Sensing things

Light waves are invisible, but our eyes can detect the effect they have on different materials because the waves produce an effect on the retina of the human eye which the brain can interpret. We cannot see heat waves either, but we can feel the effect they have on our skin. Gamma rays and X-rays are also invisible, but their detrimental effects on human tissue are well known. It is not surprising, then, that we cannot see radio waves. We cannot sense them, either, until we produce a device upon which they have an effect. That device is a radio receiver; it is able to process certain characteristics of radio waves, and make these characteristics audible by generating sound waves from the loudspeaker or headphones. Other characteristics of the same waves may be turned into light as a TV picture on a cathode-ray tube, or as a fax image on a sheet of paper.

Visible waves

Let’s start our description with some waves that we can actually see! When a small stone is thrown into a pond, we see circular water waves radiating from the point where the stone fell into the water, as Figure 1 shows. (Notice that we use the word radiating, even with water waves; it is not a radio term, but one which describes any motion where the radius of a circle is increasing. In this case, it is the radius of the circular waves which is increasing.) If you were to watch the water waves down at the water level, perpendicular to the direction in which they are travelling, you would see something like the illustration in Figure 2.

The horizontal line represents the water level before the wave started, and the vertical line represents the direction in which the water is displaced at any instant.

All waves are described in the same way

‘Freezing’ the motion of the water in this way allows us to define two very important characteristics of a wave, characteristics which we talk about every day – wavelength and amplitude. The wavelength of a wave is simply the distance (measured in metres) from any point on one wave to the same point on the adjacent wave. Look at the diagram and you will see what is meant by ‘the same point on the adjacent wave’. The amplitude of a wave is always measured from the centre (undisturbed) position of a wave to the peak (or the trough) of the wave. Both these positions are shown, the arrow indicating that the measurement is taken from the centre to the peak or trough. The amplitude of a wave is defined as the maximum displacement of the wave from the centre position – the direction (up or down) of that displacement does not matter. Waves of greater amplitude carry more energy with them.

If we now ‘unfreeze’ the wave, we will see it travel from left to right (or right to left, depending on where we are looking). The speed at which it moves is called its velocity, and is measured in metres per second.

Another useful word is propagate; it means travel. We talk about radio waves propagating from a transmitter to a receiver. This velocity of propagation (for electromagnetic waves) is very fast indeed – they will cover 300 million metres in one second. This is virtually incomprehensible, so think of a radio wave travelling around the earth – it can travel 7½ times round the earth in one second! We use the symbol c for the velocity of radio waves (which is the same as the velocity of light, of course – all electromagnetic waves travel at this speed through air and space).

The last thing we need to know about the wave is its frequency. Imagine a cork floating on the water in the path of the wave; it will bob up and down. If we were able to count the number of times it went through its highest position in one second, then that number would be its frequency. Any periodic motion like this is said to go through one cycle each time one complete wave passes a point (in this case, our cork). We are thus counting the number of cycles per second of the cork’s motion. The unit of frequency is thus ‘cycles per second’; this unit is now named after Hertz, a radio pioneer, and is abbreviated to Hz.

Our description of the wave is now quite simple – we need only four quantities:

The basic formula

Whatever may happen to a wave while it travels through different media (vacuum, air, brick, wood, etc.), one thing and only one thing remains constant – its frequency. Its wavelength, amplitude and velocity may change, but its frequency never does. Three of the four characteristics already identified are connected by the simple relationship

Remember that c is constant if the wave travels in air or in a vacuum. This means that waves having higher frequencies (f large) must have smaller wavelengths (λ small) and vice versa. You can imagine frequency and wavelengths being on opposite ends of a see-saw!

Divisions of units

Because the frequencies of radio waves are so high (despite them having the lowest frequencies in the electromagnetic spectrum!) we have a problem with writing them down. Do you write in your log book that you have just heard a station on 14 100 000 Hz? Of course not, you write it as 14.1 MHz, knowing that the prefix mega (M) means ‘one million’. The prefixes which you need to know (when applied to frequency) are:

Notice that the ‘k’ in kilohertz is a lower case letter. It is incorrect to write it as an upper case letter. ‘K’ is a computer-related prefix meaning not 1 000 but 1 024!

When we come on to discuss heat and light waves, we will use wavelengths rather than frequencies, because of the see-saw effect – as the frequencies get larger and larger, the wavelengths get smaller, and hence are numbers which are more manageable, both to talk about and to write down!

Bands

Gamma rays, X-rays, ultra-violet waves, light waves, infra-red waves are all part of the electromagnetic spectrum, but we divide them up because they have different properties. This is why we divide up our radio frequencies into different bands. The radio waves of top-band signals (around 2 MHz) have completely different properties compared with those in the 20 metre band, so we are dividing up the radio spectrum in the same way – by property.