I am working on a simulation for a loop antenna in LTspice and I can't
figure out why the signal strength features are what they are. The
model uses a pair of loosely coupled inductors to model the transmitter
and antenna loop with a separate pair of tightly coupled inductors to
model the coupling transformer. A cap on the primary circuit is the
tuning cap and a cap on the secondary is parasitic effects of the
circuit board leading to the inputs on the IC.

There is a resonance near the frequency I would expect, but it is not so
close actually. I can't figure why it is about 5% off. There is a
second resonance fairly high up that I can't figure at all. None of the
component values seem to combine appropriately to produce this peak.
When looking at the tuning capacitor voltage there is an anti-resonance
that is exactly at the frequency corresponding to the secondary
resonance with the transformer and the parasitic capacitance. That
makes sense to me, but it is pretty much the only part that jibes with
what I can figure out.

I have uploaded a zip file with the schematic and a measurement file.

http://arius.com/temp/Antenna_trans_LTspice.zip

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Rick