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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|