On Nov 15, 1:26*am, Wimpie wrote:
On 15 nov, 02:59, "amdx" wrote:

Hi Guys,
*Assuming I have a tank circuit on a crystal radio with a Z at resonance of
1.5 megaohms.
How would I make an antenna and extract maximum signal and keep the Z at
750,000 ohms.
* If don't think that's what I want to do, tell me that too. :-)
* * * * * * * * * * * * * * * * * * *MikeK

Hello Mike,

Assuming you have a long wire outside and a ground provision, you may
use an inductive coupling. *By changing the distance between the
antenna coil and the receiver coil, you modify the impedance
transformation.

Other method is capacitive coupling. *Probably positioning the end of
the antenna wire close to the high impedance side of the tank circuit
will give the desired effect. Changing the distance changes the
coupling.

For inductive coupling, your receiver circuit can be floating, for
capacitive coupling, the receiver should be grounded.

By changing the coupling you can optimize for maximum selectivity
(with reduced sensitivity) or maximum sensitivity (with reduces
selectivity).

Regarding the antenna, assuming LW and AM reception, long combined
with high gives strongest signal, hence you can reduce the coupling to
get best selectivity.

Best regards,

Wim
PA3DJSwww.tetech.nl
without abc, PM will reach me very likely

I like your very practical advice here, Wim. For Mike, as Wim noted
in another post, you can simulate this in Spice quite easily. While
you're playing with it in Spice, you might also look at two high-Q
tanks, tuned to the same frequency, with a non-zero coupling
coefficient between the inductors. Excite the first with a voltage
generator in series with the RLC tank, or a current source across it,
and observe the frequency response at the second tank. Vary the
coefficient of coupling between the coils and notice how small it is
to get critical coupling. If you use LTSpice, you can use a .step
statement to run a set of simulations over a range of k values, for
example.

The flip side of this is that in LC filters that are very narrow-band
that you design assuming no coupling among the resonators, expect to
have to work some to insure that there really is no coupling among
them in your implementation! Sometimes it gets difficult to shield
well enough between adjacent resonators to get the performance you
want.

Cheers,
Tom