OT: 75 to 50 Ohm Transformer?

[Cowboy]

Anyone come across one of these recently?

I remember some old old old QST articles on 75 to 50 ohm matching transformers...

Just curious if there's anything still commonly available as a project...

[Dan562]

Hello Cowboy,

I just used Google to find the following webpage:

http://www.repeater-builder.com/rbtip/m ... stubs.html

Dan

[Cowboy]

Hello Cowboy,

I just used Google to find the following webpage:

http://www.repeater-builder.com/rbtip/m ... stubs.html

Dan

Bingo that's what I was looking for Shame it's not a commercially available product - I have thousands of feet of 75 ohm cable laying around here...

[W6JK]

Shame it's not a commercially available product

That's probably not pracitical, due to the wide variations in available CATV coax. But there used to be a guy who'd custom-build them for you. You'd send him a few inches of your cable and he'd make the stubs to fit. He advertised for a long time, perhaps in QST. Maybe he's still around.

Jeff W6JK

[kf4sqb]

Honestly, the slight mismatch between 50 Ohm and 75 Ohm feedline isn't enough to be concerned about, unless this is for a system that would barely work with comparable 50 Ohm feedline. IIRC, the mismatch will only equate to a 1.3 - 1.4 : 1 SWR over what the same system would be with 50 Ohm feedline. That is to say, if your system had a 1.3 : 1 SWR with 50 Ohm feedline, you might expect a 1.6 - 1.7 : 1 SWR with 75 Ohm feedline. My numbers may not be exactly on the money, but they should be somewhere close. I discussed this very subject with an RF engineer several years ago (sadly, I didn't take notes), and he basically told me that the slight loss incurred wasn't worth worrying about. To make a connector to allow 75 Ohm hardline to be used with standard 50 Ohm connector styles, use a compression-type tubing connector (you should be able to get one that is a "proper fit" to the outer shell of the hardline), and find a local machine shop to make an adapter sleeve to adapt the compression connector's body to the RF connector's body.

[Cowboy]

Honestly, the slight mismatch between 50 Ohm and 75 Ohm feedline isn't enough to be concerned about, unless this is for a system that would barely work with comparable 50 Ohm feedline. IIRC, the mismatch will only equate to a 1.3 - 1.4 : 1 SWR over what the same system would be with 50 Ohm feedline. That is to say, if your system had a 1.3 : 1 SWR with 50 Ohm feedline, you might expect a 1.6 - 1.7 : 1 SWR with 75 Ohm feedline. My numbers may not be exactly on the money, but they should be somewhere close. I discussed this very subject with an RF engineer several years ago (sadly, I didn't take notes), and he basically told me that the slight loss incurred wasn't worth worrying about. To make a connector to allow 75 Ohm hardline to be used with standard 50 Ohm connector styles, use a compression-type tubing connector (you should be able to get one that is a "proper fit" to the outer shell of the hardline), and find a local machine shop to make an adapter sleeve to adapt the compression connector's body to the RF connector's body.

With that slight mismatch in mind, I've got some experiementing to do...

The majority of the 75 ohm cable I have to work with is just quad shield RG-11, same outer diameter as RG8/RG213/LMR400. I *have* put PL-259s on these in the past; just required soldering the center conductor as its still quite a bit smaller than 50 ohm cable counterparts.

I do have a few lengths of QR 875 hardline (.875" OD) with pin connectors installed - I could possibly use this for the main run and use a feedthrough housing feeding a Pin to F to feed a flex jumper (F(m) to N(m))...

Hmmm...