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72 Ohm 2 Conductor Shielded Cable - Twisted Pair?




 
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Author Topic: 72 Ohm 2 Conductor Shielded Cable - Twisted Pair?  (Read 558 times)
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Todd, KA1KAQ
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« on: March 22, 2018, 09:52:28 AM »

Still in the process of assembling all the bits for the Meissner project before ripping into it. One of the items I'm in need of is new RF cabling for the exciter-to-transmitter connection as well as replacing the line inside the transmitter.

The manual calls for shielded cable w/two 18 gauge conductors, 72 ohms, cloth covering. None of the external connecting cables came with the set, but the original piece of cable in the RF deck is still in place, albeit a bit hacked up. It doesn't appear to be anything special - two 18 ga. stranded wires with an external shield and typical old buzzard cloth covering. The two conductors are twisted, though.

I'm curious as to how they came up with 72 ohms as a spec and if it's terribly important to the bigger picture. I found some Twinax cable, also twisted, 100 ohm impedance.

Looking at the original pieces left inside, my instincts tell me to just find some 18 ga. shielded cable and use it. The overall run will be short (manual calls for 5 foot connecting cable, internal piece is around 16-18"). Just don't want to introduce any additional problems by ass-u-ming. The cloth covering matters not one bit of course, would be cool to find but not holding my breath.

What do you folks with external VFOs generally use for cable?
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KD6VXI
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« Reply #1 on: March 22, 2018, 10:07:55 AM »

A piece of coax from an old car.

It's spec'ed as 'low capacitance coaxial cable'.

--Shane
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WA2SQQ
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« Reply #2 on: March 22, 2018, 10:37:57 AM »

I needed a short length of low capacitance cable for a capacitor analyzer. I used a short length of the cable once used for network connections. I forget the exact impedance, but I'm thinking ~90 ohms?
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w8khk
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« Reply #3 on: March 22, 2018, 12:06:15 PM »


The manual calls for shielded cable w/two 18 gauge conductors, 72 ohms, cloth covering.

I think the key issue here is two twisted conductors with a shield, not simple coax.  You might consider using balanced microphone cable, as made by Belden years ago.  Most of the balanced microphone cable today is four very small conductors with a shield.  I have plenty of the Belden cable which should work well for your application.  Not sure what model Meissner you are working on.  Could you share more info, maybe a schematic?  If my cable will work for you, I could mail as much as you need.
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Rick / W8KHK  ex WB2HKX, WB4GNR
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« Reply #4 on: March 22, 2018, 01:57:32 PM »

I needed a short length of low capacitance cable for a capacitor analyzer. I used a short length of the cable once used for network connections. I forget the exact impedance, but I'm thinking ~90 ohms?

50, 75, 80 and 93 ohms is commonly available.

Easiest thing for OP to do may be skin some coax for the shrill, throw a set of wires in the drill chuck and vise and make a twisted pair.

Thread through the newly skinned braid, voila!  Balanced, shielded wire.  Heat shrink or teflon ice maker supply line sheath for insulation.

OR!  CAT5 cable in the same sheath.  Now there is 4 shielded pairs!

--Shane
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« Reply #5 on: March 22, 2018, 07:47:37 PM »

I have 3 conductor twisted cable with a shield black cloth cover also have it with white cloth cover.
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W1ITT
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« Reply #6 on: March 22, 2018, 10:14:55 PM »

Todd...
I have some lengths of TwinAx back at home.  How much do you need?  It's left over from a job long ago where we used it in a multistation environment to feed DC from detectors back to a console.  If you are willing to stand the cost of mailing, you can have a few feet.  Just say how much.  Even a few ounces removed from my garage attic will increase the structural security of the building!  The twinax I have has a heavy blue jacket, high density braided shield, and two twisted conductors.  There may even be a ground drain wire too.  We used it with some connectors that looked like BNC connectors, but had two "inners", a pain to make up.  My most recent use has been as a shielded DSL cable from my customer demarc in to my modem. 

I'm over in India now, putting up some HF antennas for aviation comms, but plan on being back next week.  Let me know how much you need, reimburse for postage after you get it and give it a try.
Norm W1ITT
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Todd, KA1KAQ
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« Reply #7 on: March 23, 2018, 09:11:59 AM »

India, Norm? Hope you're taking advantage of some excellent tandoori chicken and Mulligatawny stew while you're there. Thank you very much for the generous Twinax offer. Were it not for the blue jacket (it's needed for a WWII transmitter), I'd snap it up. Still have some Twinax connectors scavenged from a AS-400 set up stashed around here somewhere for the R-390s...

Fred: I do need some 4 conductor shielded for the exciter power cable, same make up. Any of that around? I gave up on finding cloth covered a while ago. Plenty of 2 conductor AC power cord around with cloth covering, repops for antique lamps, appliances, and radios.

Did think of the twist-and-shield approach, Shane - even found some tinned copper jacket for sale online. Figured I'd ask the experts first then roll my own as a last resort.

Rick, the transmitter is a Meissner 150-B, early WWII. Uses a repackaged Signal Shifter for the exciter/VFO. Here's a link to the manual and schematic that Rob/K5UJ scanned and put online a few years back. Photos near top, parts list and schematic are near the bottom:

http://home.fnal.gov/~atkinson/lowres150b.pdf

I did look at some Belden mic cable (8412, 8422 etc) but think I could only find 20-22 gauge stuff. Probably would work just fine? The exciter has provisions for adjustment and calibration to the transmitter, of course. I'm not looking for a museum-correct restoration per say, mainly concerned with things working right electrically. There are also things like cable clamps inside the transmitter that require a certain cable size to clamp onto and so on. Black or even gray jacket is fine. I think the transmitter uses 16-18 inches inside at most, and the manual calls for a 5 foot RF connecting cable. It uses the Amphenol 80MC2 connectors which are sitting here ready to go.

TNX for all the offers and suggestions, guys. Still a few parts away from blast off, but getting there.
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