Electronics > RF, Microwave, Ham Radio

120 or 125 ohm coax

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Could someone please recommend the "type" and/or source of the 120 or 125 Ohm coaxial cable that is not too thick?

I know of RG63 "type", but it is a little too thick for my purpose. I need it to experiment with matching a folded dipole to 50 Ohm coax using the "stepped impedance matching" method. So obviously, I only need small lengths of this cable.

Why not use twisted pair? It's also balanced which is nice for the antenna, and has the right impedance (or nearby).  Oh, maybe it's a balun and you're using the shield mode for a 1/4 wave transformer? Dunno.


Yeah, 1/4 wave transformer aka "stepped impedance matching". The piece of 125 Ohm coax would fit neatly inside the pipe that forms the folded dipole, if it wasn't that thick.

RG-63 is the only standard coax I'm aware of with Z0>100ohms. High Z0 requires a large OD/ID ratio, so a small cable diameter is asking for a lot...

What frequency are you working at? If <1GHz I would suggest a lumped element phase shifter. If much higher, maybe a PCB transmission line would work?

Well, fitting coax inside a pipe isn't going to help with shield-mode balun action.  You're making a tri-ax assembly, where the second dielectric is a mix of air and whatever the coax jacket is (usually PVC).  So the lengths will be different, and losses likely high.

But at this point we require a diagram to say anything for sure.



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