On Tue, 17 Jul 2012 23:55:22 +0000 (UTC), gregz
wrote:

Unless you use shielded twin lead. I used it on an antenna way back. Seems
like strange stuff, but didn't require transformers.

I think you mean Twinax:
http://en.wikipedia.org/wiki/Twinaxial_cabling
The stuff is about 75 ohms between pairs. Strictly speaking, it
doesn't require a matching transformer, but really should have a 1:1
balun to deal with the balanced Twinax to unbalanced (F connector)
system.

The stuff was designed for data, not RF. Impedance, balance,
capacitances, and data performance are specified in MlL-STD-1553B,
attenuation at RF frequencies is not. I'm surprised that it worked at
FM/TV(?) frequencies.

You could fake shielded twinlead with two runs of 75 ohm line. That
will give you 150 ohms between center conductors, which can be
converted to either 75 or 300 ohms with a 2:1 (trifilar) balun.
Unfortunately, the loss would be twice that of a single run of RG6/u,
so there's no benefit.

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Jeff Liebermann
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