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Ross Revenge
Tx Room

Five and Ten
Ampliphase Ten

100kw (RNI)
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*** Welcome to the Transmitter Hold ***

Radio Caroline and her sister stations broadcast from the Ross Revenge from 1983 to 1991 using three different AM transmitters. However, here in the online transmitter room we have put together comprehensive information, not only on those three trasnmitters, but also on many others used by other offshore radio stations, as well as similar transmitters used elsewhere. Without doubt this online Ross Revenge transmitter room contains the webs most comprehensive source of information about RCA's "Ampliphase" line of transmitters. Not only do we discuss the principles of operation of this rather obscure system but we trace the history of this technology with rare information on "Ampliphase" style transmitters from long before they were made by RCA and long before the "Ampliphase" name had been adopted and take the story right up to the present time where the technology has been adapted into many of todays cutting edge digital communications devices. We also have extensive information on the Continental Electronics range, as used on the previous Radio Caroline ships, the Fredericia and Mi Amigo. Feel free to explore around and follow the links on the left hand menu.

Upon entering the transmitter room from the main deck, your view is dominated by the fifty thousand watt RCA Ampliphase set as shown in the above picture, but along two sides of the room there are smaller five and ten kilowatt RCA sets. You can just see the end panel of the five set, behind the component drawers to the right of the above picture. Did you know the BTA-5G five kilowatt set, as used for shortwave broadcasts originally came from a classical music station in Washintgton DC? During its 19 years there, it survived through a studio fire, planes crashing into the mast, and was listened to by Pres. J.F. Kennedy. And did you know the Continental Electronics set ordered by Radio Caroline in 1965 for the Mi Amigo never actually made it to the ship? It in fact went to Africa for a number of years and on its return to the Uk was used to jam the broadcasts of Radio North Sea International in 1970?

The main transmitter is the RCA Ampliphase BTA-50H, manufactured in 1963, purchased from Quebec in Canada in 1982, and used until it was taken out of service in such an undignified way in 1989. If you have only ever seen Carolines blue Ampliphase, you may be surprised to know that most of them were brown. And if you've only ever seen brown ampliphases before, you may be surprised to see that Carolines is blue. You may be equally surprised at the amount of steelwork and angle-iron required to form a frame to hold the transmitters in place in the above picture. Transmitters of course, usually either sit ontop of mountains, or beside rivers in flat fields in a nice stationary building. They are not normally expected to be installed onboard a rocking and rolling ship in the North Sea and thus have no means of fixing to the floor. If your transmitter room floor often sits at an angle of 20 or more degres to the horizontal, and is known to make the accasional foray to 45 degrees, then something more substantial than gravity is required to stop everything from moving around.

And here I am standing in front of a brown one!

When the station returned in 1983 the Ampliphase was operated on 963Khz for the only service then broadcast from the ship. During this time it was normally run at about 35kw of power, though occasionally it was operated at the full 50kw. The reduction in power does not make a huge difference in reception to most listeners and in many cases would not even be noticed. However, it makes a significant difference to the life expectancy of the output valves in the transmitter, as well as reducing the voltages and currents which have to be radiated by the antenna system. In an atmosphere with a high saltwater spray content any reduction in radiated voltages and current is a welcome one. A reduction in power can also lead to an improvement in fuel consumption on the generators and a reduction in maintenance costs for them. From day one, a ten kilowatt set was available as a standby should the fifty develop any major faults, but this was not often required to be put on air. During 1984 some test transmission were radiated on other frequencies and as from late that year, a new Dutch programme service, Radio Monique hired the 50kw transmitter on 963 and the english language programmes of Radio Caroline moved to a new frequency of 576Khz, radiated from the ten kilowatt set. At about this time, an identical ten kilowatt set was obtained from South Coast Radio in Ireland and parts from this allowed some upgrades to be made to the existing ten, and for the five kilowatt set to be repaired. Prior to that the five had been in a poor and much dismembered condition and it is generally believed it was obtained ex-gratia at the same time as the ten and fifty were purchased.

To allow both services to be radiated from one antenna system, a "diplexer" had to be built. Although simple in theory, a diplexer to combine two high powered radio transmitters and match these into a single antenna is not a simple beast to design or set up. This is made even more difficult when operation is required on a ship, with the limited technical facilities and budget available. Within the components of the diplexer RF currents as high as 100 amps and 25,000 volts may be present so its construction is a feat of engineering. Following the introduction of this second service, the fifty was reduced to approx 27 kilowatts of power, and the ten was operated at about 7 kilowatts. Although there was a significant difference in operating power, the lower frequency of the english service allowed it to cover a substantial distance and in some areas coverage was improved despite the drop in power. Tests were conducted on 585Khz at times, presumably to test its suitability as a frequency during darkness, but no doubt also to increase the separation from RTE on 567Khz. However, following the demise of Laser 558 in November 1985 Caroline quickly adopted that channel and for the next few years was to go through a relatively stable and successful period.

The Diplexer as used to combine 963 and 585Khz in March 1985.

One of the most striking features of the ship upon her arrival upon the North Sea in 1983 ws the magnificent 300 foot mast. Generally regarded as the tallest mast ever upon a ship this was a remarkable feat of engineering. The base of the mast ran thorugh the ships main deck, down to the lowest level of the former cargo hold. Here it was held in place by 300 tonnes of concrete - acting not only as a counterbalance for the 28 tonnes of tower but also as a level, stable and solid floor upon which to mount the transmitters and generators. With the base of the tower physically and electrically grounded to the base of the ship, it was electrically operated as a shunt fed folded unipole. From the massive main insulator, a "skirt" of antenna wires rose to a point approximately three quarters up the tower where they were electrically bonded to the mast. Although slightly less efficient than a "conventional" base insulated vertical radiator, it was mechacnically much easier to erect without the need to stand on an insulator, and being directly grounded gave less problems with static build-up and lightning strikes. However, the closeness of the skirt to the structure could have significant effects on the reactance of the system, and ultimately led to a restricted bandwidth when the second service was introduced, firstly on 576 and later on 558 Khz. Shortly before the 300foot mast collapsed in 1987 the deicsion was made to introduce a commercial shortwave service. Various ad-hoc shortwave transmissions had been made previously, some of them "unauthorised" such as the "Caroline goes DX" broadcasts in the 48 metre band on a Sunday morning. These were made from the ships original ship-to-shore HF transmitter system and a simple longwire antenna, but for a commercial service, the five kilowatt transmitter was modified for SW use and a new antenna built. The shortwave antenna took the form of an "Inverted V" suspended from the lower cross-tree on the main mast. Airtime on this service was sold to a number of international religious clients.

Tower of Power - 1983
Mastless - November 1987

However this was not to last, as following hurricane force gales which wreaked havoc across southern England in late October 1987 it was noticed that some structural damage had occured to the mast. Many of the "egg" insulators had been cracked due to the stress imparted, leaving the guys wires slack. Other guy wires were simply stretched leaving the mast to vibrate and oscillate. Plans were immediately drawn up for major maintenance, however before this could be effected, just a few weeks later during a North-Easterly force 11, at approx 2:30am the base of the mast cracked, causing the whole structure to topple over the side of the ship, still attached by numerous guy wires. At first light the ships engineers had no option but to cut the remaining wires with the oxy-torch and angle grinders and allow the mast to sink to the seabed. During the hours when the mast had been over the side, the ship had been severely listing, taking much water over the deck, and the crew had been on a full state of alert, ready to abandon ship if the situation had deteriorated further. Over the next two years a number of temporary antennas were used, but none of them could come close to replacing the signal radiated by the original tower.

The Processing Rack c.1993
Audio for the transmitters was processed by a variety of different systems. In the single service days a then revolutionary unit, known as an Orban Optimod, type 9000A/1 was used to dynamically compress and pre-process the audio prior to transmission. By compressing the dynamic ramnge in different frequency bands as well as applying overall broadband gain control the average programme density could be significantly increased. This has the audible effect of making the programme sound louder thus masking noise and interference picked up en-route to the listener. By thus increasing the average average loudness and modulation depth, the overall envelope power of an AM transmitter is also increased, giving more watts into the air. This also has the advantage of further reducing the background noise and appearing to improve the range of the signal. The Optimod also applies some frequency selective pre-emphasis onto the audio, to give the bass more punch without overload, and to compensate for high-freqeuncy roll-off in the majority of consumer radio receievers. These features coupled with the use of low-distortion phase compensated clippers, gave the Optimod a classic processing sound. When used with the good audio response of the ampliphase transmitter, the wide bandwidth single service antenna and Carolines lack of 5KHz audio filters as used by other European stations, during 1983 and 1984 Caroline achieved better audio on AM than many of the low budget fledgeling local stations did on FM at that time. Although there are some who state that Carolines Optimod was one of the first in Europe this is probably not true. Both Luxembourg and Sunshine Radio in Dublin already had these units installed before Caroline returned to the air, and the BBC had them installed at a small number of local stations where the penetration of the signal and audience retention was poor. Both BBC Manchester and BBC London are believed to have had Optimods fitted in 1983. By 1984 or so, Radios 1 and 2 both used Optimods, though they used single units at Broadcasting House prior to programme distributuion and not a unit interfaced into each transmitter. However, Carolines 9000 is possibly the oldest in Europe, as, apart from Luxembourg, most others were of the later 9100 model type.

With the introduction of a second programme service a second later generation 9100 processor was obtained and this was used on the 558 service. In addition to these two "state of the art" (back then) processors, a number of older compressors such as Marti CL40 limiters and a tri-band Pacific Recorders Multimax were also available. These were used either as pre-processor, or for the shortwave service. However, feeding heavily processed audio to older transmitters can have drawbacks, particularly to the plate modulated sets. As these were never designed to cope with such densely processed material at a high level, failure of parts of the modulation amplifier were not uncommon. In the early days of a split service a modulation transformer had to be replaced, and the modulation choke frequently flashed over to the grounded metal frame. Eventually, the choke was mounted "outboard" on a set of ceramic insulators, from where it could flash over as much as it liked without getting upset. Latterly the processors were all mounted in an equipment rack alongside the ampliphase transmitter, but in the early days, the single Optimod was fitted in a smaller rack, which simply sat in the corner of the room. It was known on occasion to slide around, and once managed to unplug itself as it wandered across the floor during a storm, thus taking the station off air. Eventually, after falling over and damaging the main meter, the new rack was built and welded into the steel framework holding the transmitter in place. The audio rack also holds the "off air" monitors which are basically high quality radio receivers without a tuning dial. These are used to monitor the antenna output of the transmitter, and the audio from these is used to power the studio monitors and DJ's headphones. Thus any distortion or other transmission problem will be immediately noticed. The original off air monitoring was by means of Gates M-5693 units, a classic 1960's design identical to the ones previously used on both the Mi Amigo and Fredericia, but these were eventually replaced by a home designed unit with dual RF demodulator stages for the two services.

Classic 1960's Gates M-5693 Modulation Monitor. As used by all three Caroline ships.

To be continued................ !!