# Radio Signals - Properties and Uses

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?

### 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.

 KHz KiloHertz 1 x 103 Hertz MHz MegaHertz 1 x 106 Hertz GHz GigaHertz 1 x 109 Hertz

#### 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:

Frequency Range Abbreviation Full Descriptive Title
300 to 3,000HzULFUltra Low Frequency
3 to 30KHzVLFVery Low Frequency
30 to 300KHzLFLow Frequency
300 to 3,000KHzMFMedium Freqency
3 to 30MHzHFHigh Frequency
30 to 300MHzVHFVery High Frequency
300 to 3,000MHzUHFUltra High Frequency
3 to 30GHzSHFSuper High Frequency
30 to 300GHzEHFExtreme High Frequency

### 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:

 Morse signal    _ . _ .    _ _ . _    _ . .    .    . _ .    . .    . _ _ | y=Signal Amplitude | |  ||| | ||| |   ||| ||| | |||    ||| | |    |    | ||| |    | |    | ||| ||| |  ||| | ||| |   ||| ||| | |||    ||| | |    |    | ||| |    | |    | ||| ||| |  ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||   x=Time |  ------------------------------------------------------------------------ |  |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||| |  ||| | ||| |   ||| ||| | |||    ||| | |    |    | ||| |    | |    | ||| ||| |  ||| | ||| |   ||| ||| | |||    ||| | |    |    | ||| |    | |    | ||| ||| |

#### 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:

 Morse signal      _______        ________        _         _______ | y=Signal Amplitude | |    ||||||||||||||||| | | | | |||||||||||||||||| | | | | |||| | | | | |||||||||||||||||| |    ||||||||||||||||| | | | | |||||||||||||||||| | | | | |||| | | | | ||||||||||||||||||   x=Time |------------------------------------------------------------------------------- |    ||||||||||||||||| | | | | |||||||||||||||||| | | | | |||| | | | | |||||||||||||||||| |    ||||||||||||||||| | | | | |||||||||||||||||| | | | | |||| | | | | |||||||||||||||||| |

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.

### 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.

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:

BandFull Descriptive NameFrequency RangeNotes
LWLong Wave150 to 529 KHzThese two bands cover the upper LF and lower MF bands, which normally span a frequency range of 30 to 3,000 KHz
AMAmplitude Modulation 530 to 1,710 KHz
SWShort Wave3 to 20 MHzCovers the majority of the HF band, however is of very limited practical use as no commercial stations broadcast in this band. Note: See the final paragraph in this entry before exploring the SW band
FMFrequency Modulation87.5 to 108 MHzCovers a very small percentage of the VHF band, marked as FM on the dial simply because the commercial stations use Frequency Modulation in order to improve reception in this band. The full VHF band stretches from 30 to 300 MHz.

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:

#### 30 to 300KHz - Low Frequency (LF)

 60kHz Time signal (UK) 136kHz Radio Ham, 2km band 153 to 279kHz Civilian LW AM broadcasts (Usually seperated by 9kHz bands) 153kHz - Germany162kHz - France171kHz - Russia, Morocco177kHz - Germany183kHz - Germany189kHz - Italy198kHz - BBC Radio 4216kHz - France, Norway234kHz - Luxembourg243kHz - Denmark252kHz - Eire (Atlantic 252)261kHz - Moscow279kHz - Belarus 283.5kHz Maritime and Aeronautical Navigation beacons

#### 3 to 30MHz - High Frequency (HF)

 3.016MHz Air Traffic Control - Mainly civilian traffic in airways 3.023MHz SAR5 RAF Search and Rescue Frequency - Nighttime use 3.023 to 3.230MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 3.401 to 3.497MHz Air Traffic Control - Mainly civilian traffic in airways 3.800 to 3.950MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 4.651 to 4.696MHz Air Traffic Control - Mainly civilian traffic in airways 4.700 to 4.995MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 5.000MHz Reference Frequency and Time Signal 5.450 to 5.480MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 5.481 to 5.676MHz Air Traffic Control - Mainly civilian traffic in airways 5.680MHz SAR, RAF Search and Rescue Frequency - Daytime use 5.684 to 5.726MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 6.526 to 6.682MHz Air Traffic Control - Mainly civilian traffic in airways 6.685 to 6.763MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 8.816 to 8.960MHz Air Traffic Control - Mainly civilian traffic in airways 8.965 to 9.037MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 10.000MHz Reference Frequency and Time Signal 10.006 to 10.096MHz Air Traffic Control - Mainly civilian traffic in airways 11.175 to 11.271MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 11.276 to 11.396MHz Air Traffic Control - Mainly civilian traffic in airways 13.200 to 13.257MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 13.261 to 13.357MHz Air Traffic Control - Mainly civilian traffic in airways 15.000MHz Reference Frequency and Time Signal 15.010 to 15.097MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 17.901 to 17.967MHz Air Traffic Control - Mainly civilian traffic in airways 17.970 to 18.027MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 20.000MHz Reference Frequency and Time Signal 21.870 to 21.924MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 21.925 to 21.997MHz Air Traffic Control - Mainly civilian traffic in airways 23.200 to 23.350MHz Air Traffic Control - Mainly military traffic, operating outside recognised airways 25.000MHz Reference Frequency and Time Signal 25.500 to 25.600MHz Used by Radio Astronomers 26.3125MHz Unapproved6 French imported cordless phones 26.965 to 27.705MHz Standardised European FM-CB7 Channels. 40 Channels in total: Channel N°1 is at 26.965MHz,Channel N°2 is at 26.975MHzetc... with channel seperations of 10kHz (0.01MHz) 27.500 to 28.000MHz Weather balloons 27.601 to 27.991MHz UK Licenced (Legal) FM-CB Channels.40 Channels in total: Channel N°1 is at 27.60125MHz,Channel N°2 is at 27.61125MHzetc... with channel seperations of 10kHz (0.01MHz) 28.000 to 29.700MHz Unlicenced Radio Ham - 10m Band

#### 30 to 300MHz - Very High Frequency (VHF)

 MHz 35.000 to 35.250MHz Model Aircraft remote controls (26x10kHz channels, N°s 60 to 85) 36.700 to 37.100MHz Cordless household audio devices (cordless headphones/speakers etc. 39.000 to 39.200MHz Radio Hams, Meteor Burst Comms (Bouncing radio signals off meteor storms to increase their range) 40.500MHz Emergency channel 40.665 to 40.995MHz Remote control model cars and boats (34x10kHz channels) 47.000 to 48.800MHz Short range devices, Pagers, vehicle alarms, cordless phones, radio microphones etc. 49.820 to 49.980MHz Baby alarms (short range - 10mW max.) 50.000 to 52.000MHz Radio Ham, 6m band (can vary dependant on the country). 70.000 to 70.500MHz Radio Ham, 4m band, Class A or B licences required (no novices) MHz MHz MHz MHz MHz MHz MHz MHz MHz MHz MHz

### 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...

1AM is sometimes marked as MW on some dials, it is named AM simply because the commercial radio stations use the older Amplitude Modulation system to transmit their signals2ILR - Independant Local Radio3INR - Independant National Radio4RSL - Restricted Service Licence. A commercial licence, granted on a short-term basis (max. 28 days) for short range transmissions (max. 3-4km)5SAR - Search and Rescue6Unapproved by British Telecom that is7CB - Citizens Band Radio - usually low power, short range transmitters, no requirement for operator licences