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

On Sun, 14 Nov 2010 19:59:44 -0600, "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


Hi Mike,

Try thinking backwards. How much current through your headphones/ear
pieces do you need to hear the signal you want? (This is assuming you
are not amplifying the signal, which if that's the case, what's the
point in building a xtal set?).

What is the impedance of your headphones/ear pieces? They will
dictate the "loaded Q" of your tank circuit.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sun, 14 Nov 2010 19:59:44 -0600, "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


Hi Mike,

Try thinking backwards. How much current through your headphones/ear
pieces do you need to hear the signal you want? (This is assuming you
are not amplifying the signal, which if that's the case, what's the
point in building a xtal set?).

What is the impedance of your headphones/ear pieces? They will
dictate the "loaded Q" of your tank circuit.

73's
Richard Clark, KB7QHC
Hi Richard,
It all affects the loaded Q, the antenna, the diode, the audio matching
transformer and headphones.
I think the ideal is to impart maximum power to the tank circuit from the
antenna, now you have an
AC power supply with a known source impedance. Then pick your diode and
transformer match
your headphone for max power out...
I'm going through a mental exercise to build the most sensitive crystal
radio I can.
The best scenario would be to build a headphone with about a megaohm of
impedance
and eliminate the matching transformer losses. Here's one form that could
be modified.
http://www.hpfriedrichs.com/images-votc/gallows.jpg
If it's not self explanatory, you connect a stethoscope type headphone to
the tube that sticks out.
So to rephrase my question, how do I raise the impedance of an antenna to a
very high impedance
with minimum losses?
MikeK

On Sun, 14 Nov 2010 23:58:47 -0600, "amdx" wrote:

http://www.hpfriedrichs.com/images-votc/gallows.jpg
If it's not self explanatory, you connect a stethoscope type headphone to
the tube that sticks out.

Not what I would call a megOhm Z candidate in any form as this is the
model for the commonplace 600 Ohm (telephone standard) to 1KOhm
headphone of antiquity. It uses a coil. You should be looking at
piezo.

So to rephrase my question, how do I raise the impedance of an antenna to a
very high impedance
with minimum losses?
MikeK

You could shorten the antenna to an inch or two for AM, but then it
would be shorter than your tank coil, and hence your tank coil should
be the antenna.

This was a toy commonly found in the late 50s when I got it for
Christmas. It was shaped like a Buck Rogers space ship with a ball
handled plunger as the nose cone that adjusted the ferrite in the core
of the coil (to be used as a tuner). Of course the longest wire was
the earphone lead (all of a foot or 18 inches which would still
constitute an extremenly high Z antenna that you seek).

I lived outside of San Francisco at the time, and AM stations were
unknown to it. However, I could get some Marine traffic like the
Coast Guard (I had a view of the Ocean from the hills around Pacific
Manor, now Pacifica).

Very uninspired performance to say the least.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sun, 14 Nov 2010 23:58:47 -0600, "amdx" wrote:

http://www.hpfriedrichs.com/images-votc/gallows.jpg
If it's not self explanatory, you connect a stethoscope type headphone to
the tube that sticks out.

Not what I would call a megOhm Z candidate in any form as this is the
model for the commonplace 600 Ohm (telephone standard) to 1KOhm
headphone of antiquity. It uses a coil. You should be looking at
piezo.

Piezo is a candidate, but also on the table is a C shape laminated core
with a
small gap. The gap will be large enough that a small magnet will fit in it.
The magnet
is connected to a lever and on the other end is a diaphragm. The core will
be large
enough to hold thousands of turns. Possibly even a E I core with the a gap
milled in
the center leg for the magnet/lever assembly. A coil can be put on each of
the outer
legs. Not sure, but I think the center leg should be cone shaped to focus
the magnetic
field at the magnet. The magnet/lever assembly should have low mass.The
coils will
have taps for a va
Anyway that is the idea I'm working with at the moment.
Regarding piezos, I envision multiple piezo with a switching arrangement to
alter the impedance to match the impedance of the radio.


So to rephrase my question, how do I raise the impedance of an antenna to
a
very high impedance
with minimum losses?
MikeK

You could shorten the antenna to an inch or two for AM, but then it
would be shorter than your tank coil, and hence your tank coil should
be the antenna.

Trying to maximum signal for contest situations want a longer antenna.
Thanks, MikeK

73's
Richard Clark, KB7QHC

On Mon, 15 Nov 2010 09:17:30 -0600, "amdx" wrote:

C shape laminated core
with a
small gap. The gap will be large enough that a small magnet will fit in it.
The magnet
is connected to a lever and on the other end is a diaphragm.

This doesn't sound like a Crystal radio project at all. You describe
nothing that comes close to even 1KOhm, much less 1MOhm in load. Your
descriptions all use appeals to sensitivity, not impedance.

Trying to maximum signal for contest situations want a longer antenna.

This confounds your desire for higher Z. In the extreme (antenna of
several wavelengths and necessarily close to ground) will be less than
1KOhm. In the mid-range, could be hi-Z IFF it is a halfwave long. In
the conventional lengths, some may pose a moderately hi-Z (maybe
KOhms). None will exhibit the Z you anticipate for your Tank.

As I said, start thinking backwards from the power delivered to your
ear. Can you express that as a number? Not much point in the rest of
this if you cannot.

73's
Richard Clark, KB7QHC

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
PA3DJS
www.tetech.nl
without abc, PM will reach me very likely

"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
PA3DJS
www.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. ??
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

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

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