RadioBanter - View Single Post

March 19th 05, 01:21 AM

wrote:

Hi Roy, I'm OK with 319+j0 half way up the coil, but at 20% of the way
up I keep getting 50+j50. I did it on the Smith Chart, and on a Spice
based circuit analysis program. Too old to do this stuff by hand.
That's not a real good match for 50 ohm coax. The are other taps that
will provide a better match, but no where did I find 50+j0.
In an earlier post I stated that it looked like the real part of the
antenna impedance needed to be less than 50 ohms to get a perfect
match, using this method.
The impedance across the whole coil is not purely resistive, it is at
the 50% point. Apparently I am modeling incorrectly, or missing
something.
Gary N4AST

In SPICE, what coefficient of coupling did you specify between the
portion of coil below the tap and the portion above the tap? The
autotransformer impedance relationship I gave is strictly true only for
a coefficient of coupling = 1. A real inductor will be a little less,
but 1 is a decent approximation for a real solenoid of typical
proportions for this application.

I'm curious how you handled coupled inductors on the Smith chart -- I
don't believe I've ever seen it done.

Roy Lewallen, W7EL