"Richard Clark" wrote in message
...
On Wed, 17 Nov 2010 09:17:51 -0600, "amdx" wrote:

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.

Hi Mike,

There are a number of problems here. Q is strictly a function of
RESISTANCE and nothing else.

Ya, loss resistance of the tank.
Can you agree that it varies with frequency?


What you call source resistance, when using the math you provide, is
actually source impedance. Impedance embraces resistance and is
solely resistance when reactance is gone - such as at resonance.

The tank is used at resonance, so yes, source resistance.

However, it would seem that the Xtal Radio community treats reactance
as resistance interchangeably.

Not so!


That is fine to a point, but it introduces serious problems in the
usage of the term "match" as there are actually several types of
"matching" and each has its peculiarity (only peculiar when you are
unfamiliar with the other types).

Are you getting at conjugate match?

It is a given that you are going to have to change either Xc or Xl if
you are going to tune to another frequency. This alters the circuit
Z, certainly; and for the AM band you have a 3:1 variation from band
end-to-end.

So your in agreement that Rp is not fixed?

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.

Without numbers, this is like anticipating worry about your MPG
driving up a hill in comparison to driving down the same hill. Yes, a
wild variation, but do you give up cars and start walking instead?
Some of these "concerns" sound like they come from the AVC needy. I
thought Xtal Radio folks would man up and get with the program.

No pick a different diode.

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.

No, actually you've either mis-read the intent (which is certainly a
problem with that author's hodge-podge writing style) or you've read
too many written sources that each only see part of the elephant.
"It's a snake! We need something bulky."
"It's a wall! We need something slim."
"It's thin and flappy! We need something sturdy."

The problem with the diode is in its forward current and reverse
current. If they are about the same, you don't have a rectifier. This
would be a weak signal problem. The forward current defines the
series resistance of the diode and how much that is in comparison to
the load defines the efficiency. You want more current to lower the
diode resistance to increase the efficiency. Unfortunately this
demands you have a higher source voltage that offers less current
(Catch-22).

However, this problem is strictly for a low Z load like a magnetic
speaker - and thus enters the consternation over competing agendas
that gives everyone the fits that seems they enjoy agonizing over when
the solution is obvious. I will touch on that next:

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.

Those are non-starters for those who love to struggle in hope against
reality.

Wire type, solid, Litz

Ah yes, the magic Litz wire that is the salvation. The solution for
the problem that doesn't matter. You do this because you can, not
because you need to. And then you return to life with the car still
in the ditch (but you changed the air in the tires).

Can you wind a coil with a Q over 1000 with solid wire?

Antenna Match technique, Tap, cap tune, inductance coupling

This has been long figured out. For nearly a century.

Diodes, Diodes, Diodes

Whose solution is found in:

Audio, Transformer matching , high impedance piezo, sound powered
headphones, crystal earphone, horn driver.

Whose choice is obvious in hi impedance.

Like I said, there are not that many variables, until you find someone
on a soap box with a stump speech about the invidious dereliction of
design introduced with the Philips head screw. The advantages of the
Bristol head anyone?

Do not confuse the multitude of competing and contradicting factors as
the multitude of variables.

Introduce figure of merit to the discussion and argue about that. If
that gets resolved, the solution will pop right out.

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.

There is no new information. It is a niche application you are
talking about, not new science. The niche has its concerns and they
are impacted by issues that are rarely a problem for the standard
usage of the components you mention, but there have been no new
devices introduced in this discussion. I see nothing innovative. In
fact, the discussion is so in-bred it lacks coverage outside of a
clubbish mainstream. I will offer something new below that is
actually quite old.

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 have designed at the extreme edges of component characteristics,
true. More often in regard to temperature variation, mechanical
vibration, noise, accuracy, resolution, speed, bandwidth, linearity -
all that went towards continuous service in a largish orange box in
some jets.

I have studied diodes to no end, especially LEDs and photodetectors.
Talk about marginal signal detection and loss. I started with 1n34s
being used as varactors back in the late 60s. PIN and Tunnel diodes
followed. There is a world of non-linear devices out there. I
especially like the surgistor and posistor. Any reports of their use
in Xtal sets?

I have maintained a number of audio standards and measurement devices.
I have had to repair detectors that measured absolute value voltage
out to 7 places, the same issues arose there too. Small currents?
Like those that rise continuously through the air in femto-ampere
levels? You would need polonium treated probes to measure them (or
just use the anti-static record brushes of the 1970s).

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

This stuff merely shows the agony of counter trending data. In other
words select the optimal Is and you get trash Rd - and vice versa.
Reporting the numbers doesn't solve the problem however.

* * * * * * * *

In this offered discussion there is nothing of how the pioneers
managed it with galena crystals and cat whiskers. That data for
comparison is starkly absent in this and their discussion.

My Fourth Edition of "Standard Handbook for Electrical Engineers"
(1915) offers Coherers and Magnetic detectors. The coverage is
likewise absent of these alternatives.

"Section 21-289 Contact Rectifiers. The rectifiers most frequently
consist of a contact between a fine wire and some variety of mineral.
Among the minerals frequently use are iron pyrites, galena, silicon,
and molybdenite. In other forms, two crystals are used in contact,
such as zinkite with chaleopyryte or bornite, or silicon with metallic
arsenic.
"For detecting the direct-current pulses, head telephones of from
1,000 to 3,000 Ohms resistance are ordinarily used."

Returning to the obscure references, for magnetic detectors, consider:
http://en.wikipedia.org/wiki/Magnetic_detector
for the coherer, consider
http://en.wikipedia.org/wiki/Coherer

* * * * * * *

You might want to reflect that this thread has not been about matching
an antenna to a crystal radio at all. That topic was rather easily
and quickly answered by J. Todd. Loss is defined by wire resistance
and the proximity of objects to the tank.

The match from the Tank is similarly simple - once you define the
load. Loss is defined by wire resistance and the proximity of objects
to the tank.

The load is presented as its two native components: the detector and
the headset.

The detector is the variable of concern, it is defined by the current
necessary to drive the headset, everything surrounding the detector
has to conform to its choice. The loss is defined by the ratio of the
detector resistance to the load resistance plus that diode resistance.

The headset is defined by a power level you identified as one picowatt
delivered to the ear. I think we can agree that was very generous.
The loss is defined by the transducer efficiency.

This is the genesis of figure of merit, and that is your solution.

73's
Richard Clark, KB7QHC

I think you finally got it Richard.
Thanks, MikeK :-)