"Richard Clark" wrote in message
...
On Tue, 16 Nov 2010 19:19:09 -0600, "amdx" wrote:
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. :-)
Hi Mike,
Unfortunately your response is not an answer. Your response suffers
equally for the proposed additional transformer - and you have added
loss for no net forward movement to the solution.
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 first blush I have to ask, "How many source resistances are there
to be found for a Xtal radio?" I am not under the impression you have
much flexibility in that regard.
Every time you change frequency you change source resistance.
XL times Q equals source resistance, Q changes with frequency,
and XL changes with frequency.
Also when you change to a different station with a different signal strength
you change the current through the diode, this changes the resistance the
diode
presents to the tank.
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).
25 million WHAT?
I don't know what that is derived from.
For minimum detector power loss at
very low signal levels with a particular diode,
If it were a "particular" diode, it would seem giving it a part number
would lend authenticity to this report.
Well, the rest reads about like an oath to motherhood and apple pie.
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.
I am less than whelmed.
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
Frankly, it looks like spaghetti math. If it suits you, then there is
a number that will supply the optimal solution - that is the nature of
math, after all. However, too much of this looks circular, especially
when I see complexity piled on top of loss just to return to the same
problem.
However, what it all boils down to is that you have to live with
whatever diode you select, and you shift the tap for the best
performance (not many variables left in the game of Xtal radios, is
there?).
There are dozens of diodes to pick from, depends on the tank Z
and signal strength. I don't know what you mean when you say there aren't
many variables, there are many, many!
Coil Type (solenoid, spider, 3D solenoid)
Coil Form Material, paper, PVC, polypropylene, etc.
Wire type, solid, Litz
Antenna Match technique, Tap, cap tune, inductance coupling
Diodes, Diodes, Diodes
Audio, Transformer matching , high impedance piezo, sound powered
headphones, crystal earphone, horn driver.
Here's some more info on the diode selection and why.
Richard I'm well aware of your great knowledge (not being facetious)
and my lack of math and knowledge on this, but I'm to the point where
it seems you are not willing to learn new information. It seems you already
know it all. This especially in regards to the idea that the diodes
characterists
are very important to losses in the radio.
I think one of these pages has a graph showing a diode not matched to the
current available has a 33db loss.
Here's more diode info.
http://www.crystal-radio.eu/endiodes.htm
http://www.bentongue.com/xtalset/17Is_n/17Is_n.html
http://www.klimaco.com/HAMRADIOPAGES/xtal_how_to.htm
I do appreciate your time and the discusion, I did learn from it.
I'm going to take a few days off.
Sincerely, MikeK
73's
Richard Clark, KB7QHC