[TenTec] 75 Ohm twin velocity factor ?

[Steve Hunt]

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 at 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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