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Terrestrial: Long and Medium Wave Radio
15 October 2009 - 10:13pm
Medium Wave and Long Wave broadcasting has existed now as a public medium now for well over 80 years. It was the first type of sound broadcasting via radio-waves available. Because of this, in its early days it was referred to as "the wireless".
Even in the 21st century broadcasts on Medium and Long Wave frequencies play an important part in the lives of millions of people across the globe even with much more advanced technology available.
Broadcasting on Medium and Long waves is done by a system of Amplitude Modulation, or AM. What happens is that the carrier wave of the frequency is mixed with the sound frequency which rises and falls. This gives a waveform with various different peaks on each cycle.
This system is easy to make and cheap to produce, though it is prone to natural and man-made noise more so than FM.
The Long wave band extends from 148.5kHz to 283.5kHz, though some radios may have spaces on the dial that go up and over 300kHz.
For broadcasting, the Long Wave band is limited to being used in Europe, North Africa (Morocco & Algeria), the Asian part of Russia and Mongolia. The main reason why it is not used elsewhere is that these low frequencies are very prone to atmospheric disturbances like thunderstorms, and in other places like near the Equator these are a very regular occurrence.
The wavelengths on Long Wave Broadcasts are huge, ranging in and around 1000 - 2000 metres. To ensure that signals can broadcast efficiently, large masts are used, often extending to over 300 metres tall. The old Warsaw Radio mast, which collapsed in 1991, was over 646 metres tall.
The Long Wave mast used bye RTÉ in Clarkestown, Meath is almost 250 metres.
Most of the Long Wave transmitters are huge beasts, pumping out effective radiated powers of up to 2500kW. The RTÉ Long Wave transmitter is capable of powers of 500kW but at the moment runs at about 300kW during the day and 100kW at night. The BBC Long Wave transmitter in Droitwich in Central England runs at 500kW nominal.
The range of a Long Wave transmission can be big. The RTÉ Long Wave transmitter on 252kHz that carries Radio 1 can easily cover the whole of Ireland, Northern Ireland and much of Britain. The France Info transmitter in Alloius in Central France covers the whole of the county with 2000kW and can even be heard in many parts of Ireland during the day. In general daytime reception ranges can carry to around 300km from the transmission site for a high powered transmitter. At night time the signals can reflect off the E Layer of the atmosphere to allow them to travel further still, though this reflection is not usually as good as that of Medium Wave signals.
In very hilly and mountainous areas, Long Wave radio stations can often be received where Medium Wave and VHF cannot. This is because Long Wave transmissions easily follow the Earth's curve and can bend around large objects. Where possible transmitter sites for Long Wave broadcasts are usually in places where soil conductivity is good.
Irish Long Wave Transmitters
* 252kHz - RTÉ Radio 1, Clarkestown, 300kW day, 100kW night
The Medium Wave band is an internationally used band for broadcasting. It ranges from 525kHz up to 1615.5kHz, though this extends up to 1705kHz in the Americas.
Medium Wave broadcasts vary in range from low powered local radio stations to high powered national and international broadcasters. Within Europe there are stations on Medium Wave that have output powers ranging from 1W to 2000kW.
The wavelengths of Medium Wave signals ranges from 185 to 565 metres therefore the height of the masts compared to Long Wave don't need to be as big, especially for the higher frequencies.
For local broadcasting, Medium Wave transmitter powers range usually from 100W to 5kW depending on the coverage to be obtained which at this power limits good coverage to around 40km or less. For National Broadcasters, larger powers are used. The RTÉ Radio 1 Medium Wave transmitter at Tullamore is capable of broadcasting at 500kW, though at the moment this is only at 100kW. The old 2FM Medium Wave transmitter at Athlone had a maximum power output of 100kW. With these powers RTÉ can cover most of Ireland, though in places like Dublin with a high density of concrete buildings, and in places like the coasts, reception can be a little more difficult. International broadcasters use large amounts of power to be able to reach listeners hundreds of kilometres away. The likes of Radio Sweden use a 600kW transmitter while the Russian allocation of 1386kHz in Kalingrad can output up to 2000kW. These transmitters can end up covering much of Europe at night time.
During the day, Medium Wave reception is restricted to "ground wave" coverage. This is because the signals from the transmitter site which escape into the atmosphere are absorbed by a layer known as the "D" layer. Ground Wave coverage is where the signal follows the curvate of the Earth like Long Wave signals. The lower part of the Medium Wave band is better at travelling on a ground wave than the higher end. As a rough guide, a transmitter on 531kHz with an ERP of 1kW would cover roughly the same area as a transmitter on 1602kHz with an ERP of 5kW.
At night the D layer disappears. The layer above it, the E layer, instead of absorbing the radio waves, reflects them back down to earth. This allows stations that are not heard during the day to be heard at night. Typically in Ireland, stations from across Europe will appear on nearly every part of the band, from hundreds of kilometres away. In good conditions and with the right set up, stations from the Middle-East, India and even North America are possible to hear, though the main problem is that with so many Medium Wave stations on air, it can be difficult to hear a weak signal if a stronger signal is on or near the same frequency.
In places where VHF radio reception cannot be got, Medium Wave can often be received instead. This often includes hilly rural areas where like Long Wave, the Medium Wave signal can bend around objects. However in some mountainous areas, even Medium Wave signals can disappear; not only because of the size of the mountains, but also because of the conductivity of the ground which in these places is usually rocky and therefore a poor conductor. Like Long Wave, transmitter sites where soil conductivity is good are usually sought.
Irish Medium Wave Transmitters
* 549kHz - ICB (Unlicensed), Monaghan, 24kW?
* 567kHz - RTÉ Radio 1, Tullamore, 100kW (Licensed 500kW)
* 729kHz - RTÉ Radio 1, Cork, 10kW
* 846kHz - Radio North (Unlicensed), Quigleys Point, 2kW
* 981kHz - Radio Star Country, Emyvale, 1kW
There are also a number of unused and vacated frequencies assigned to Ireland. These will be published shortly.
Northern Ireland Medium Wave Transmitters
* 693kHz - BBC Radio 5 Live, Enniskillen, 1kW
* 720kHz - BBC Radio 4, Lisnagarvey, 10kW
* 720kHz - BBC Radio 4, L/Derry, 250W
* 774kHz - BBC Radio 4, Enniskillen, 1kW
* 792kHz - BBC Radio Foyle / Ulster, L/Derry, 1kW
* 873kHz - BBC Radio Ulster, Enniskillen, 1kW
* 909kHz - BBC Radio 5 Live, Lisnagarvey, 10kW
* 909kHz - BBC Radio 5 Live, L/Derry, 1kW
* 1026kHz - Downtown Radio, Knock Breckan, 1.7kW
* 1053kHz - Talk Sport, L/Derry, 1kW
* 1089kHz - Talk Sport, Lisnagarvey, 12.5kW
* 1215kHz - Virgin Radio, Lisnagarvey, 16kW
* 1287kHz - BFBS Radio 1, Various sites based in Army Barracks, 1W each site
* 1341kHz - BBC Radio Ulster, Lisnagarvey, 100kW
(Notes by Watty:
The Lisnagarvey mast was originally symetrical. The top was chopped to re-tune it for a frequency change many years ago before it was Radio Ulster, I think in BBC Home Service days.
Article originally written by Northern Correspondent on ICDG Wiki.