Roy Lewallen wrote:

We can use the inductor as an autotransformer. If we tap up on the
inductor some fraction k of the whole way up, the impedance we'll see
at
that tap will be very nearly 1275 * k^2, and it'll be purely
resistive
(no reactance) because the impedance across the whole coil is purely
resistive. For example, half way up the coil we'll see 1275 * (.5)^2
=
1275 / 4 = 319 ohms. So to get 50 ohms, we tap up sqrt(50/1275) = 20%
of
the way up the coil.

Roy Lewallen, W7EL

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

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

Roy Lewallen wrote:
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

Hi Roy, This is where I am probably missing the boat. In both models,
I used 2 separate inductors, the sum of both being the required
inductance, in this case .35uh. I assumed that the tapped coil could
be modeled this way. I see now that this is not an auto-transformer,
it is simply 2 inductors.
The Smith Chart program has a standard transformer feature, and after
you cancel out the reactance, a transformer with a ratio of 1:0.2 gives
50 ohms. The Spice program has coupled inductors, but I will have to
do a little research to see if I can apply them to this model.
Thanks for the explanation, and thanks to others who replied by email
with suggestions.
Gary N4AST

wrote:
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.

Maybe it's the same as the parallel tuning coil installed
at the base of a 75m mobile antenna which has a real part
equal to 12-25 ohms (depending on the quality of the antenna).
--
73, Cecil http://www.qsl.net/w5dxp

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wrote:
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.

EZNEC comes with VERT1.EZ, a 40m vertical. I just ran it
at 19 MHz and EZNEC reports a 50-j160 ohm feedpoint impedance
with a maximum gain of 2.23 dBi and a TOA of 16 deg. Seems all
that is needed for matching to 50 ohms is a series base coil.
--
73, Cecil http://www.qsl.net/w5dxp

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Cecil Moore wrote:
wrote:
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.

Maybe it's the same as the parallel tuning coil installed
at the base of a 75m mobile antenna which has a real part
equal to 12-25 ohms (depending on the quality of the antenna).
--
73, Cecil http://www.qsl.net/w5dxp

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If you use an "L" network consisting of two inductors as a matching
device, the R must be less than 50 ohms. I was incorrectly modeling
the tapped coil as a two inductor "L" network.
Gary N4AST