Radiowaves. What are they?

We listen to the radio and watch the television every day of our lives, we make mobile telephone calls, we even cook using radio waves, but have you ever considered how it all happens?

This article is an introduction into the world of radiowaves, their uses and how they are propagated throughout the atmosphere.

Frequency versus Wavelength

Traditionally, radio signals have been quantified using two units of measurement, wavelength (measured in metres) and frequency (measured in Hertz - hereafter referred to by the standard abbreviation Hz).

Frequency

Firstly we will deal with frequency measurement. A radio wave can be visualised in the form of a sine-wave, travelling at the speed of light, the frequency of the wave can be considered to be the number of complete wave forms passing a fixed point in one second.

Wavelength

This is an older method of quantifying radio waves, by measuring the physical distance between two adjacent peaks on the sine wave. Measured in metres, radiowaves can have wavelengths varying from several kilometres in length (very low frequency transmissions) to just millimetres (microwave signals).

A quick method of translating wavelength to frequency (and visa versa) is by using the following equation:

Wavelength (in metres) = 300 / Frequency (in MHz)

Abbreviations

Now would probably be a good moment to mention some of the many technical terms and abbreviations used when talking about radio signals

Frequencies

Having just stated that frequencies are measured in Hertz (Hz): the number of cycles-per-second is normally far too high for Hz to be used on its own.
Normally a prefix is added to the abbreviation Hz. These abbreviations are the same as those used in computing and other fields: ie.

Frequency Bands

The radio spectrum has been broken up, by international convention, into a series of bands, each one covering a certain range of frequencies, as follows:

Modulation

Here is where things start to get a little more complicated.

Basically, when you listen to a radio broadcast, what you are actually hearing is sound waves, which have been converted to electrical energy (via a microphone for example) and then 'piggy-backed' onto a sinusoidal radio wave. When your radio receives this signal it strips the piggy-backed signal from its carrier wave and re-converts this into sound waves - coming out of the loudspeaker.

There are two methods of attaching the audio signal onto its carrier wave, known as signal modulation. The two methods are:

Amplitude Modulation (AM)

This is where the audio signal is added to the amplitude of the carrier wave. The easiest way to show this is to take the example of a morse code signal, the simplest of all radio transmissions. A morse signal is just a 1KHz audio tone - being alternately turned on and off, attached to a carrier wave.

An AM morse signal would look something like this:

Frequency Modulation (FM)

This is slightly more complicated, frequency modulation consists of adding the frequency of the audio signal to that of the carrier wave, so that a 1KHz morse signal, when added to a 5MHz carrier wave would give a signal that would alternate between 5.000MHz and 5.001MHz. An oscilloscope representation of the modulated signal would look something like this:

Note: In the second diagram, the time scale (x axis) has been greatly extended in comparison with that of the first diagram. Also, the difference in frequencies has been exaggerated in order to clearly show the variations.

Early radios used the much-simpler Amplitude Modulation. However, this type of transmission has one serious flaw... The amplitude of the signal may vary during transmission for a multitude of factors; weather conditions, movement of receiving or transmitting aerials, interference from other stations etc. As the signal strength changes, the modulated information looses quality on reception.

Frequency modulation does not suffer this problem, even if the strength of the signal changes, the frequency variations will remain stable, hence the reason most modern, commercial radio transmisisons are now transmitting FM signals - and the reason why when you listen to an FM radio station the sound quality is so much better than an AM station.

Types of radio signals

Voice Transmissions

Morse Code

Telex

Data Transmissions

Frequency Ranges and their properties

ULF-LF

These lower frequency ranges (300Hz to 300KHz) are not generally used by commercial traffic. The particular properties of Ultra Low Frequency radiowaves means that they will travel great distances, actually following the curvature of the earth. Also, they will penetrate several metres into the ground and are capable of passing through a certain depth of water.

The major disadvantage of this sort of radio transmission is that, due to the incredibly long wavelengths involved, the receiving and transmitting aerials must be of an enormous size. As well as this, again due to the long wavelengths, the data transmission speeds are extremely limited, a relatively short text message can take up to ten minutes to transmit.

MF-HF

Generally used for long range radio transmissions. Shorter wavelengths give an advantage over ULF/VLF bands in that data transmission speeds can be much higher. Radio waves of in these bands no longer follow the curvature of the earth, but travel in straight lines, the reason they travel such great distances is that electromagnetic waves of below 30MHz are reflected back to earth by the Ionosphere, which is made up of several layers of electrically charged (ionised) particles, between 80 and 170 km above the ground. The radio waves can sometimes bounce between these layers and the ground several times before becoming too weak to detect. Hence the extremely long range of HF signals.

VHF-SHF

Once we get into the higher frequency bands the radio signals can be considered to be truly 'line-of-sight' signals. Above 30MHz, radio signals will quite easily pass through the ionosphere and continue on into space. The disadvantage is that the higher the frequency, the lower the penetrative power of the radio waves when dealing with solid objects, hills, buildings, walls etc. SHF signals will quite happily make a round trip of over 70,000 km to a communications satellite and back, but will have great difficulty in penetrating a thin brick wall when they get back to earth. Hence the reason there is no such thing as an set-top satellite dish.

Commercially available radio receivers

When buying a commercially available radio, either for the car or for the home (as part of a HIFI system) we rarely consider the frequency range covered. This is because commercial radio stations all broadcast within preset frequency ranges and 99% of commercially available tuners just cover these frequencies.

When thinking about standard commercial radio, generally the frequency bands are labelled slightly differently than previously stated. A basic radio will have just two bands, marked:

• AM¹
• FM

The more expensive radio tuners will have one or two extra bands available, the normal maximum being:

• LW
• AM
• SW
• FM

A brief explanation of these bands would be:

When we look more closely at these frequencies one thing stands out almost immediately. There are enormous gaps in the coverage. So just what are all these other frequencies used for?

Frequency Ranges and their uses

As previously stated, there is a whole world of radio signals out there that can not be picked up by a standard, hi-fi radio tuner. Here is a non-exhaustive list of some frequencies and their uses:

300 to 3,000Hz ULF Ultra Low Frequency

3 to 30KHz VLF Very Low Frequency

30 to 300KHz - Low Frequency (LF)

300 to 3000kHz - Medium Freqency (MF)

3 to 30MHz - High Frequency (HF)

30 to 300MHz - Very High Frequency (VHF)

300 to 3,000MHzUHFUltra High Frequency

3 to 30GHzSHFSuper High Frequency

30 to 300GHzEHFExtreme High Frequency

Above and below the RF Spectrum

Other useless information that I haven't decided what to call yet

Important Note

WARNING: In the UK, if you listen to anything other than licenced broadcast stations or Amateur Radio (including CB) without a special licence, you are breaking the law. Even having a private frequency stored in the memory of a radio receiver is considered enough proof that you have been intercepting messages not intended for you. Penalties for infractions include heavy fines and imprisonment...