"Richard Clark" wrote in message
...
On Tue, 16 Nov 2010 14:38:12 -0600, "amdx" wrote:

First,
I'll rephrase what I think you meant to say, I am surprised about YOU
bemoaning the source material transformation loss of using a transformer
to connect their speaker to the Tank.

Why?

I was trying to make sure I understood what you said, that's all.


Even I worded that poorly, but are you suggesting I should not work for
1.5db?

Why start with a 1.5dB deficit when the Tank is already there to do
the work of matching at no loss?

You have the diode to drive before the audio section and you want to match
the
diodes impedance. Tapping down will allow you to match diode impedance, but
I'm not sure tapped down is where you will find the best diode efficiency.
Your response to my "not sure " is anticipated. :-)


Second,
Tapping down on the tank coil may not work as well as first thought.

You are going to have to explain that better, because what follows
doesn't.


You still have diode characteristics to overcome, and as you tap down the
voltage also decreases.

"Here's a quote from Ben Tongue's webpage;
Many times the question is asked, "What is the best diode to use?" The
answer depends on the specific RF source resistance and audio load impedance
of the Crystal Set in question. At low signal levels the RF input
resistance and audio output resistance of a detector diode are equal to
25,700,000*n/Is Ohms (current in nA). For minimum detector power loss at
very low signal levels with a particular diode, all one has to do is
impedance match the RF source resistance to the diode and impedance match
the diodes' audio output resistance to the headphones by using an
appropriate audio transformer. The lower the Is of the diode, the higher
will be the weak signal sensitivity (volume) from the Crystal Set, provided
it is properly impedance matched to it's circuit (see article #1). This
does not affect strong signal volume. There is one caveat to this, however.
It is assumed that the RF tuned circuits and audio transformer losses don't
change. This can be hard to accomplish. It is assumed that the Rs, diode
junction capacitance, n and reverse leakage are reasonable. If the diode
you want to use has a higher Is than the optimum value, tap it down on the
tuned circuit. If the diode you want to use has a lower Is than the optimum
value, change the tank circuit to one with a higher L and lower C so that
the antenna impedance can be transformed to a higher value and repeat step
#1. "

And yes I note the "tap it down" which is used to match the impedance of the
diode.

Changed voltage / changed current = changed Z

That's why the Tank is also called a transformer. You could displace
that function into a secondary, lossy transformer, but the same thing
will happen insofar as your last complaint. In other words, you can
add more loss and get the same grief.

I"ll add the contest guru's that I have noted use high
impedance taps to the detector. Although depending on signal strength they
switch in diodes that better match the current the radio signal is
delivering.

Do they offer a case for fumbling through a selection of diodes? At
first blush, the best is going to be the best - hands down.

See above.
And yes they sometimes multiple diodes on there radio.

Still hyave concern about diode characteristics.

Name one characteristic that presents a concern.

Saturation current and
axis-crossing resistance equal to Rr
Sorry that's two, but their related.

See Ben's Page http://www.bentongue.com/xtalset/4opd_xfr/4opd_xfr.html

Thanks, MikeK


73's
Richard Clark, KB7QHC