Open Wire "Coax"

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Chuck Pool:
Hi to all,

If two coax cables were tie wrapped together to form a balanced transmission line what would be the correct method to calculate this balanced line impedance, i.e. two 50 ohm cables tied together? 

Should both ends of this balanced system have their ground/shields tied together?

Or, should just one end have their ground/shield tied together?

Would spacing the two cables further apart have and effect on the lines impedance?

Thank you,

Chuck 

Bob Lewis:
The impedance would be 100 Ohms.
The shields should be connected at both ends.
Changing the distance would not change the impedance. The impedance is determined by the center conductor to shield separation.
The loss per foot will be very high when compared with real open-wire line or ladder line.

Clark McDonald:
And the correct terminology for such is, "shielded balanced line"...

Nelson Derks:
Quote from: AA4PB on February 26, 2013, 08:13:08 AM

The loss per foot will be very high when compared with real open-wire line or ladder line.

Not sure I'd say "very", but the shielded balanced line will have approximately the same characteristics as the coax it's made from in terms of loss and voltage ratings. Should be somewhat more forgiving of a mismatch as well. And don't forget that you can do this with RG-59 or RG-6 for a 150 Ohm balanced line. Given the cost and availability of RG-6 this might be worth considering. Especially if the line needs to run underground or in a setting that would be difficult for ladder line. The shields should be tied together at both ends and some references will tell you they should be grounded at both ends. I prefer grounding only one end as that's an old audio trick for avoiding a ground loop, which could be completely irrelevant to this concept....

BTW:  Now that I think about it, "very" high loss if we're talking about RG-174 in parallel.   ;D

Scott Schrader:
sounds like the IBM Token-Ring coax of a couple decades ago.  it was two wires TP inside the coax shield.  don't remember if it was air core or not, like the old ugly Motorola car antenna coax stuff.  iirc from some studies in a college 500 class, that was 125 ohm impedance and I would not want to run more than QRP down that stuff.

so it's been done, kinda looked like a two-pin N connector to join it.  you would be much happier with LMR and a balun.

now, IBM found the limitations early, including serviceability, and they moved to twin RG-58s welded together at the outer insulation, and used BNCs for connectors.  you get the same effect tying all your lines together going up a tower so the same woodpecker gets them all at the same time ;)

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