[TowerTalk] 75 or 70 Ohm twinlead or ladderline cable - does it exist?

[Jim Brown]

On 3/24/2014 5:41 PM, Dan Hearn wrote:
> I remember 75 ohm twinlead built specifically for RF use. It used 
> rather large stranded wire  quite close spaced. It was rather stiff 
> and the plastic coating was not the stuff used in Zip cord. I believe 
> it was capable of running quite high power 

I remember low loss 300 ohm line made by Belden (which implies beefy 
conductors, and thus low loss, and my copy of the 1971 lists several 
versions of it, including one rated for 1kW.

Remember that Zo is determined by conductor diameter, spacing, and the 
dielectric, and that both power handling and loss are determined by 
conductor diameter and Zo. Thus, the kW rated 300 ohm cables used 
smaller conductors than the 75 ohm lines, and were wider spaced.

Note that my previous post describing Belden 8210 is neither conjecture 
nor distant memory -- I was looking at both the specs in the 1971 Belden 
catalog and the physical cable itself, which is still in my stash of 
goodies!

BTW -- 72 ohms was chosen because it was the Z at resonance of a dipole 
in free space. We paid attention to that in those days, overlooking the 
fact that a low dipole over good ground had a lower Z at resonance. :)

73, Jim K9YC