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How A Crystal Radio Works


In order to understand the crystal radio receiver you first have to understand were the "signal" or radio wave comes from and how it was made.

The Transmitter
A radio station is allowed to broadcast a radio wave. The radio wave is sort of like dropping a pebble into a pond of water. The ripples or waves radiate outward from where the pebble was dropped in the water. Radio wave radiate outward from the radio stations antenna sort of like the waves in the pond. Radio waves are electromagnetic waves that travel through the air.

In our figure #1 below, we have a wave. The wave below shows one cycle. How "strong" a wave is we call its amplitude.

radio wave
Figure#1



In our second example in Figure #2, we have a radio wave that has cycled 3 times in one second. This is called the "frequency" of the radio wave. In this case the frequency is 3 cycles per second. We measure frequency using the term hertz or Hz which is the amount the radio wave cycles in one second. The term "hertz" came from the German physicist Heinrich Hertz, born on Feb 22, 1857. It was named in his honor for the work he did with radio waves research.

So below in Figure #2 we have a radio wave with a frequency of 3 Hz (3 cycles per second).


radio wave 2
Figure #2



The AM radio band is from 530,000 Hz to 1,710,000 Hz. We use the designation k for 1000, so it would be written as 530 kHz to 1710 kHz.

The AM radio station can broadcast a radio wave on one frequency between 530 kHz and 1710 kHz. The FCC regulates which frequency they can use.

Now for a sample radio station. It is "radio station 610 on the AM dial". This means the radio station is broadcasting a radio wave of 610 kHz or as we now know, a radio wave that cycles 610,000 times a seconds! Thatís correct, 610 thousand times each second!

OK, then how does that wave carry the sound?

The radio station has equipment that varies the "strength" or "amplitude" of the radio wave (see Figure below). It still cycles at the same rate, but it gets stronger or weaker according to the sound.


radio wave1
Figure #1 (again)





Notice in Figure #3 below that both waves are cycling at 2 cycles per second (2Hz), but the amplitude in wave B is much higher than in wave A.

radio wave 3
Figure #3


If that amplitude is controlled by the sound say from a microphone (with other equipment), going up and down but is cycled at a fixed rate a second, you have a radio wave like the type coming from a radio station! The station is "modulating" or varying the amplitude of the radio wave. This is called "Amplitude Modulation" or AM.

In Figure #3a below you will see a "modulated" wave at 8 Hz (cycles per second) on the top example and a "modulated" wave also at 8 Hz (cycles per second) in the bottom example. Please note that the cycles per second remain the same in both signals, but the "amplitude" changes.

Un-modulated Wave (signal)

fig6.GIF
"Modulated" Wave (signal)

Figure #3a



Now we have this radio wave flying through the air hitting everything! That's right including you! Radio waves can travel at 186,000 miles a second in the air!

Interesting fact: a sound wave travels over 600 miles an hour (speed of sound), but a radio wave can travel at 186,000 miles a second! If you record a singer in a concert hall in New York and transmit it over radio waves, The radio wave could reach San Francisco before the sound wave from the singer reaches the back of the concert hall.


 We have to catch it and do something with it. Letís take your crystal radio and see if we can change it back into a sound that you can hear.


The Crystal Radio Receiver

We are going to use a very simple crystal radio for this explanation. Figure #4 below shows a very simple crystal set. A small amount of energy from the radio wave is captured by the antenna wire and is taken to the coil. The coil has to be designed just right to capture only the frequency we are trying to receive. In our case we are trying to receive our radio station above at 610 kHz. By winding just the right amount of wire on just the right diameter coil form, the coil will be what we call "resonant" and "ring". In other words it will be able to store the energy of the radio wave we want to hear.  All other radio waves not "resonant" will pass through the coil and out the other side to the ground.


crystal set1

Figure #4

A small amount of the radio wave energy stored in the coil (our 610 kHz or 610,000 cycles per second) moves to the detector or the device called a diode. The energy is an alternating current signal (AC) at this point. The detector (diode) rejects half of the alternating current signal and the signal looks like figure #5 below. Now the signal is a pulsating direct current (DC) signal.

rectified radio wave
Figure #5


This allows the earphone to use the energy. If both sides of the wave were used, they would cancel each other out as they are opposite of each other. As this energy go into the earphone, the amplitude or strength of the signal varies because the wave is "modulated". This energy is converted by mechanical means in the internal workings of the earphone. The sound waves exit the earphone which you perceive as the original sounds from the radio station.

Not discussed here is tuning the coil to get different frequencies. That can be done by adding more turns of wire around the coil form. Or removing some. On a simple radio, this is done by moving the diode up or down the taps on the radio in effect making the coil longer or shorter. This changes the "inductance" of the coil or makes it resonant to different frequencies.

Another way to do this is to add a variable capacitor across the coil. This adds or removes capacitance to the coil and changed the resonant frequency the coil will tune to.




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