An RSL is a Restricted Service Licence.
There are two types of RSL - 28 day licence and long-term licence
28 day licence
Most RSLs on FM (but have been a few on Medium Wave) have had a 28 day licence. On FM, up to 25 Watts is generally allowed with such a licence, enough to cover a relatively small but still sizeable area from a decent location and on a clear frequency.
In London, in the early 1990s, high-power (kilowatts) 28-day licences were allocated to some foreign broadcasters.
28-day RSL stations tend to fall into one of 4 types:
Prospective full time licence stations
Prospective full time licence stations use RSLs to raise their profile, gain experience in broadcasting, and encourage listeners to lobby for a full time licence to be granted. Examples (past and present) have included:
Angel FM (Portsmouth area), Challenge FM (Wolverhampton) , Palace FM (Tamworth) , Vixen 87 (Market Weighton)
Special event stations
Some RSL's are for short term events. Religious organisations use RSL stations celebrate religious festivals such as the Muslim festival of Ramadan or the Hindu festival of Diwali. Some special events, such as air shows, have RSL stations to publicise the event, broadcast what is happening at the event, and give traffic and weather reports to people travelling to the event. In the run-up to Christmas, some shopping centres such as Meadowhall in Sheffield have had RSL stations to give traffic reports etc. Student radio stations often have RSL broadcasts, particularly near the start of the student year ("Freshers week"), to give advice to new students.
Some RSLs are to raise money for charity, examples have included the Radio Cracker network in the early 1990s, and Radio for New Infants (RNI). A number of prospective full-time licence stations, and religious stations, have also donated some or all of their revenue to charity.
Non profit stations
Finally, non-profit stations such as hospital radio and student radio stations sometimes have RSL broadcasts to raise the station's profile in the community.
University radio stations have often had RSL's during Freshers Week, when students first go back to Uni after the summer term.
Some RSL's, generally on Medium Wave, transmit all year or on certain days throughout the year, but use very low power which is (theoretically) only receivable within one or two miles. Some football clubs broadcast on match days, giving traffic and other information to people travelling to matches at the stadium. Some hospital and student radio stations use low power RSLs as a means of transmitting throughout the hospital grounds or university campus.