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

"Richard Clark" wrote in message
...
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

Hi Richard,
I have probably confused things, I have 4 or 5 threads running at this
time.

The C core EI core thing is a starting attempt to build a speaker with a
high impedance
to eliminate the losses of a matching transformer. The whole excercise is to
build a crystal
radio that will eack out the most sensitivity.

1 picowatt to the earphone is a good number.

For sensitivity the starting point has got to be the tank circuit, you
want to build
an inductor with very high Q and then mate that to a good quality capacitor.
A Q of 1000 is possible over much of the AM BCB.
Can we agree on that?

Now you need to couple in energy from an antenna. If this is adjusted for
maximum power transfer, we have reduced the Q by 1/2 or Q=500.
Assuming a 240uh inductor and frequency of 1 Mhz the XL is 1507 ohms,
multiply that by the antenna loaded tank Q of 500 and we have an Rp =
753,500 ohms.
Does that work for you?

I think I found a good site for the antenna matching;
http://www.crystal-radio.eu/enantunittest1.htm


Now we need to detect and tranfer the signal to a transducer

I'll stop here till I get some feedback, I don't know my question anymore? \
:-)
Mike.

amdx wrote:


The C core EI core thing is a starting attempt to build a speaker with a
high impedance
to eliminate the losses of a matching transformer. The whole excercise is
to build a crystal
radio that will eack out the most sensitivity.

1 picowatt to the earphone is a good number.

You may want to make sure this enough to be audible. An iPod may output 30
milliwatts which is probably more than you would ever need/get. 1 picowatt
to a headphone may not be sufficient.


For sensitivity the starting point has got to be the tank circuit, you
want to build
an inductor with very high Q and then mate that to a good quality
capacitor. A Q of 1000 is possible over much of the AM BCB.
Can we agree on that?

Have you considered what Q=1000 does to the bandwidth of the received
signal?



Now you need to couple in energy from an antenna. If this is adjusted for
maximum power transfer, we have reduced the Q by 1/2 or Q=500.
Assuming a 240uh inductor and frequency of 1 Mhz the XL is 1507 ohms,
multiply that by the antenna loaded tank Q of 500 and we have an Rp =
753,500 ohms.
Does that work for you?

Won't both the antenna and the load serve to reduce the Q? You need to be
looking at how power is transferred from the antenna to the earphone. Trying
to optimize one part at a time may not yield the best result.



I think I found a good site for the antenna matching;
http://www.crystal-radio.eu/enantunittest1.htm


Now we need to detect and tranfer the signal to a transducer

I'll stop here till I get some feedback, I don't know my question anymore?
\
:-)
Mike.

Hello Mike,


On 15 nov, 22:54, "amdx" wrote:
"Richard Clark" wrote in message

...



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

* Hi Richard,
*I have probably confused things, I have 4 or 5 threads running at this
time.

*The C core EI core thing is a starting attempt to build a speaker with a
high impedance
to eliminate the losses of a matching transformer. The whole excercise is to
build a crystal
radio that will eack out the most sensitivity.

*1 picowatt to the earphone is a good number.

* For sensitivity the starting point has got to be the tank circuit, you
want to build
an inductor with very high Q and then mate that to a good quality capacitor.
A Q of 1000 is possible over much of the AM BCB.
* Can we agree on that?

Now you need to couple in energy from an antenna. If this is adjusted for
maximum power transfer, we have reduced the Q by 1/2 or Q=500.
Assuming a 240uh inductor and frequency of 1 Mhz the XL is 1507 ohms,
multiply that by the antenna loaded tank Q of 500 and we have an Rp =
753,500 ohms.
*Does that work for you?

I think I found a good site for the antenna matching;http://www.crystal-radio.eu/enantunittest1.htm

Now we need to detect and tranfer the signal to a transducer

The transducer will be the difficult part (the antenna is quite simple
compared to this). As you may know, below 150mVRFpk across the
junction, rectified output voltage drops quadratically with RF input
voltage, hence detection efficiency.

So to get maximum voltage across the junction, you need a high
impedance rectifier (that means low "Is"). Disadvantage of this is
you need a transducer with same (very high) impedance (as I assume
you don't want to use electronic LF amplification). I did my
experiments with LF electronic amplification.

If you can't find / make one in the several hundred kOhms range, you
will probably need to use rectifiers with higher Is. If so, you also
need to transform the RF impedance to a lower value to get best RF
power transfer to the rectifier.

Regarding diodes, years ago I did experiments around 7 MHz with tuned
detectors where the diode capacitance is no longer small with respect
to the tuning capacitance.

When using hybrid schottky rectifiers (that are the ones with relative
high reverse blocking voltage), strange hysteresis effects occurred in
the DCout versus RFinput voltage curve. When using real schottky
rectifiers (like BAT15, 14, etc), this effect wasn't present. I didn't
document it (only some notes), so I can't share the full details with
you.


I'll stop here till I get some feedback, I don't know my question anymore? \
:-)
* * * * * * * * * * * * * * * * * * *Mike.


Good luck finding/designing the best transducer,


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