Electronics > RF, Microwave, Ham Radio

120 or 125 ohm coax

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tchicago:
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.

T3sl4co1l:
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.

Tim

tchicago:
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.

mtwieg:
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?

T3sl4co1l:
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.

Tim

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