wireless communication - radio waves.

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Long wavelengths travel well through Earth’s atmosphere.

Radio waves and microwaves are good at transmitting information over long distances.

This is because they don’t get absorbed by the Earth’s atmosphere as much as waves in the middle of the EM spectrum (like heat for example), or those at the high frequency end of the spectrum (e.g. gamma rays and X-rays).

Radio waves are used mainly for communications.

Radio waves are EM radiation with wavelengths longer than 10cm.

Different wavelengths of radio wave refract and diffract in different ways.

Long-wave radio (wavelengths of 1 – 10km) can be transmitted from one place and received halfway around the world because they diffract (bend) around the curved surface of the Earth.

Radio waves used for TV and FM radio transitions have very short wavelength (10cm -10m). To get reception, you must be in direct sight of the transmitter – the signal doesn’t bend around the hills or travel far through buildings.

Short-wave radio signals with wave lengths of about 10m -100m can be received at long distances from the transmitter because of reflection in the ionosphere. Medium-wave signals (well, the shorter ones) can also reflect from the ionosphere, depending on atmospheric conditions and time of day.

Diffraction makes a difference to signal strength.

Diffraction is when waves spread out at the edges when they pass through a gap or past an object.

The amount of diffraction depends on the wavelength of the wave, relative to the size of the gap or obstacle.

Longer wavelengths can encounter a lot of diffraction because they are large compared with the gap o obstacle.

This means that they are able to bend around corners and any obstacles – such as hills, tall buildings and etc.

So longer wavelength radio waves can travel long distances between the transmitter and receiver without them having to be in the line of sight of each other. Shorter wavelength radio waves and mico0waves don’t diffract very much, so the transmitters need to be located high up to avoid obstacles (even the then cannot cover short distances).

Some areas have trouble receiving shorter wavelength radio (and microwaves) signals – e.g. if you live at the foot of a mountain you will receive poor signal strength.

Diffraction can also occur at the edges of the dishes used to transmit signals. This result in signal loss – the wave is more spread out so the signal is weaker.

Refraction can help radio waves travel further.

When a wave comes up against something that has a different density it changes speed. If the waves hits the new substance at an angle, it changes direction. This is refraction. When it happens high up in the atmosphere, it can help waves travel further for long communication.

UV radiation from the sun creates layers of ionised atoms (atoms that have either gained or lost electrons) in the Earth’s atmosphere. These electronically charged layers are called the ionosphere.

Radio waves travel faster through ionised parts of the atmosphere then non-ionised parts. This causes refraction.

Short wave and medium waves radio signals are refracted most in the ionosphere – they are effectively bounced back or reflected back to Earth. This means that short and medium wave radio signals can be received a long way from the transmitter.

The amount a wave is refracted in the ionosphere depends on its frequency and angle elevation. High frequency/ short wavelength signals such as short-wave radio don’t refract as much as medium wave.

Radio waves ‘bounce’ off the ionosphere in a similar way to how light waves totally internally reflect inside optical fibres.

Refraction’s not always good enough though. It can disrupt a signal by bending it away from the receiver dish.

Digital radio help reduce interference.

There’s a limited number of radio wave frequencies that can be used to transmit a good analogue signal – so radio stations often broadcast using waves of very similar frequencies.

These analogue signals often suffer with interference because of this – similar waves covering a similar area can combine, which causes noise. This is why radio stations near to each other use different frequencies – so they don’t interfere as much.

Digital audio broadcasting (DAB) works in a different way to traditional radio broadcast – its digital to start with.

With Dab, different signals are compressed, then transmitted as a single wave – this is known as multiplexing.

They are transmitted across a relatively small frequency bandwidth and separated out by the receivers at the other end. You need a Dab radio set to pick up and decode the signals.

DAB suffers less interference than traditional radio broadcasts, and since the signals from many stations can be broadcast at the same frequency (multiplexing), it means an increase in the potential number of radio stations available.

At the moment there are a limited number of Dab transmitters in the UK (and the world) so some areas don’t receive radio signals at all.

Even if you can receive DAB, the sound quality is often not as good as a traditional FM radio broadcast, due to the compression of the signal.