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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).

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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:

Name Abbreviation Multiplier
kilo k times 1000
mega M times 1,000,000
giga G times 1,000,000,000
tera T times 1,000,000,000,000

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:

 

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The longest (in wavelength terms) form of EM waves are radio waves. Long, medium and short waves are all radio waves. Long wave should be familiar as Radio Four broadcasts in this band.

Medium waves are shorter than long waves, and also carry radio programmes.

Short waves are shorter still, and some short waves can be transmitted over long distances by reflecting off layers high in the atmosphere. They are used to carry radio programmes and for amateur use.

Microwaves are used for cooking and for specialist communication purposes.

Infra-red radiation is better known to us as heat.

As you can see, (pardon the pun) visible light is but a small part of the EM spectrum. Red light has a longer wavelength than blue light.

Shorter in wavelength than blue light is ultra-violet radiation. UV is what gives us sun-tans.

X-rays and gamma rays are harmful to humans in large doses. Gamma rays are produced by some radioactive materials and some cosmic events.

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:

Long Wave (LW, LF) 150-285 kHz (2000-1063m)  
Medium Wave (MW,MF) 525-1605 kHz (571-187m)  
Short Wave (SW, HF) 3950-4000 kHz (75m band)
5950-6200 kHz (49m band)
7100-7300 kHz (41m band)
9500-9775 kHz (31m band)
11700-11975 kHz (25m band)
17700-17900 kHz (16m band)
21450-21750 kHz (13m band)
25600-26100 kHz (11m band)
 
Band I (VHF) 41-68 MHz Five channels, each 5 MHz wide, which were used to transmit BBC 405-line television (channels 1-5).
Band II (VHF) 87.5-108 MHz Used for VHF FM transmissions.
Band III (VHF) 174-216 MHz Eight channels, each 5 MHz wide, which were used for BBC and ITA/IBA 405-line television channels 6-13).
Band IV (UHF) 470-582 MHz 14 channels, each 8 MHz wide, currently used for terrestrial analogue and digital television (channels 21-34).
(Sub-bands) 590-598 MHz and 606-614 MHz Were reserved for use by VCRs, video games etc. but were released by the Government for analogue transmissions of Channel 5 (channels 35 and 37).
Band V (UHF) 614-854 MHz 30 channels, each 8MHz wide, currently used for terrestrial analogue and digital television (channels (38-68).
Band VI (SHF) approx. 10-12 GHz Used for direct satellite-to-home transmissions.

Key to abbreviations: LF - low frequency; MF - medium frequency; HF - high frequency; VHF - very high frequency; UHF - ultra-high frequency; SHF - super-high frequency.

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Basic Antenna Theory

Practical Examples of Aerial Design

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