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Electromagnetic spectrum: 3. The properties and uses of radio waves

Doc Brown's Physics exam study revision notes

3. The properties and uses of radio waves

The sources and properties of radio waves

Making radio waves - transmitter: Radio waves, like all EM radiations are oscillating electric and magnetic fields, and are produced by alternating electric currents (a.c.).

Alternating currents automatically produce an oscillating electric and magnetic field that makes the electric charges (electrons) oscillate and this automatically produces radio waves if the oscillations of that frequency.

The frequencies of the waves produced will equal the frequencies of the alternating current - however complex the signal transmitted.

The diagram below illustrates the principles of how radio signals are generated, transmitted and received.

The oscillating charges produces and emit an EM wave of radiation - radio waves. The EM wave has the same frequency as the a.c. current that produced it - so a radio transmitter circuit uses a.c. frequencies in the EM radiation band to create radio waves of the desired frequencies.

Receiving radio waves: EM waves can cause charged particles like electrons to oscillate at the same frequency. If these electrons are part of a 'receiver' electrical circuit, an alternating current is induced at the same frequency as the EM wave - this is what a radio receiver aerial (antenna) does and the rest of the electronics produces the sound and pictures.

You can demonstrate by feeding a signal from a microphone or signal generator into an oscilloscope and into a radio transmitter circuit.

The signal from the radio receiver can then be fed into a 2nd oscilloscope.

You can then compare the generated signal with the received signal and they should be the same, however complex the signal may be.

In real radio transmission-reception situations the complex signal frequencies use a carrier wave.

carrier wave is a pure wave of constant frequency, a bit like a sine wave and has imposed on it the more complex wave of the signal information. The process of imposing an input signal onto a carrier wave is called modulation.

The combination of transmitter (e.g. radio mast) and receiver (e.g aerial) allows you to encode information onto a radio signal and transmit information from one place to another - its essentially a data transfer system which might be a radio signal or TV .

Radio waves do penetrate liquids and solids but are gradually absorbed and end up as heat energy - increasing the thermal energy store of the surroundings.

Radio waves pass straight through the human body without causing harm.

Long-wave radio waves can be transmitted over long distances, in fact all the way around the Earth. They have long wavelengths of 1-10 km and can diffract around the surface of the Earth and around hills too and reflect/refract off the ionosphere - see diagram and comments below.

This means radio signals can be received even if the transmitter and receiver are not in direct line of sight.

Short-wave radio signals of wavelength ~10 m to ~104 m can also be transmitted and received over long distances because they are reflected off the ionosphere (which is an electrically charged layer of the Earth's upper atmosphere).

The lower atmosphere is not electrically charged and does not inhibit the transmission of radio waves or microwaves.

However, part of the upper atmosphere, called the ionosphere, contains electrically charged particles (ions) that interfere with radio waves e.g. reflect them, and so they zig-zag through the lower atmosphere travelling over thousands of km from transmitter to receiver.

Radio waves are used to transmit information from one location to another e.g. to your TV and radio 'appliances' for you to view and/or listen to.

TV and FM radio use very short radio waves and other radio transmissions use medium and longer wavelengths (MW and LW). To get a good reception, you do need to be in direct sight of the transmitter because these waves do not diffract (bend) around obstacles and they don't travel through buildings.

Bluetooth devices use low power short wavelength radio waves to communicate and send data between devices like mobile phones or computers relatively close together - so the radio waves only have to travel short distances. The distances between devices are quite short i.e. within a few metres of each other.

This avoids the use of wires e.g. the wireless headset you use to use your phone while driving a car.

You can pick up a long wavelength (lower frequency long wave) radio signal with your radio receiver without being in a direct line with the radio transmitter because these radio waves are diffracted by hills and other large obstacles.

The wavelengths of 'long wave' radio waves can be over a km, which is similar to the width of hills.

You can also pick up long wave radio signals over large distances because they can diffract around the Earth's surface.

For a good reception of higher frequency (shorter wavelength, 'short wave') TV and FM radio signals, you need to be in direct line with the transmitter, unlike 'long wave radio', the signal shows little diffraction ('bending').

Short wavelength, high frequency radio signals, can also be received over large distances because these signals can bounce of the Earth's surface AND the ionosphere - an electrically charged layer of the Earth's upper atmosphere - the waves effectively zig-zag between the Earth's surface and the ionosphere (== /\/\/\ ==>).

Unlike microwaves, radio waves are refracted by some layers of the atmosphere, so cannot be used for satellite communications.

You can now use high-frequency radio wave scanners to security checks on passengers and luggage at airports. Radio waves are much safer than using X-rays.

I don't know of any? Does political propaganda count?!!! (just a joke!)

Radio waves are of long wavelength and low frequencies and so carry little energy and pass through our body without doing any harm - as far as I know?

Keywords, phrases and learning objectives for electromagnetic radiation spectrum

Be able to describe the properties of radio waves and its sources.

Know that radio waves are a very useful part of the electromagnetic spectrum of radiation.

Know and describe the uses of radio waves in TV and radio communication systems.

Know that radio waves are not only reflected and refracted, but they are diffraction over hills so can listen to the radio in a valley.

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