coax for ham radio

Post by SolarMax » Sun Mar 28, 2021 11:01 am

It will work just fine. Make sure good, proper connectors are properly attached. While many antennas are theoretically 50 ohms, actual measured feed point impedance can vary up or down from that, depending on height above ground, ground conductivity, apex angle and many other things. Good quality 75 ohm CATV coax will for the most part be a better choice than cheap RG58 or RG8X.



innate-in-you
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Re: Using 75 ohm CCTV coax for ham radio

Post by innate-in-you » Tue Apr 06, 2021 12:02 am

Mike Oxlong wrote:
Sat Mar 27, 2021 9:35 am
There's been an on going off and on discussion with a few ham friends about the following:

One of them has a hefty spool of high quality 75 ohm CATV outdoor coax left at their house by Comcast. I guess the stuff cost a bit per foot from what he says. Anyway, he wants to use it for ham radio. Some say it will work, some say it won't.

Let's say I got a 50 ohm coax and a 50 ohm radio. The match is off. So that means the impedance that the radio is seeing is not 50 ohms. So what do I do? Adjust the length of the antenna so that the match is 1:1 and the impedance the radio sees is 50 ohms. I'm sure we are all in agreement.

So let's say I use the 75 ohm coax with an antenna and a 50 ohm radio. And of course the match is off.
Do you see any reason why, using the 75 ohm, the length of the antenna can't be adjusted so the radio sees a 1:1 match?

In a nutshell, that's the discussion. Some say it should work. Some say it won't.
What sayeth you?
I wonder if you could make modifications to the antenna to give it a 75Ω characteristic impedance.

I do know that short lengths of coax are sometimes used to adapt one characteristic impedance to another.



Mike Oxlong
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Re: Using 75 ohm CCTV coax for ham radio

Post by Mike Oxlong » Tue Apr 06, 2021 5:04 pm

innate-in-you wrote:
Tue Apr 06, 2021 12:02 am
I wonder if you could make modifications to the antenna to give it a 75Ω characteristic impedance.

I do know that short lengths of coax are sometimes used to adapt one characteristic impedance to another.
What we are wondering is, using 75 ohm CATV cable, if the length of the antenna can be so that the radio "sees" 50 ohms.


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audiophile
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Re: Using 75 ohm CCTV coax for ham radio

Post by audiophile » Tue Apr 06, 2021 8:22 pm

If it is cut to an even number of wavelength(s) the match will be not be affected at one frequency. Zin = Zout

If it is odd 1/4 wavelength then it will act as a transformer and cause impedance to be increased to 115 ohms at the radio.

In summary on single frequency use, when the cable length is cut to match, it will work good. Otherwise it's a bad idea!


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Mike Oxlong
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Re: Using 75 ohm CCTV coax for ham radio

Post by Mike Oxlong » Tue Apr 06, 2021 10:59 pm

audiophile wrote:
Tue Apr 06, 2021 8:22 pm
If it is cut to an even number of wavelength(s) the match will be not be affected at one frequency. Zin = Zout

If it is odd 1/4 wavelength then it will act as a transformer and cause impedance to be increased to 115 ohms at the radio.

In summary on single frequency use, when the cable length is cut to match, it will work good. Otherwise it's a bad idea!
Ok thanks.


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