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# The very Basic Basics of How Radio's work.

Posted Aug13-10 at 10:48 AM by jambaugh

Recall that moving electric charges (currents) induce magnetic fields. Put a (DC) current carrying wire next to a magnetic compass and you'll see the needle move.

Also changing magnetic fields induce voltages i.e. electric fields.

These two together are how a transformer works. The AC current in one coil causes a changing magnetic field which in turn causes an electric field (voltage) in the other coil which can then push a current. With different winding ratios you can actually step up to a higher voltage (at the cost of your ability to drive a current so you can't ever get more energy out then you put in).

Maxwell's big insight was that changing electric fields also induce magnetic fields all by themselves without actual moving charges.

An electro-magnetic wave can be understood roughly as a changing electric field causing a changing magnetic field causing a changing electric field .... which radiates outward from the source.

We think of waves such as water waves or sound waves as undulations of some medium. Electromagnetic waves are in some sense their own medium... the electric wave carries the magnetic wave and vis versa. Thus radio waves can travel through the vacuum of space.

So picture this. You attach one wire of an AC power source to the ground and the other to a piece of wire, (the antenna). The AC power drives a cyclical changing current in the wire which induces an alternating electro-magnetic wave radiating out into space from the wire.

For best results the antenna should be half the wavelength of the resulting wave which you get from wavelength times frequency = c =300,000,000 meters/second (the speed of light). Note that to keep the antenna from being miles long you need pretty high frequencies. 1Km wavelength = 300,000 cycles per second or 300 kilohertz. A 1 meter wavelength corresponds to 30,000 kiloHertz or 30 MHz. The wavelength and frequency are inversely proportional so doubling one halves the other.

What happens at the transmitting antenna also happens in reverse at the receiving antenna. The electric field of the EM wave induces an alternating current in another wire. In a way you have a very inefficient transformer with the primary and secondary windings spaced very far apart and straight instead of wound in a coil.

The receiver's antenna current is typically very very small but you can amplify it and that allows you to detect when the transmitter is turned on. Put a key switch on the transmitter and you'll be able to send Morse code to the receiver. You have a wireless telegraph, the first type of radio used.

To send voice or music you can use something called Amplitude Modulation (AM radios). As mentioned above the frequency of practical radio waves need to be quite high relative to say the frequencies of sound (20Hz-20KHz). At those frequencies the changing current or voltage of a microphone is like a very slow undulation. You can then use the microphone signal to add power to your high frequency transmitter and the strength (amplitude) of the radio waves are then varying with the pressure of the sound.

At the receiving end you need a high speed rectifier (the crystal of a crystal radio) to detect just the amplitude of the radio signal.

This will vary in the same way as the pressure waves entering the microphone and so when (further amplified and) attached to a speaker you can hear the sound repeated.

Now as it stands just hooking an amplifier and detector to an antenna would give you every electromagnetic wave being broadcast including random EM noise. This is actually happening when a cheap stereo amplifier picks up the 60Hz hum of your AC power.

To pick a particular station you use a tuning circuit which resonates only at a particular frequency which you can adjust. Then you hear only what is being broadcast by say the AM950 station and not everybody else including your power lines.

One problem with AM radio is that the weaker the radio signal the less loud will be the sound you pick up at the receiver. One way to control that is to automatically control the amplification of the sound signal after detection so that it is more uniform. That's called automatic gain control AGC.

Another problem is that even with good circuits AM receivers tend to pick up noise and broadcasts at other frequencies from stronger (nearer) transmitters. There is also inherent distortion in the signal due to the way it is amplified and sent. For good fidelity we can use other tricks.

The next stage is frequency modulation (FM radio). Instead of letting the microphone control the strength of the radio signal you let it control the frequency at which it broadcasts. That way you have a constant radio signal strength. FM gives a much clearer transmission of the audio signal which is why it is used now by most music stations.

There are other tricks to get the best signal such as super-hetrodyning, and using side-bands, and ultimately sending digital signals encoded and decoded by computer circuits.

For more on that you should begin studying up on electronics. A good starting point is to check out Amateur (Ham) Radio websites and books.

Here's one final afterthought. You could conceivably send AC electrical power over a single wire instead of two by using the ground as the 2nd wire. But doing this makes the power transmission wire one big 60Hz broadcast antenna and you'd loose all that energy into space. To transmit power efficiently over distances we use 3 phase AC with three wires in parallel so the radiating effects will (mostly) cancel. We in other words make the transmission lines as bad an antenna as possible.

But back in the early years of electrical power Nikola Tesla tried it the other way around. He experimented with using transmitters and receivers to send electrical power. Unfortunately it wasn't practical due to losses and also unfortunately Tesla didn't have Maxwell's last key equation to realize he was building a radio although I believe he did patent the first wireless telegraph device. Unfortunately Marconi got all the credit.
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