"rickman" wrote in message
...
High Q isn't the goal, high radiation resistance is -- the bigger the
loop, the better it couples with free space, until it's a wave length
around.

I'm not clear on why you keep referring to radiation resistance for a
receiving antenna. Does this result in a larger received signal? I am
concerned with maximizing the voltage at the input to the receiver.

You're also not concerned about that -- you're concerned about maximizing
SNR at the receiver.

A Q of a million will get you gobs of "gain", but if it doesn't couple
into free space, it's only the thermal noise of the loss generating that
signal.

An antenna with high (expressed as ESR) radiation resistance might have a
modest Q, but gives far better SNR because it couples to free space.

Raw volts don't matter, you can always throw more amplifiers at it (as
long as they don't corrupt the SNR also!).

Yes, a higher stepup ratio gets larger signal up to a point. That point
is determined by the parasitic capacitance of the receiver input. That
capacitance is reflected back through the transformer and affects the
antenna tuning. In my simulations it creates a filter with two
resonances.

Oooh, capacitance! I like capacitance. Capacitance is easy to
cancel...inductors are good at that.

What's a nearby inductor working against that capacitance? The current
transformer in your simulation, if its inductance can be controlled, would
be an excellent candidate. The circuit effectively becomes a double tuned
interstage transformer, like,

http://www.jrmagnetics.com/rf/doubtune/doubccl_c.php
This is two resonators coupled with a cap, but any coupling method will
do. Capacitive, magnetic (putting the coils end-to-end) or
electromagnetic (coils side-by-side) coupling does equally well; normal
arrangements have them all in phase, so in practice, unshielded coils will
need smaller coupling capacitance than designed, etc.

If you line up that 10p resonance with the operating frequency, you should
get gobs more gain. In fact, because the reactances cancel, the driven
impedance will be much higher than you were expecting, and so will the
gain. The CT might go from, say, 1:8 up to, who knows, 1:20? 1:100?

The bandwidth of that coupling (not necessarily of the antenna itself, so
they should be similar bandwidths) is determined by the coupling
coefficient (in the coupled-inductors case, simply k) and Q of the
components.

If your receiver datasheet specifies an equivalent input circuit, you
might be able to estimate the equivalent loss and optimize gain.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com