This page last updated on Friday, October 13, 2006 at 6:45:58 AM
The Physics of Aerial Design - an Introduction
Have you ever wondered why radio and television aerials are the shape and size that they are? This introduction is a brief guide...
The Electromagnetic Spectrum
Television and radio signals are transmitted from the transmitter to the receiving aerial by radio waves.
Radio waves are a form of electromagnetic radiation, as is the visible light that enters our eyes so that we can see objects around us. The difference between radio waves and visible light is their frequency (or their wavelength).
The above is a picture of a wave. You can see that it consists of crests and troughs. If you imagine this to be a diagram of a water wave, then the horizontal line represents what the water would be like if it was completely still.
The distance between one crest to another is called the wavelength.
The width of a wave from top to bottom is called the amplitude.
Electromagnetic waves, like water waves, transfer energy from one place to another.
Imagine that this wave is moving from right to left on the screen. As it moves off the left of the screen, the time it takes between each crest to move off the screen is called the period.
If the crests disappear off the screen at a rate of 1 per second, then the wave has a frequency of 1 Hertz (Hz). The frequency is a measure of how frequent the crests pass by a fixed point.
I expect that as you imagined the wave moving off the screen, in your mind's eye it was moving at quite a slow speed, like a water wave. However, electromagnetic waves, like radio waves and light, move at nearly 300,000,000 metres per second (186,000 miles per second). It is difficult to imagine such a speed. The Sun is 96 million miles away, yet it takes light only 8 minutes to reach the Earth from the Sun.
No matter what the frequency, the speed the electromagnetic wave travels at is the same.
The above example had the wave's
frequency as being 1Hz. If it had a frequency of
1000Hz we could just as correctly say that it had a
frequency of 1 kilohertz (1kHz). In physics, we use
prefixes like kilo as shorthand when
describing very large or very small numbers:
For example, for a VHF frequency of 94Mhz, 94,000,000 crests would be going by each second.
Here is a good time to dispel a myth! VHF (very high frequency) is not the same as FM. VHF refers to what frequency range a transmitting station is in, and FM (frequency modulation) is how the signal is encoded onto the wave. Similarly, medium wave is not the same as AM (amplitude modulation).
If a wave has a long wavelength then it follows that it must have a low frequency. Conversely, if a wave has a short wavelength it has a high frequency.
Let's take a look at how the different forms of electromagnetic radiation fit into the electromagnetic spectrum:
Radio waves, then, take up a much larger space on the EM spectrum than visible light does. The International Telecommunications Union allocates space on the EM spectrum for telecommunications applications.
Let's take a closer look at the bands
used for broadcasting in the UK:
Key to abbreviations: LF - low frequency; MF - medium frequency; HF - high frequency; VHF - very high frequency; UHF - ultra-high frequency; SHF - super-high frequency.