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.