On Wed, 27 Feb 2013 22:07:51 -0500, rickman wrote:

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

Re-read what Tim Williams said.

A way to translate what he's saying into your simulation is to include
the radiation resistance of the antenna into your simulation, and reduce
the coupling -- 1e-6 is probably good enough. I am, frankly, not sure
where this is best put in your radiation resistance, but just increasing
the series resistance on L3 is probably sufficient; putting it in as a
parallel resistance in L3 is probably more accurate, but would be more
useful as a way of separating the radiation resistance effect from the
winding resistance of L3 (which is, I assume, where your figure comes
from).

Do you have the ability to measure the Q of your antenna as built, and
compare it to the Q calculated from the known L, C, and winding
resistance? That should give you a good estimate of the radiation
resistance, or at least radiation resistance + other losses.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com