In article ,
says...
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.
If don't think that's what I want to do, tell me that too. :-)
MikeK



Put an alligator clip on the antenna lead and run it down the tank to
find max volume, then clip it on.

"J. Todd" wrote in message
...
In article ,
says...
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.
If don't think that's what I want to do, tell me that too. :-)
MikeK



Put an alligator clip on the antenna lead and run it down the tank to
find max volume, then clip it on.
Use a fork to scoop water out of a glass to satisfy your thirst.
I will work, just not well.
MikeK

On Mon, 15 Nov 2010 09:59:06 -0600, "amdx" wrote:

Put an alligator clip on the antenna lead and run it down the tank to
find max volume, then clip it on.
Use a fork to scoop water out of a glass to satisfy your thirst.
I will work, just not well.

Hi Mike,

Being facetious in the face of an answer that responds exactly to
what you want is not very smart.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Mon, 15 Nov 2010 09:59:06 -0600, "amdx" wrote:

Put an alligator clip on the antenna lead and run it down the tank to
find max volume, then clip it on.
Use a fork to scoop water out of a glass to satisfy your thirst.
I will work, just not well.

Hi Mike,

Being facetious in the face of an answer that responds exactly to
what you want is not very smart.

73's
Richard Clark, KB7QHC

Richard you are right, I read it wrong.
I thought he was just being smart because it's a crystal radio and he wanted
me
to just grab a alligator clip and connect the antenna ( I missed the taps
part)
I apologize J.Todd.
Connecting the antenna to taps on the coil is common, I don't believe this
method transfers maximum energy into the tank circuit, But I am open to more
info about that.
Also I expect any high performance crystal radio to be made with
litz wire, I suppose taps could be made to litz but I have looked at a lot
of
radios and don't recall ever seeing taps on a litz wire coil.
J.Todd I apologize for my smart alec remark.
MikeK

"amdx" wrote in
:

....
Connecting the antenna to taps on the coil is common, I don't believe
this
method transfers maximum energy into the tank circuit, But I am open
to more info about that.

Well, you have to move beyond "believing" and do some analysis.

Broadly, adjusting the tap point, and retuning for maximum output is
finding the point of maximum power transfer for the given components and
source impedance.

I am quite skeptical (that is like "don't belive") that your scheme for
headphones is going to deliver what you suggest.

When I was a kid, these things here ALL used a crystal (or piezo)
earpiece, which has an impedance that looks like extreme R and a very
small shunt C, in all a very high impedance at audio frequencies of
interest.

Look at http://www.jaycar.com.au/productView.asp?ID=AS3305
&keywords=earpiece&form=KEYWORD for an example. (You probably will need
to fix the split URL. They also have a simple crystal radio kit in the
catalogue, no doubt suppliers in your country do also.

Traditionally, these were used with improvised smallish antennas, eg bed
iron, and that brought in too many stations. To obtain benefit from
enhanced sensitivity, you might need to be in a very quiet location, and
have extreme selectivity... both are practical issues.

Owen

*Use a fork to scoop water out of a glass to satisfy your thirst.
I will work, just not well.
* * * * * * * * MikeK


True, but operating crystal radios is much like sailing.

Maybe multiple taps is not that bad an idea.

- I never saw a crystal radio optimized for such a small tuning ration
that the LC had a fixed impedance. A 1-to-3 tuning ratio is quite
normal, and tank impedance will therefore vary considerably.

- On medium wave the impedance of the electrically very short wire
antenna will vary, but not that much. It will stay probably in the
order of a kiloohm or more. On shortwave it will swing considerably.

- If the receiver has to resolve different stations or at least
clearly separate different SW broadcast bands (to listen to the most
powerful broadcaster in each band, more is not doable), it may make
sense to reduce the loading below optimum energy transfer at the
resonant frequency.

This suggests that a coil with multiple taps could be useful. If the
radio has a tapped coil and 3 croc clips (one each for the tuning cap,
antenna, and detector) the user has to optimize over 3 dimensions in,
and 2 dimensions out - wanted signal level and interference.

Great fun, but not easy.

The human ear is not linear and tends to be a bad level measuring
instrument. It helps if the radio also has a microamp-meter to make
relative linear comparisons in total signal level. A 50uA meter may
have a DC resistance of 2kohm and load about the same as a magnetic
headset, but a 500uA may be sensitive enough and load less.

The combination of separate improvements that the ear alone could not
judge may in the end be considerable.

A digital voltmeter would be so high impedance as to be practically
invisible even at very high impedances too, but it has a battery
powered DC amplifier. IMO it is aesthetically incongruent with a RF-
powered radio.

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.

Say you shoot for resonance at 1 MHz: a 2.5 mH inductor (already
impractically large) and a 10 pf capacitor (already impossibly small)
will resonate close to 1 MHz but have an impedance of 16 kilohm or so,
100 times lower than your assumption.

For 10 MHz, it would be 1 pF and 250uH, again for 16 kohm.

In practical circuits the impedance is typically in the order of
300-2000 ohm.

A typical old-style mediumwave tuning capacitor has a maximum
capacitance of 360 pF to tune about 520 kHz at 850 ohm with a 260 uH
coil.

Assuming you use a much smaller than usual 100pF variable, say 130 pF
including tray capacitance in the coil, at the bottom of the band you
will need 760hH, and get 2.5 kiloohm.

On shortwave, let's say yo want to tune a lowest frequency of 6MHz
with only 60pF all included - again, nobody does that - you need 12uH
inductance but only get 450 ohm impedance.

"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

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.

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