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Technical Topics => The RF Workbench => Topic started by: JimIO on December 23, 2019, 2156 UTC

Title: 50 to 75 ohm 1/12 wavelength coaxial transformer
Post by: JimIO on December 23, 2019, 2156 UTC: I have a VHF FM transmitter with a 50 ohm output. I want to make a 50 to 75 ohm 1/12 wavelength coaxial transformer so I can use 75 ohm coax to connect it to an antenna. I want to use F type compression connectors to join the coax sections together. I need some 50 ohm coax with most of the dimensions of RG6 (except the O.D. of the center conductor). Looking around I see RG5 and maybe LMR300. Looking some more I can't find a source for small quantities of either one. Does anyone know if LMR300 would be compatible wit F compression connectors and a source for RG5 or LMR300 by the foot?

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Title: Re: 50 to 75 ohm 1/12 wavelength coaxial transformer
Post by: redhat on December 24, 2019, 0119 UTC: Why the interest in F connectors? If your equipment has them already that's one thing. Otherwise, adapters from F to N, BNC, and SMA exist.

+-RH
Title: Re: 50 to 75 ohm 1/12 wavelength coaxial transformer
Post by: JimIO on December 24, 2019, 0259 UTC: I know there are other ways to do it. I have the coax and F connectors and tools.
I have F chassis connectors and RG58. I could make it with that but I would need something to mount those to and drill holes and solder. If I can get the proper coax I can make what I need in less time than it takes a soldering iron to heat up.
The transmitter will put out 5 watts or 15 watts. What do you think about using just 75 ohm coax at 5 watts?

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Title: Re: 50 to 75 ohm 1/12 wavelength coaxial transformer
Post by: redhat on December 24, 2019, 2234 UTC: If you're feeding a dipole, feedpoint impedance is going to be 70 ohms anyway. Vswr will be around 1.3:1 under those conditions.

+-RH
Title: Re: 50 to 75 ohm 1/12 wavelength coaxial transformer
Post by: pinto vortando on December 25, 2019, 2103 UTC: Built a 1/12 wave coax matching section here several years ago to use RG6 with a 6 meter rig (50 ohm output),
didn't seem to make enough difference with the SWR to be worth the effort.
If you do decide to go ahead with the 1/12 wave match, be sure to include the velocity factor in your calculations.
Title: Re: 50 to 75 ohm 1/12 wavelength coaxial transformer
Post by: JimIO on December 26, 2019, 0029 UTC: I made a post a little earlier but its not here. Must have been a wifi dropout.
Anyway, I'm going to just screw the RG6 to the F to BNC adapter and be done.

I want to make a coaxial colinear antenna out of RG6. What's a simple cheap and strong way to do the shield to center conductor cross over connections?

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Title: Re: 50 to 75 ohm 1/12 wavelength coaxial transformer
Post by: redhat on December 26, 2019, 0142 UTC: Quote from: JimIO on December 26, 2019, 0029 UTC
I want to make a coaxial colinear antenna out of RG6. What's a simple cheap and strong way to do the shield to center conductor cross over connections?
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Due to the aluminum jacket, you won't be able to make solder connections. I would use something like RG-400 or 0.141" coax to make a colinear. The jacket is tinned, making assembly quite easy.

+-RH
Title: Re: 50 to 75 ohm 1/12 wavelength coaxial transformer
Post by: Pigmeat on December 26, 2019, 1245 UTC: I've seen RG-58 and 59 with a copper jacket and F connectors for cable tv applications. It matched up well for HF work w/o a tuner. It hard to find and wants to kink. I finally cut the connectors off and switched back to BNC's.

75 ft. is the shortest run of the F-connector stuff I've ever ran across at stores that carry CATV coax. If you've got a buddy who's a cable guy, see if you can get a reel end from him? You can put any connector you want on it then.