[TenTec] 75 Ohm twin velocity factor ?

[Dr. Gerald N. Johnson]

On Sat, 2007-01-27 at 09:07 +0000, Steve Hunt wrote:
> Jerry,
> 
> Despite using the wrong formula I guess I came to the right conclusion .... you can't get down to 75 Ohms without some "non-air" dielectric.
> 
> I have three HP41C calculators (one in the house, one in the shack, and a spare) and was surprised something so sophisticated doesn't have hyperbolic functions; but I believe they were available on one of the plug-in math modules.
> 
> 73,
> Steve

How do you get that? I tried 0.1" diameter conductors 0.11" center to
center and it comes up 53.2 ohms impedance. That's 0.010" gap, fairly
closely spaced. Same conductors 0.105" center to center (0.005" gap) is
37.79 ohms impedance. Cut that gap to 0.001", 0.101 gives 16.96 ohms
impedance. That's 120 arccosh (S/d). S is center to center spacing, d is
conductor diameter. Working closer, 0.0001" gap for 0.1001" center to
center gives 5.37 ohms. So lower impedances are possible, maybe not
practical, but possible. 10 millionths gap makes 1.7 ohms, 1 millionth
gap makes half ohm impedance. But yet the conductors don't quite
intersect or touch unless the gap gets all the way to nothing. And then
the impedance is zero.

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer