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Re: TC radiates

Original poster: Paul Nicholson <paul@xxxxxxxxxxxxxxxxxxx>

Herwig Roscher wrote:

[base impedance]
> I would like to get a feeling for the magnitude of this
> impedance - is it some tens or some 100 Ohms? And is there
> a method to measure this impedance w/o a breakout?

It is typically between 100 ohms and 1k ohms.

A TC radiates very little, so all but a fraction of a percent
of this resistance represents the resonator's loss referred
to the base of the coil.  The input impedance is related to
the Q factor by

    Rin = 2 * pi * F * L/Q

at the resonant frequency F.   Q is maximum, and Rin minimum,
when the coil is unloaded.  As loading (breakout, etc) is
applied, Q falls and Rin rises.  This provides a self-limiting
action when the base is driven from a source which approximates
a constant voltage supply.

For measuring input resistance, I use a bridge arrangement
in which balance is detected by nulling the net current in
the bifilar primary of a current transformer.  The circuit
is in


Adjust Rb for minimum reading on the scope at resonance,
then Rb equals the Rin of the coil.  I use an old flyback
core for the transformer, a couple of turns of 2-core
makes the bifilar primary and a dozen or so turns of hookup
wire on the opposite side of the core forms the secondary.
Rs and Rx pad the source impedance down to a low value so
that I can measure the coil's Q factor at the same time.

The balance is a little distorted by capacitance between the
bifilar primary and the core.  A twisted pair primary might
help there.  Also, a resistor across the CT secondary may
be needed to kill its self-resonance if that gets in
the way.

Rin always measures quite a bit higher than you would expect
based on an estimate of the coil's wire resistance alone.
For example a large coil which should have an AC resistance
of around 40 ohms gives an Rin between 120 and 300 ohms,
the extra loss being attributable to the ground return
resistance and various other losses occuring due to coupling
of the coil's field to the immediate surroundings.
Paul Nicholson
Manchester, UK.