[TenTec] 75 Ohm twin velocity factor ?

[Dr. Gerald N. Johnson]

On Sat, 2007-02-03 at 14:48 -0500, Charles Greene wrote:
> Stuart,
> 
> New, high quality 450 ohm ladder line has a velocity factor of 
> 0.90.  Someone said his handbook said transmitting twin lead had a 
> velocity factor of .67.  Suggest you follow up on this lead, as it 
> sounds reasonable
> 
> 73,  Chas W1CG
> 
The fields around high impedance ladder line are mostly in air, so the
velocity is high. The fields around the 72 ohm transmitting twin lead
are mostly in dielectric, its very close spaced, so the velocity is
determined by the dielectric and the books say 0.67. I suspect they did
know how to determine VP back in 1955 when Amphenol was making the low
impedance twinlead. In any case its always worth checking the velocity
of propagation when used in a resonant impedance transformer because Vp
can vary if the conductor location varies a little in the dielectric or
the extruder pump caused small variations in dimensions or density.

73, Jerry, K0CQ