On Fri, 2007-01-26 at 21:35 +0000, Steve Hunt wrote:
> Folks,
>
> Thanks for all the responses on the Velocity Factor issue.
>
> I would expect 75 Ohm twin to have a lower VF than 300 Ohm or 450 Ohm line.
> With nothing but air between the conductors the limit on characteristic
> impedance is 83 Ohms, so to achieve 75 Ohm the line must have a significant
> amount of dielectric material as a separator; this will tend to lower the VF.
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.
> I have an MFJ259B and ZM30 so I could measure it easily enough if I had some.
> Being a "cheapskate" I didn't want to go buying it before knowing it would do
> what I want. I guess I'll just have to buy a few metres to test :)
>
> Thanks again.
>
> Steve G3TXQ
> _______________________________________________
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|