75 Ohm C-type coaxial connectors

I'm looking for a few hundred 75 Ohm C-type coaxial connectors. Apparently, these became obsolete in the

1980's, but I inherited a lot of expensive hardware that still uses them. Failing that, I'd still be interested in a drawing of the interface. The copies of MIL-STD348 I could get my hands on only show the 50 Ohm variant. Unfortunately, the 50 Ohm plug cannot mate with a 75 Ohm socket.

For those too young to know, a C-type connector looks like an over-sized BNC.

Thanks, Jeroen Belleman

Reply to
Jeroen Belleman
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Shown here,

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I wonder if maybe you could still find an adapter to something else?

George H.

Reply to
George Herold

I've been rescuing 75 Ohm C-to-BNC adapters, but I don't have nearly enough. What surprises me is that I find hardly any trace of 75 Ohm C connectors ever having existed!

Jeroen Belleman

Reply to
Jeroen Belleman

Well I'd never heard of it. (but I'm not much of an RF guy.) Do they have Ham radio 'shows' in Europe? Find some at swap meet.

Hey this is a fun fact, C connector was invented by C. Concelman

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who is also the C in BNC
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(It's easy to get lost in wiki. :^)

Reply to
George Herold

Quick google search shows that

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claims to have them, worth a call. If nothing else maybe they will share a copy of MIL-STD348.

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Regards, 
Carl Ijames
Reply to
Carl

?? I see only the 50 Ohm version. I *have* a copy of MIL-STD348. I checked the issues from 1986 to present. The 75 Ohm C-type isn't in there.

Jeroen Belleman

Reply to
Jeroen Belleman

The page I linked to is titled "75 Ohm Video Cable Connectors", and the first paragraph is:

PIC Wire & Cable has an extensive line of high quality 75 Ohm video cable connectors and contacts for its cable offering, including TNC, BNC, N, HN, C, SMA, ARINC, M39029 and D-Sub. In addition, PIC has many innovative connectors that improve termination, installation, maintenance and reliability. To ensure proper field installation, termination instructions and crimp die sets are available for most connectors. PIC also offers complete certified cable assemblies built to your requirements.

This clearly says 75 Ohm and "... , C, ...", and 75 Ohm C is what I thought you were looking for? I know you said that you had a copy of MIL_STD348, but then you said that it barely mentioned 75 Ohm so I thought maybe they had a more complete copy. I've never dealt with picwire, just found them and thought they might be a source for you.

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Regards, 
Carl Ijames
Reply to
Carl

I had to page down some to get to the 75 ohm connectors... and then more to find the C-type. George H.

Reply to
George Herold

Well, there's a photograph of one on the Wikipedia page:

I vaguely recall seeing connectors with red insulation in my junk pile. I'll dig.

I like Amphenol's poor excuse for not making a real 75 ohm connector: It has a constant 50 ohm impedance and performs up to 11 GHz. However, it may be used with 75 ohm cable at lower frequencies (below 300 MHz).

MIL-STD-348B Y're right. The table of contents only shows 75 ohm versions for BNC, TNC, and N connectors.

It looks like the coaxial OD might be the same for 50 and 75 ohms. Only the center pin and receptacle diameters are different. I wonder if you could rebuild 50 ohm C connectors with 75 ohm size pins and receptacles? You might also need to add an insulated sleeve and turn down some insulation on a lathe to make it fit.

Or, if your application is impedance sensitive but can tolerate an extra 5.6dB loss, use a 50 ohm coax cable and replace all the 75 ohm (panel) connectors with 50 ohm. Then add a 50->75 ohm "minimum loss pad" to get a 1:1 impedance match.

Or, if your application can tolerate a 1.5:1 VSWR (14 dB return loss) but not the 2*5.6dB loss, then just use 50 ohm cable and connectors (if you can make them fit).

Or, if your existing equipment uses flange mount panel C connectors, just replace them with the far more common 75 ohm N connectors.

Or, if have a CNC lathe, the make your own 75 ohm C connectors and sell the excess to pay for your efforts.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

Now I'm getting confused. Both you and Carl claim to have found 75 Ohm C-type connectors, but I don't see them! I can find 75 Ohm connectors, but there is no C-type. I can find C-type connectors, but they are all 50 Ohms.

What do you see that I don't?

Jeroen Belleman

Reply to
Jeroen Belleman

Thanks Jeff. I think we'll end up reducing the centre pin diameter of 50 Ohm connectors. I can tolerate the impedance bump of a mis-matched connector, but not the loss caused by matching pads. If I can't find an official interface drawing, I'll just measure one. I have a handful 75 Ohm C connectors left over to serve as examples.

Replacing the existing cabling, or even replacing just the connectors on the existing cabling are not options I'm willing to consider (yet). These cables date from the 1960's. They are something weird, a 1/2" solid aluminium coax. I'd love to rip them out and install new ones, but that's far too expensive. There are 250 of them and they're in a very awkward place. The equipment at both ends would need to be modified or replaced too. The troubles of keeping ageing installations alive.

Two years ago, somebody accidentally cut one, and we replaced it by a 50 Ohm cable with matching transformers at both ends. Ugly, but it worked very well.

Jeroen Belleman

Reply to
Jeroen Belleman

Sorry my mistake... I was looking at the Mil-C-81695 (which I guess is not what you want.)

George H.

Reply to
George Herold

Indeed no. Another hope squashed.

Jeroen Belleman

Reply to
Jeroen Belleman

.500 75 ohm hardline is still used in CATV systems. We always had scraps le ft from the ends of reels. It is typically used between line taps, while .7

50 or larger is used for trunklines, or supertrunks. (A supertrunk is a tru nkline with no bridgers, of feeders. It is used to tie large segments of CA TV systems together.)

.412 was used at one time, which replaced the RG11 used in very early 12 channel systems. .412 was mostly obsolete by 1970, for new construction.

Reply to
Michael Terrell

That's possibly the easiest way. However, I have yet another idea. If your coax is carrying signals over a limited frequency range, then you can use a 1/4 wave or 1/12 wave matching section to match 50 and 75 ohm system. Something like this: If you're carrying the entire CATV spectrum, that won't work. However, if you're carrying only a few 6MHz wide CATV channels, such a match should work.

I have access to a CNC lathe and probably make some center pins and matching receptacles. I couldn't find a drawing online.

I know it well. I was scrounging the stuff from the local CATV underground construction people. I was making antennas out of the

1/2" stuff, but never have pieces long enough to use as transmission lines.

Yeah, that's a problem when something is built well and designed to not fail. Such things refuse to die prematurely and tend to run forever. Fortunately, that won't happen with modern equipment, which is designed blow up immediately after the warranty expires.

The uglier it is, the better it works. I like ugly.

If you can use a transformer, I would guess that there's not much RF power involved and you're operating at lower frequencies. So, dump the coax cables and switch to twisted pair as in CAT5e cable. Plenty of BNC to CAT5e baluns available:

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

On Jul 20, 2019, Jeff Liebermann wrote (in article):

The data is at work, but I recall that the mechanical interface control diagrams were in the MIL-PRF-39012 slash sheets for the specific connector type in question. The slash sheets are on the web for free.

Joe Gwinn

Reply to
Joseph Gwinn

Thanks, but nothing found. MIL-PRF are performance specs. Plenty on testing but no dimensions. Methinks this is the latest: "MIL-PRF-39012F (W/ AMENDMENT-3), PERFORMANCE SPECIFICATION: CONNECTORS, COAXIAL, RADIO FREQUENCY, GENERAL SPECIFICATION FOR (18-MAY-2017)., This specification covers the general requirements and tests for radio frequency connectors used with flexible RF cables and certain other types of coaxial transmission lines."

The problem is that it's easy to find the MIL specs for for the 50 ohm version (MIL-STD-348B), the document does not contain any mention of a

75 ohm version. The 50 ohm dimensions for the pins are on Pg 38 - 41.
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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Jeff Liebermann

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