On 15 nov, 16:23, "amdx" wrote:
"Wimpie" wrote in message

....
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.

Wim
PA3DJSwww.tetech.nl
without abc, PM will reach me very likely
*It seem a common method to couple the antenna is just a single series
variable capacitor. I don't know if this method couples maximum energy to
the radio. ??

Yes, with the correct value, it will.

*Normally we transform our antenna to 50 ohms, in this case we would like
to see 1 million ohms, I'm not sure that can be done, and if it can the
losses
may be higher than having a mismatch.
* * * * * * * * * * * Thanks, MikeK

Hello Mike,

Regarding the 1M Ohm impedance.
If you can make a tank circuit with 1MOhm impedance at resonance, you
can transform your antenna impedance to that value. You use your
resonant circuit as part of the impedance transformer. Even in case
of a ohmic 50 Ohms antenna impedance, you can transform this to 1 Mohm
(inductive or capacitive coupling). Maybe you have to combine the
coupling capacitor with a tap on the coil.

Regarding your detector.
In the detector circuit you transform a high impedance (around 1 MOhm)
to a lower impedance to find the sweet spot for your detector diode /
LF load combination.

If you have a simulator (for example SPICE type SW), you can model
your antenna as a voltage source in series with a capacitance (pF
range) and some resistance (1..100 Ohm range, depending on ground
provision).

By changing the coupling capacitance from the antenna to the top of
your resonant circuit, you can see what happens (in an .AC sweep).
You will see impedance transformation (that is more output voltage
than input voltage) and of course some detuning of your circuit
because of the additional capacitance.

If you want to do some hand calculation, search for: L impedance
transformation network.

Best regards,


Wim
PA3DJS
www.tetech.nl
please remove abc first in case of PM