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1.5 megaohms

Am I somehow wrongheaded or am I missing messages from the thread??? I
reread the whole thread and I think I didn't see anyone else
addressing the impossible absolute impedance value. And you just don't
transform over a 100:1 or more impedance ratio with impunity using
passive devices only.

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On Nov 17, 11:42*pm, FCC per Anna Scarpetta wrote:
1.5 megaohms

Am I somehow wrongheaded or am I missing messages from the thread??? I
reread the whole thread and I think I didn't see anyone else
addressing the impossible absolute impedance value. And you just don't
transform over a 100:1 or more impedance ratio with impunity using
passive devices only.

Well, NOW I do see messages on that.
AND I see I am somehow logged in with a totally dufus name, to
boot. :-(

Filippo / spamhog / N1JPR

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"FCC per Anna Scarpetta" wrote in message
...
On Nov 15, 1:59 am, "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.

I did take part in the discussion of matching, but I am afraid you are
off by several orders of magnitude regarding the impedance value.

Hi FCC,
You have something wrong in your thinking, but tonight I'm not willing go
to far in
the description, Maybe tomorrow. We will use 240uh inductor and about 107pf
cap.
and assume a combined Q for the tank of 1000. and a parallel resonant
circuit.
The formula is 2 x pi x freq x L x Q. So at 1Mhz we have,
2 .x 3.14 x 1.000.000 x .00024 x 1000 = 1,507,200 ohms.
Look here http://www.crystal-radio.eu/engev.htm
There are more involved formulas, but this is close.
MikeK

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On Wed, 17 Nov 2010 14:09:11 -0600, "amdx" wrote:

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?

Ok One more, with a little help I figured it out.
It has to to with the Thermal Voltage of the diode.
Vt=KT/q
with k=1.38E-23 and q=1.6E-19. T is absolute temperature in degrees Kelvin,
k is Boltzmann's constant and q is the charge of an electron. VT is close
to 0.025 volts at 20 degrees Celsius.

With a slightly increased temperature the .025 is raised to .0257,
and .0257 / 1 nanoamp =25,700,000
Thanks, MikeK


Hi Mike,

So, for the complete expression:
25,700,000*n/Is Ohms (current in nA).
you take the same current and divide it out to yield
0.025*n
where n, as I observe (but fail to see the significance of) is a
multiplier of nearly 1.

Putting this back into the complete expression in the complete
statement:
At low signal levels the RF input
resistance and audio output resistance of a detector diode are equal to
0.025 Ohms (current in nA).
or with temperature change:
0.0257 Ohms (current in nA).

Sounds fairly trivial when we are talking about tenths of milliOhms
per degree facing into source Z of a million times that, and feeding a
load resistance that is at least a thousand times larger.

This temperature dependency, too, is something I worked with 30 odd
years ago. The temperature characteristic has been around as long as
the solid state diode. I used it specifically for measuring
temperature, as does every inexpensive electronic thermometer. My
design used a constant current LED (to indicate a complete circuit)
and a common diode in series, with that diode placed at the point of
interest where temperature was a production flow variable. The
voltage across that diode, minus an offset for Kelvin, was a linear
indication of temperature, usually accurate to within 1 degree C, if
not slightly better. The coefficient is roughly 2mV/degree C.

The one oddity I find with the original material you cite is that it
specifies Is which is a reverse current, and the temperature
dependency is for forward voltage characteristic curve.

Such things make me question the authenticity of these sources.

73's
Richard Clark, KB7QHC

Matching antenna to crystal radio
 

On Tue, 16 Nov 2010 12:47:02 -0800, Jim wrote:

Here is some more info for you guys to chew on...

http://www.midnightscience.com/downl...es/anatomy.pdf

OK Jim,

I gave it a look.

I have to wonder from the start, and in the context of Xtal sets that
are usually employed for BCB, where the author decided that a typical
antenna exhibited 50 Ohms resistance and 40pF capacitance.

That was the first of a number of similarly random values assigned to
components that has me drawing up cold. From that, I have to wonder
what the value is in the practice of transforming those values.

73's
Richard Clark, KB7QHC

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On 11/18/2010 8:39 AM, Richard Clark wrote:
On Tue, 16 Nov 2010 12:47:02 -0800, wrote:

Here is some more info for you guys to chew on...

http://www.midnightscience.com/downl...es/anatomy.pdf

OK Jim,

I gave it a look.

I have to wonder from the start, and in the context of Xtal sets that
are usually employed for BCB, where the author decided that a typical
antenna exhibited 50 Ohms resistance and 40pF capacitance.

That was the first of a number of similarly random values assigned to
components that has me drawing up cold. From that, I have to wonder
what the value is in the practice of transforming those values.

73's
Richard Clark, KB7QHC

I think that you need to ask the author. I just wanted to add some other
opinions.

I would say that he used some ideal values as a place to start with.
Kind of a 'what if.'

-Jim

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On Nov 18, 2:45*pm, Jim wrote:
On 11/18/2010 8:39 AM, Richard Clark wrote:





On Tue, 16 Nov 2010 12:47:02 -0800, *wrote:

Here is some more info for you guys to chew on...

http://www.midnightscience.com/downl...es/anatomy.pdf

OK Jim,

I gave it a look.

I have to wonder from the start, and in the context of Xtal sets that
are usually employed for BCB, where the author decided that a typical
antenna exhibited 50 Ohms resistance and 40pF capacitance.

That was the first of a number of similarly random values assigned to
components that has me drawing up cold. *From that, I have to wonder
what the value is in the practice of transforming those values.

73's
Richard Clark, KB7QHC

I think that you need to ask the author. I just wanted to add some other
opinions.

I would say that he used some ideal values as a place to start with.
Kind of a 'what if.'

* * -Jim- Hide quoted text -

- Show quoted text -

When I was a kid I used to build crystal receivers that had a sliding
tap on the coil titerally wound on a toilet paper tube and adjusting
a variable cap. every station you received had a "best " posotion for
the cap and tap and this also seemed to change from day to day or even
minute to minute. This was my first lesson in the interaction between
impedance and resonance.

Jimmie

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On Thu, 18 Nov 2010 10:45:57 -0800, Jim wrote:

I think that you need to ask the author.

Hi Jim,

There are too many indifferent scribblers to merit reading them, much
less entering into a dialog when they already demonstrate a lack of
communication skill. I tried that for years with Art.

Having said that, your suggested link did reveal someone who looked at
the problem carefully and laid out the steps taken to achieve a
result. What was missing was motivation, and a connection to reality
(not being smart-assed, just wondering about the extra components
decorating the schematic is all).

I would say that he used some ideal values as a place to start with.
Kind of a 'what if.'

The "What if" is demonstrated, and it certainly gives scope. However,
ideal circuit values are far from apparent. It is just as easy to use
values any Xtal set owner might find exhibited by his window screen,
as contrasted starting with a full size BCB antenna with an odd
reactive component tossed in for spice (excuse the pun, but it works
at all levels). In other words, it takes absolutely no more effort to
model what one can reasonably expect to find at home, than to go to a
broadcast station and borrow their sky hook (and then antagonize the
Field Engineer by tossing in a haphazard capacitor).

73's
Richard Clark, KB7QHC

Matching antenna to crystal radio
 

Richard Clark wrote:

On Thu, 18 Nov 2010 10:45:57 -0800, Jim wrote:

I think that you need to ask the author.

Hi Jim,

There are too many indifferent scribblers to merit reading them, much
less entering into a dialog when they already demonstrate a lack of
communication skill. I tried that for years with Art.

How do you know this author would not be interested in your comments? He has
presented much more than Art has in the past 10 years.


Having said that, your suggested link did reveal someone who looked at
the problem carefully and laid out the steps taken to achieve a
result. What was missing was motivation, and a connection to reality
(not being smart-assed, just wondering about the extra components
decorating the schematic is all).

Which components did you determine to be 'extra' and why?



I would say that he used some ideal values as a place to start with.
Kind of a 'what if.'

The "What if" is demonstrated, and it certainly gives scope. However,
ideal circuit values are far from apparent. It is just as easy to use
values any Xtal set owner might find exhibited by his window screen,
as contrasted starting with a full size BCB antenna with an odd
reactive component tossed in for spice (excuse the pun, but it works
at all levels).

So, what values would you suggest?

In other words, it takes absolutely no more effort to
model what one can reasonably expect to find at home, than to go to a
broadcast station and borrow their sky hook (and then antagonize the
Field Engineer by tossing in a haphazard capacitor).

73's
Richard Clark, KB7QHC

You talk like the pdf had significant faults, but your reply is so vague
that you don't really add anything. Please share you understanding so that
others may learn.

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On Fri, 19 Nov 2010 07:43:35 -0600, joe wrote:

Which components did you determine to be 'extra' and why?

Hi Joe,

To enumerate:
Ra, 50 Ohms
Ca, 40pF
R1, 308 KOhms
D1, a selection criteria of 200nA Ir
Co, 200pF
and
Ro, 154 KOhms

The question why? you apply to me is as easily asked of the author
because he says nothing on their choice, which give every appearance
of being capricious.

Let's look at the last first, in the audio output we have a high
frequency cutoff knee of 32 KHz, why? I can think of nothing to
justify that selection that is 10 times outside of the passband.

We have an audio load of 154 KOhms, why? I can well imagine this
being a piezo headset, but is it an optimal load (it would appear that
the diode needs a heavier current draw than that). It is not a
standard resistor value, so there must be some motivation for this
value - but that is left to our imagination.

D1, a selection criteria of 200nA Ir, why? Is this a good selection,
or a bad one? Again, lacking the information of motivation....

R1 appears to be inserted on the basis of an anticipated Q, a topic
that is wholly absent from the computations and discussion! Why? This
is the component I thought of as being "extra."

Ca and Ra have already been decried, and the Ca placement looks
suspiciously on the wrong side of Ra.

So, what values would you suggest?

I cannot imagine trying to figure out the agenda for the author. The
piece is wholly unmotivated beyond being an etude of computation.

The values for Ra and XCa are infinite in possibilities. For the
average BCBer with limited antenna options, Ra would typically be low,
maybe an Ohm at the very highest (and probably much less). XCa would
be high, maybe a KOhm (but not suspiciously high like the current 7
KOhms). As you can see, the differences from the original are
considerable.

You talk like the pdf had significant faults, but your reply is so vague
that you don't really add anything. Please share you understanding so that
others may learn.

Please reread my comments for praise where praise was due. If the
rest sounded vague, it was entirely due to the vague material offered.
Consider that also.

Further, asking me why don't I do ______ (fill in the blank), I have
no interest in pushing that rock up the hill - but thanx for asking.

I am far more interested in the detector side of this, but the Xtal
radio brotherhood approach this like Penitentes continuously flogging
themselves in order to attain a religious high.

I would choose a quasi-digital solution with a shift ring register
commutation style of detection. But that means I need a battery
(ANATHEMA!!! I hear the cowled acolytes sputtering). If I use a
battery I could as easily, sinfully listen to a transistor radio - and
my MP3 player already suits my needs with its built in radio function.

40 years ago I worked on BaseBand sets and designed with synchronous
detectors. This is all very interesting to me from my former
devotion, but this XTAL splinter is rather provincial.

73's
Richard Clark, KB7QHC