Radio is one of the most important branches of electronics. At one time, it was the focal point of electronic devices; If you made anything as a garage electronics hobbyist, chances were it was a radio or something radio-related. As computers and similar information-processing devices have taken over, this is no longer the case, but radio waves still form a very serious part of the serious electronic engineer's knowledge; A great many devices that we use today operate on radio waves, and indeed, radio waves are very useful for all manner of communication and control electronics.
When working with radio circuits, there are two types of general-purpose circuits which are absolutely crucial for effective work with radio. These circuits are:
Amplifier circuits As you can imagine, amplifier circuits are, simply amplifiers. They make weak signals stronger.
Filter circuits A filter circuit is a circuit that allows only electric signals of a certain frequency to pass through, while blocking other frequencies. (Technically, a filter circuit can actually block any component of an electric circuit; It doesn't have to be frequency. However, filter circuits are usually used for allowing only certain frequencies through.)
As you probably know, there are two basic types of modulation used with radio transmissions: Amplitude Modulation (AM) and Frequency Modulation (FM). There are other types of radio modulation used today, but AM and FM are the two classic, basic types that are the bread-and-butter of everyday radio modulation.
AM creates the radio signal by changing the amplitude (strength) of the radio signal. AM stays on one exact frequency. However, it changes the strength of the signal slightly to create signal waves that actually carry the radio transmission. The changes in amplitude are quite small compared to the overall amplitude, but they are large enough to be detected and used in a radio receiver.
FM creates the radio signal by changing its frequency. This may seem confusing at first, because you tune a radio to a specific frequency; How can the receiver stay in tune if the transmission is constantly changing its frequency? The answer is simple: Frequency deviation is expected and required in FM transmissions, and FM receivers are made to handle it. The frequency of the base FM signal is very high (in the range of 100 MHz) compared to the frequency deviation used to create the actual transmission signal; This frequency deviation is usually on the scale of only 10 to 30 KHz (which is 0.01 to 0.03 MHz). FM radio waves always stay on the same amplitude, but they change their frequency slightly to create the signal.
When you're first starting out with radio electronics, I recommend that you start using AM, because it tends to be more intuitive than FM. It's generally easier to change the strength of a circuit than the frequency of an oscillating circuit.
For radio waves to do anything particularly useful, there must be two separate electronic devices at work: A transmitter and a receiver. The transmitter is the device which creates the radio waves and sends them out over an antenna; The receiver is the device which receives those radio waves and decodes them into useful signals. Sounds simple enough so far, right?
It stays pretty simple for now. Let's start with making a basic radio transmitter. As it turns out, creating radio waves is one of the easiest things in the world to do. To create radio waves, all you have to do is absolutely anything. Your body's nervous system creates electricity. Even if you sit perfectly still, there is electricity running through your brain, and these electrical pulses create radio waves. Electricity creates magnetism, and vice-versa; This is a key principle behind the workings of electric generators and motors. Any kind of an electrical field creates electromagnetic waves, of which radio waves are one type.
Of course, the electricity running through your brain is of too small a magnitude to be measured by most electrical equipment, so we can take this concept a step further by simply creating any kind of electrical circuit that creates a changing voltage state. A perfect and simple example is an oscillator circuit, which simply moves up and down in a constant, steady rhythm, forming a wave pattern (such as a sine wave or square wave). A changing voltage state is necessary, because radio waves are not generated unless the electric field is CHANGING.
On the other side of a radio transmission, making a radio receiver can also be quite easy. All you really need to make one is a piece of metal. Presto, there you have a device which is receiving radio signals. Whether it's a tiny piece of wire or a giant antenna tower a mile high, any piece of metal is being hit with radio waves in the air all around you, and it's absorbing them. Of course, the larger the piece of metal, the better a receiving antenna it will make because it will catch more of the radio signals, but the point is that an antenna really is nothing more than just a piece of metal.
You can demonstrate the phenomenon of radio transmission fairly easily if you have an oscillator circuit and a fairly sensitive oscilloscope. All you need to do is turn on the oscillating circuit, turn the oscilloscope to its highest sensitivity, and hold the tip of the probe close to the oscillator (without actually touching it). As you move the tip closer, you'll actually begin to see the wave of the oscillationg showing up on the screen, even before the probe is actually touching anything. This is because it's picking up the radio waves generated by the oscillator circuit. You can try adjusting the voltage of the oscillator circuit to see how it affects the readout on the scope, and you can also try attaching a long wire to the output of the oscillator circuit and try moving that wire around to see how the scope reacts. What's actually happening is, the wire has become an antenna, and as the oscillation goes through it, it strengthens the radio waves as they move through the air, because you have increased the amount of metal that is exposed to them.
Of course, for a radio receiver to be really useful, it usually needs to be a little more than just an antenna. The problem with a simple metal stick as an antenna is that it receives all frequencies of radio transmission. When you tune a radio, you want to just receive one particular frequency; You want to focus on just one transmission, and block out all the others. This is not possible with a simple antenna. And so we come to...
An antenna connects to a radio receiver, which is the device that receives radio waves and turns them into useful electrical signals (typically audio).
There are three basic stages in a radio receiver:
The tuner selects one frequency and allows only that frequency to pass through, while blocking all others. This is important, because an antenna is simply a piece of metal; It has no ability to choose frequencies, and in fact, an antenna receives every frequency. The tuner is a filter circuit; It filters through only the desired frequency.
For many dedicated-purpose radios, the "tuner" is set to one single frequency. On an everyday household radio receiver, however, the tuner is tunable, meaning it can be adjusted to pass different frequencies, allowing you to tune in to different radio stations.
Because the radio wave is not an exact audio wave, it must be decoded into an audio signal. In the case of AM, the decoder is extremely simple: It's basically a diode. In FM transmission, the decoder is a little more complicated.
Because the signals actually received by the antenna are usually much too weak to power a speaker to the point where human ears could hear anything, they must be amplified quite a lot to create a reasonably audible sound.
If the radio signal is audio, then from the amplifier, the electricity is sent to a speaker, and the speaker makes the sounds! The radio reception process is complete.
I wanted to do something really simple as a first radio electronics project. My first idea was to make a circuit that would simply control a LED remotely; You could turn the LED on or off with the push of a remote-control button. Unfortunately, as it turned out, creating a simple remote-control radio device like that is actually more complicated than making a simple radio device that transmits audio. Since I am mostly an information-electronics kind of guy and am not too interested in transmitting audio signals via radio, I did not want to get involved in building that kind of a project. However, you can find a lot of really excellent ideas, how-to steps, and project photos at http://www.scitoys.com/scitoys/scitoys/radio/radio.html. Check out the "Building a very simple AM voice transmitter" section for a truly simple device that will take audio input and make a radio transmission out of it. Building a radio receiver is actually somewhat more complicated than building a transmitter, because a transmitter really just takes an input signal, modulates an oscillator with it, and sends it to an antenna, while a receiver must filter the desired frequency. Even so, there are several excellent ideas for receivers on that site as well.
I am still hoping to do some useful work with radio transmission of information (such as 802.11 stuff) in the future; However, as far as I can tell, that kind of work is not something that can be begun with a truly simple electronics project, and so I don't anticipate doing anything with it, or putting anything else on this web page, for a while.