Chapter 2
How radio works
Making waves
If you tap a screwdriver across the terminals of a 9-volt dry battery, it will produce tiny sparks. The sparks are difficult to see, except in complete darkness, but if you do it close to an ordinary transistor radio, they should be easy enough to hear, as a distinct crackle on the radio. That, in effect, is what Heinrich Hertz did in 1888, when he became the first person to demonstrate the existence of what we now know as radio.
The sparks create an electric field which builds up quickly but then collapses. This rapidly changing electric field generates a magnetic field, which also grows, and collapses. This combination of constantly changing electric and magnetic fields produces what we know as electromagnetic radiation, or radio waves.
More sophisticated radio transmitters work by supplying a rapidly changing electric current to an antenna (aerial) to create the electric field. The speed at which the current changes controls the speed at which the electric field around the antenna changes, and is measured in Hertz (usually abbreviated to Hz).
1 Hz = 1 cycle per second;
1kHz = 1kiloHertz = 1 thousand cycles per second;
1MHz = 1 Megahertz = 1 million cycles per second;
1GHz = 1 Gigahertz = 1 billion cycles per second.
If you find it difficult to visualize a fluctuating electric field, imagine, instead, that someone has just dropped a pebble into a pond. The pebble represents the transmitter, and the ripples radiating out from it represent the fluctuating electrical and magnetic field.
Frequency and wavelength
One big difference between the waves on the pond and the electromagnetic waves of a radio transmission is that radio waves travel outwards in three dimensions, not just two. Another, even more important, is that all electromagnetic waves travel at 300,000,000 metres per second, (roughly 186,000 miles per second or 162,000 nautical miles per second).
300,000,000 metres per second is known as “the speed of light” because light is one of many kinds of electromagnetic wave.
Imagine that you can see electromagnetic waves as clearly as you can see the waves on the sea. You know that they are travelling at 300 metres per microsecond, so if 30 of them pass you in one microsecond, the first one must be 300 metres away by the time the thirtieth one has past you. From this, you can see that each wave must be 10 metres long. In other words, they have a wavelength of 10 metres. Similarly, If 300 of them pass you in 1 microsecond, they must have a wavelength of 1 metre.
Wavelength = 300,000,000 / Frequency;
Frequency = 300,000,000 / Wavelength;
Marine VHF operates at a frequency of about 156MHz;
So it has a wavelength of about 300 / 156 = 1.92 metres.
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