A finite natural resource with a wide range of uses
The radio spectrum is a finite national resource. It is the backbone for a wide range of activities in sectors including telecommunications, broadcasting, transport, defence, public security, emergency services, research and development, scientific services and hobbies/leisure uses. Such radio services play an important role in the communications infrastructure of a modern economy.
Transcending national borders
Radio waves do not stop at national boundaries, yet we expect to use the radio spectrum without suffering interference. Interference free use is possible due to international agreements on the use of the radio spectrum which national administrations implement in their day-to-day management of the spectrum.
Different properties at different frequencies
Different uses make use of different radio frequencies due to the properties of the radio waves operating at those frequencies. Hence low frequencies were used for large area transmissions (LW and MW radio, shipping and aircraft radio beacons)
Local transmission can be achieved using higher frequencies (VHF-FM radio, private business two-way radio).
The lower frequencies penetrate walls better than higher frequencies, but require very large aerials for effective transmission. Higher frequencies do not penetrate walls as easily, but require smaller aerials for effective transmission.
Different bandwidths for different uses
The broadcast radio and two-way business radio cases only carry one voice/sound stream in a transmission. These do not take up too much spectrum. Services such as TV signals require a larger bandwidth, that is they require more spectrum per stream. Therefore TV transmissions had to be placed at higher frequencies.
Newer digital mobile phone systems combine many voice and data streams from different users into a single transmitter signal and thus also require higher frequencies. The more data we want to fit onto a radio signal the larger its bandwidth and the more spectrum it needs.
The varying demands of large data bandwidth, penetration into buildings and small aerials all have to be balanced in the choice of radio frequency for a given use.
Decisions on frequencies and uses
Manufacturers, operators and governments must make decisions on the most appropriate frequencies, that is parts of the radio spectrum, to use for different requirements. Economies of scale in manufacture suggest that globally valid decisions must be made. These global decisions are made under the auspices of the International Telecommunications Union at World Radiocommunications Conferences.
Some decisions remain at national level and European level. National spectrum policy sets out the desired results of national level decisions.
Microwaves and dishes
Satellite and Radio link systems are a specific subset of equipment which use frequencies offering high bandwidth but which do not penetrate well. However by using aerials called dishes, which are sometimes very large, the signals can cover large distances once they have clear vision (line of sight) between the transmitting and receiving dishes. Satellite signals travel up to satellites which are generally located about 36000km from earth.
Short waves; a special frequency range
One specific range of frequencies exhibiting properties which are unusual and run contrary to the general rule are called short waves. Short Waves (SW) were used in the early days of broadcasting before satellites were invented to transmit radio programmes to locations in other continents. Though there are still some SW broadcasts, they are not the main focus of the sector these days. SW spectrum is still used by defence, maritime and aviation sectors and by amateur radio (Ham Radio) enthusiasts. Radio amateurs are private individuals who are licensed to use various radio frequencies for self-education and training in both hobby and humanitarian/civil society assistance contexts. Further information is available from the Irish Radio Transmitters Society. They are licensed by the Commission for Communications Regulations (ComReg).
Terminology used over time
Over the years different frequency bands have been given various names by which the general public may know them, however sometimes these have changed with time, as in the early days of radio the frequency was associated with its wavelength. The wavelength and frequency are inversely proportional. Nowadays it is more common to refer to a frequency or frequency band.
Long Wave (LW) (for example 1500 meters) mighty now be called Low Frequency (LF), at around 200kHz, Medium Wave (MW) (for example 300 meters) is now called Medium Frequency (MF) at around 1000kHz. On broadcast radio receivers these bands, though mainly MW, may both be labelled "AM". Short Wave bands from 80 meters to 10 meters are referred to collectively as High Frequency (HF) bands by radio amateurs. When broadcast radio services around 100MHz first appeared they were said to be in the Very High frequency (VHF) band, though these days this is more likely labelled FM. Nobody has ever referred to these frequencies as in the 3 meter band!
When initial TV services appeared consumers may have "tuned" to channels A to J which were on the VHF bands (near 55 MHz or 200MHz), staring the trend of not quoting a frequency or wavelength. Later on TV services were put in the Ultra High Frequency (UHF) bands around 500-800 MHz (that would be wavelengths of about 50cm broadly speaking). The UHF TV services were often more commonly referred to by "channel numbers" between 21 and 69. More recently the frequencies above 800MHz (channels numbers 61-69) and by 2022 those above 700MHz (channel numbers 49-60) have been given over to supplement the bands for mobile phone services. This has been called the digital dividend.
Modern equipment does not need the user to select a service by frequency
Frequencies need not be explicitly known by consumers for systems using frequencies high than those mentioned above. Modern equipment such as Saorview TV receivers and other digital broadcasting service receivers will often offer a general scan to memory without the user needing to "tune in" each service each time they wish to change radio service.