Quote from: Localjoe on December 22, 2007, 04:47:35 AM

Hans i guess im more interested in the math or science behind what exactly makes the specific  crystal in a crystal radio optimum for that freq spread and what factors define it ...With that knowledge I would possibly pay a company to design ones to specs that would suit our needs or the freq spread i would like it to be reactive to meaning the uv or higher... So i guess thats what im hunting for.
                                                                                                                                              Joe

It doesn't Joe, the frequency is determined by the resonant circuit in front of it, the crystal is just a rectifier that blocks RF and lets LF through.

(http://home.arcor.de/radio-freak/images/schaltung.jpg)

The coil and the variable condenser here make up the resonant circuit, the variable condenser allows you to tune to a specific frequency. There are other ways of doing it like with a variable coil but essentially it's as simple as this.

Hans

Alright now im confused .. lf is low freq mf is medium and hf is high freq .. doesent rf encoumpas at least one or two of those bands>?

hansvonlieven, how does it block RF and let LF thru?

a galena crystal is a semiconductor or diode. It lets electricity flow only in one direction, so therefore blocks AC. RF is AC. The superimposed signal that is left is a DC component which passes through, hence the separation.

This is a rough and ready explanation but close enough to the mark.

Hans

@ Joe, think of the LF signal in terms of pulsed DC rather than AC

What do you want to filter?

Hans

Are you telling me you want to filter all oscillations out of some electric current?

If yes you need a full wave rectifier, a Galena crystal is only useful for very small currents.

Tell me exactly what you want to do with what.

Hans von Lieven

.
Hello , i hope i can help ,  with simple unprofessional words better than complicated "sounds" around this. /My Father have had an radiorepairshop , and i (7 years old) was learning by asking.
So it was very simple to learn (60 Years are gone now)
---------------------

AC is an changing  current that change the polarity from positiv  to negativ , cross the Zero(volt) , and go again to positiv . tge dioode , rectifier , semiconductor is like an valve than conduct in one direction (as an closed switch)
and in other direction its like an open switch.
- Only one polarity will PASS the Diode  ! -
(This is valid for any AC like Radio- Audio-or line-frequencies)

SO NOW, YOU HAVE UNDERSTAND the diode.

On the output from diode you have an "pulsed DC ,
that say: the DC is not proper and swap (like the input frequency, from Zero to the peak Voltage of the High-frequeny (RF) or the
Audio Frequency (AF) or even to the Line frequency 50 or 60 Hz = cps (cycles per second - that was used in US )

You will ONLY have an "proper" DC , IF YOU used an capacitor
connected from this output to ground.

(in Low Frequeny Application USE: MIKOFARADS range in RF (radiofrequeny) in Pico-farads.

OK - NOW YOU WILL USE an crystal (diode) Receiver to rectify the power (or Messages) from an AM Radio-Transmitter.
(Exampel: 1000Khz = 1 Mega-Hertz)

Antenna Coil will calibrate with (variable) condensor so
that its are in HARMONICS with the transmitter station , so
the L-C will have the maximum Voltage (high resistance) that
can have (only) an few microwatts.

But with the 1000Khz (1Mhz)you can do nothing. BUT after
recifying this . It have DC (positive Voltage)
-or negativ by turning the diode in against direction-

-you can mesure it, if it used 1000pf Condensor (as output) ,
 so you have an proper DC. - use this-in receiver- to have an better signal in any way!!!!

BUT this condensor it is SO small , that it will only
make an proper DC for the RF Signal .
The AUDIO Signal-from radio-station- that contains musik or Speach, will pass this condensor (with nears NO LOSSES) , and you can receive on 2  Kilo-Ohm earephone the AUDIO (Low Frequency modulation of the Signals) ,
also to an crystal/piezo Earephon), to an old crystal microphon (made from: Seignette-salt and aluminium-foil) also to als aoutput transoforner from an old tube receiver ( have 3-4 kohn to 8 ohm
for loudspeakers - will not be louder !)

If You use insted the 1000pf  for example 1000mf (uF) you will
hear nothing , Why?
The strong capacitor will also short the low frequencies !
(and make an Proper DC from the audio-signal)
-------------

Why the diode make not "disortion" to the audio frequency ?

BECAUSE the RF Signal from AM-Transitter is AMPLITUDE modulated !!
that say : The DC that you have after the Diode, have
an DC Voltage (positiv) and THIS DC is only fixed . It transmitter have NO AUDIO SIGNAL !!
If MUSIK (it wil be OVERLAYED , than THIS DC swap from ZERO to the dubble Voltage. (Because the Transmitter will change his output power,
now from (near) ZERO to the 2 times power. (Amplitude Modulation)
So the DC that make your crystal set will overlayed with
AUDIO Signal , that make an AUDIO frequency , that can used for earephone , also to use for amplifiers (sound-card)


I ONE wil change this in an better english , and send it to me.
I will change this thread (if need to understand)

Some people that have an better spelling, i hope to understand now better understanding the electronic

Pese

    Crystal atoms are in a Mexican stand-off so to speak.  The neuclei of the atoms don't roll to align with the magnetic field change of the imposed wave but move linearly.  This causes mechanical vibration of the crystal. The vibration of the crystal is also felt by the electron field.  The vibration can shake-off an electron from it's atomic orbital and generate a current. If the mechanical properties of the crystal are such that it gives rise to a resonating vibration at a given frequency you get preferential reception.  It is a very inefficient means of generating an electrical current.  Perhaps if the crystal collector was mechanically attached to a magnetic field propogator within whose field lies a conductor there could be electrical current generated in the conductor directly from vibration of the crystal.
   
@Koen1

      I think one was talking about storing the energy in a magnetic circuit.  By removing the magnetic "keeper" the magnetic field collapses and you get the input energy that made it back out.

Radio Towers transmit in the Megawatt range. No free energy here. Every piece of wire on the planet picks up radio waves. If you could use a similar device to pick up natural signals, and make it do work, then we'd have something to talk about.

@One
how much energy do you suppose this device could "harvest"?