Jerry,
Thanks for the correction. I've never come across the Zo formula involving a
cosh function and for all these years have been using the log formula in
"ignorant bliss".
Just shows you're never too old to learn something new :)
73,
Steve G3TXQ
----- Original Message -----
From: Dr. Gerald N. Johnson
To: tentec@contesting.com
Sent: Friday, January 26, 2007 10:26 PM
Subject: Re: [TenTec] 75 Ohm twin velocity factor ?
That 83 ohm limit is untrue. Its where the conductors would have to overlap
if you use the log formula which is only accurate above 200 ohms. The proper
formula for all impedances and spacings that works down to .01 ohm
characteristic impedance is 120 cosh^-1 b/a (that's the inverse hyperbolic
cosine, not often in a calculator or set of tables) as I recall. That shows
curved lines on a log log chart.
My 1959 ARRL handbook shows Amphenol 214-080 had a vf of 0.68 and 19.0
pf / foot. That must be the receiving size. Then 214-023 had a vf of
0.71 and 20.0 pf / ft and the footnote says 214-023 is made for
transmitting purposes. Amphenol number 214-079 was 150 ohm
characteristic impedance with vf of 0.77 and 10.0 pf / ft. What the
characteristics of the British versions, I can't be sure. I'm sure the
vf depends on the plastic used and whether is foam or solid or a
composite with some solid and some foam (as in some oval 300 ohm lines)
So if you need a particular phase angle for a phased array the only way
is to measure it. If you are using it for a quarter wave transformer
then the vf and the length have some tolerance since the quarter wave
transformer has a large bandwidth.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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