United Kingdom - The First RSLs

written by Norman McLeod - published in Medium Wave News around 1999
Republished with kind permission of the Medium Wave Circle

Nowadays, there are over 300 RSLs (restricted service license) a year and we tend to take them for granted. Almost any excuse can be offered for a temporary radio service and almost any person or group capable of raising the funds can secure a licence. But it was not always so. Back in 1983 there was no such thing as a temporary licence for normal, freely-radiating transmissions, but it didn't stop radio journalist Peter Laverock from applying to cover the Greenbelt Christian arts festival ni Northamptonshire using the only technology available to him - an inductive or 'leaky feeder' system akin to those used by university radios.

Induction loops and leaky feeders

Since 1968, where the first station was established at the University of York, students on a campus-based university or college have been able to broadcast to each other on AM using a system which effectively confines the signal to the area within the perimeter of the campus boundary. Since the early eighties hospitals have also been able to exploit this system. Most often stations use loop aerials just under 1m square and in the Wireless Workshop system (of which there are about 40 installations across the UK) each loop is driven with 50mW of carrier power from a small two-transistor amplifier, fed with DC and RF from a coaxial cable, and placed in the corner of the loop. This provides useful reception on ordinary radios up to 20 or 30m away from each loop depending on the characteristics of the building where the loops are installed.

The aim of the installation is to provide signals comfortably above 70dB/uV inside the buildings to provide some protection from interference, particularly at night, while restraining the field strength beyound the boundary of the licensee's premises to 48dB/uV or less in order to meet the licence conditions, which were obsessive about the need to protect outside parties from the horrors of student broadcasting. These days experiments are being run to see if freely-radiating transmitters can provide a service to student and hospital radio, but in the early eighties this was not even a remote possibility. Commercial radio was strictly regulated by the IBA, while the Home Office (dealing with programming) and the DTI (on engineering) ruled student and hospital radio directly.

However, in the absence of buildings the induction field performs poorly, and Peter Laverock's 1983 experiment in leaky feeder to cover an open-air festival site was not a success from a technical point of view. The bid for 1984 was for a freely-radiating system without the handicaps of induction which would for the first time break the BBC/IBA monopoly on the airwaves.

Negotiating the breakthrough

It was during the period between summer 1983 and summer 1984 that I spent some time, together with my colleagues in the then Wireless Workshop (Alan Brown and Tim Foulsham), negotiating the conditions under which Greenbelt 1984 would be licensed. It's fair to say that both the Home Office and the DTI were very concerned about covering themselves against both real and imaginary complications resulting from letting the hoi polloi loose on the airwaves. But at the same time they saw the potential of radio services to add to the enjoyment of open-air events, and in the end we got our licence - for 50 milliwats of ermp on 1602 khz - and Special Event Radio, as it was then called, was born.
Geluidsfragment Extract from Green Belt Festival Radio from 27 August 1984 on 1602 kHz (courtesy of Clive Rooms)   00:36, 16 kbps, 72 kB (MP3)

Power and distance

We had spent some time developing a 10W FET linear amplifier to add to our induction loop radio system. This was capable of providing up to 10W of carrier power, adjustable in 1dB steps, to the antenna, which was restricted to 10m in height. To improve the efficiency of the antenna, toploading was applied using an inductor at the top of the mast and a capacity hat. Typically we found that to produce 50 mW of effective monopole radiated power, something we agreed with the DTI to be equivalent to 66dB/uV/m at a distance of 1km, we needed something of the order of 2 W of RF power into the antenna. Power is (sensibly) measured in terms of the field strentgh at 1km. 66dB (microvolt/metre) corresponds to 50mW emrp, while 79db represents 1W ermp. With the mast restricted to 10m height, aerial efficiencies of the order of a few per cent are typical, so it is not unknown for up to 50 W of carrier power to be used up in producing just 1 W of ermp, although more usually 10 or 20 W will suffice.

Evaluating the results

In feedback to our lords and masters we reported that the low-power assignment of 50mW might not be sufficient for all purposes and we suggested that up to 1W might be required for larger events. This point was eventually conceded, although at a higher licence fee than the 50mW option. FM RSL's came along in 1987 and the system was formalised in legislation setting up the Radio Authority in the late eighties. But in the early days, caution was the watchword and every licence had to be individually negotiated.

The second licence - Silverstone

Although I played a major part in securing the Greenbelt 84 licence, ill-health prevented me from attending the broadcast itself. I was, however, present during the second-ever RSL, the following year at the Grand Prix race track at Silverstone, for which we had secured the truly awe-inspiring power allocation of 100 mW!

I remember setting up the transmitter the night before and playing some test tapes over the air while securing reception reports via the local 2m repeater using my amateur radio callsign (G4PAQ) to contact local amateurs with communications receivers who were able to give me good reports at up to 30 or 40 km distant. This highlights the vast difference between domestic quality receivers (which require at least 60dB/uV/m for half-reasonable reception) and professional equipment connected to a good aerial (which can dig out signals 40 or 50 dB weaker). We did not want the DTI to be aware that people could hear the station this far away as it would have fed their paranoia to know this, even though only professional equipment could detect the signal!

The next day I was minding the shop in the studio when three gentlemen came in and announced that they were investigating a pirate radio station. I asked for their identification and found that they were indeed from the local RIS, and they had never heard of Special Event Radio. We, for our part, didn't actually have any paperwork on us which proved we could do what we were doing, but eventually we persuaded them by dropping the names of the engineers in Waterloo Bridge House in London that we were allowed to do what we were doing. Were they tipped off by an over-zealous radio amateur listening in to my reception reports the night before? I'll never know...

Representations for the future

In our feedback, we also put forward the idea  that what was then called Special Event Radio didn't have to be tied to a particular event, but could be used by just about anybody - including trial broadcasts by groups who wanted to try out a particular form of programming not necessarily tied to a 'special event'. As the number of SER stations grew without any serious programming or technical problems surfacing, the ground was set for the widening of the definition we know today, where RSL's can be set up for almost any reason, and quasi-permanent installations have been established at football grounds. Over 2000 RSL's have taken place since the pioneering days of 1984, and it is gratifying to know that so many people have been able to get a taste of real radio broadcasting.

External links

Green Belt Festival