DIRECTORATE OF TELECOMMUNICATIONS
Sub Surface Radio
By: Peter Bridgeman
With the 1960’s came a new generation of portable radios such as the ULTRA 3A4 VHF packset and, at the end of the decade, the UHF PYE Pocketfone.
These new radios offered new opportunities for specialised radio communications including innovations to support fire fighters working below ground or within highly screened environments.....
My first experience of this work was with Andy Twose at Shapwick where as two young wireless technicians we were dispatched by the then depot C.W.T Harry Bollands to an overnight Fire Brigade exercise in the Severn Railway Tunnel. Our role was to erect a scaffold gantry across the tunnel entrance at Severn Tunnel East, the Bristol Pilning Junction entrance. Then to rig an 18 element yagi suitably arranged to beam signals into the tunnel mouth from a W15U Westminster. A test train with a Brush diesel locomotive and flat bed wagons then took the brigade fire fighters equipped with pocketphones into the tunnel and the fun began.
Needless to say communications were excellent for a mile or so, until the train passed beyond the first corner and then signals were lost abruptly, occulted as efficiently as the rear red light on the last wagon. Andy and I blamed the ½” of soot on the tunnel wall.
Sadly nothing remains to record this exercise or I suspect of any other experiments carried out by the Directorate around this time.
Similar Severn Tunnel exercises have since been re-enacted with a recent exercise being supported by RAYNET.
Around the same time Directorate HQ (then at Kew) received from ACFO an operational requirement for a direct point to point radio communications system specifically to support fire fighters working in basements, tunnels or other screened environments. This led to a feasibility study managed by Wyn Crompton and Bruce Thompson, with Bruce becoming project officer within R & D section. The study was awarded to Plessey Roke Manor Research in 1971. The main objectives were to determine composition and specification of an equipment to meet the operational requirements then produce a prototype apparatus suitable for engineering and operational trials. The equipment was to become known as a Fire Ground Apparatus for Radio Operations, or by the more romantic anachronism F.I.G.A.R.O.
To practically assess the magnitude of building attenuation, Plessey made reception assessments of broadcast stations in the Home Office CCE Harrow (1) basement areas. This demonstrated that only MF and VLF stations could penetrate to the basement and be heard. A further practical trial was conducted using a 26 kHz SSB transmitter energising a 50ft diameter ground loop to cover the basement areas. Good coverage was obtained over the upper and lower basement areas with a portable receiver using a 2ft loop, however high levels of noise from the nearby electrified railway proved to be a significant problem.
Theoretical studies and calculations suggested frequencies of 150 kHz and 3 MHz would offer theoretical advantages, and thus further experiments were conducted on these frequencies. Finally significant reductions in noise were observed when using small square magnetic loops at the 3 MHz frequency.
At the culmination of the study Plessey finally recommended that frequencies between 2 and 8 MHz should be chosen for the system and Frequency and Site planning Group began to search for available spectrum. Subsequent negotiations with the CAA produced three pairs of aeronautical frequencies around 2.7 and 3.4 MHz which could be shared with sub surface applications using the magnetic loop antennas. The final proposal for FIGARO was a 3 channel 10 watt PEP SSB transceiver using 1 metre square loops, one variant worn as a manpack with an integral loop stitched in a jerkin, the other as a base station using two independent fixed loops fixed a right angles for transmit and receive. Two frequency operation provided the option for the base station to act as a repeater. A “confidence” PIP tone was added to the base transmitter to confirm range. By 1974 Plessey had delivered several prototype equipments listed as PRD2200 and 2201. The project officer was now ready to commence engineering and operational trials.
Around this time Alan Taylor and I joined R & D section, being recruited to help with various projects including assisting Bruce with FIGARO. Initially engineering trials were conducted in the CCE basements to replicate the earlier Plessey tests. Later operational trials were undertaken together with London Fire Brigade (LFB) in LT tube tunnels and deep shelters, with Manchester Fire in factories and with Cornwall Fire in tin mines. As time went on tests were also conducted in shopping centres, factories, the Fire Service College, MoD bunkers, war ships, cargo ships and at UKWMO sites.
For Alan and I there began numerous adventures in sometimes lonely, usually dark and dripping wet locations, sometimes flooded underground spaces, smelling musty with the ever present water noises and strange rumbles coming through the ground … all the time one straining to hear those somewhat ghostly pips permeating through the tons of earth and rock above. Certainly not a task for the superstitious. The trials went on until from 1974 to late 1975 after which other projects began to take precedence for us both.
FIGARO propagation relied heavily on the magnetic fields produced by the loops and the normal range, whether below ground or in the open, was restricted to a few of hundred metres. However in practice the fields often coupled into cables, pipes or other conductive media which would carry the signal further and often far underground. Unfortunately this made the performance of FIGARO very dependent on the good positioning of the base station loop and success or failure in a situation could be a matter of a few metres one way or another. During one notable trial in the old City and South London railway tunnels the range to the base station was no more than 10 yards and well within voice range, such being the attenuation in a steel tube below its cut-off frequency. However, when we tried to measure this distance using a steel measuring tape the signals were restored as the tape neared the receiver. We found that propagation continued for another 5 yards from the end of the tape.
By way of example, placing the base station loops near to the pit head cables permitted signals to reach the lower levels of South Crofty Tin mine but moving them away into open ground caused the signals to fade. South Crofty was a very wet mine served by two tier cage, known tenderfoots enjoyed an exhilarating free fall drop of some 1000 ft … once we stood on the lower workings our guide calmly informed us that if the power failed the pumps would stop and it was impossible to climb out quickly enough using the emergency ladders before one drowned !
Similar results were obtained at many other locations and it became evident that successful deployment in most situations depended on the base station loop antenna being placed by an experienced person. Also, in practice the packset and its jerkin proved to be a rather unwieldy and cumbersome burden which worried users already adorned with heavy jackets and Breathing Apparatus. Nevertheless FIGARO did provide voice communications to highly screened environments in situations where VHF/ UHF were unsuitable.
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FIGARO was deployed by London Fire Brigade at the scene of the Moorgate disaster where it supplemented communications from the station entrance to personnel working on the wreck below. FIGARO was also tried by the New York City Fire Service and reported in an article entitled “The marriage of FIGARO”. Here a telephone line was used to remotely control the base station over the streets of New York. In the late 1970’s operational trials continued with the production units supported by HQ and Field Service staff. This work was supported by Cliff Wilson, including trials at power stations and another trial in the Severn Tunnel. The activities are reported in Intercom No 10.
The photograph above shows the prototype FIGARO equipment delivered in 1974. Prototype antennas not shown . The equipment could not be made intrinsically safe due to the high transmitter power.
One advantage of ssb was that operators could transmit at the same time and still be heard. The interference being non destructive and is similar to listening to several speakers in the same room.
The last production FIGARO set is now on display in the Fire Service College museum. All prototype units eventually went for disposal.
The yellow production version of FIGARO shown above was very robustly made although very similar electrically to the prototype. The top picture shows the base station and loop antenna.
The lower picture show the packset jerkin with loop stitched into the edges and pouch for packset worn at chest height. The packset was VOX operated
After the D/Tels trials Plessey produced a production version of FIGARO at their own cost and the equipment was made available to the market. Unfortunately the equipment proved expensive and this compounded with the uncertainties of base station antenna deployment gave brigades with a somewhat difficult investment decision.
Consequently very few were actually sold, however because of the obvious risks in the London area, one set was purchased by LFB and another by HMI (Fire) more or less as a build standard. London Fire Brigade maintained FIGARO in a state of readiness for several years.
It is believed FIGARO was stood down in the mid 1980’s when the growing use of radiating coax generally rendered the system obsolete.
THE HOME OFFICE LINE LINKED REPEATER
By 1990 radiating coax (leaky feeder) systems were installed at a majority of high risk public locations thus permitting the use of standard handheld and intrinsically safe Fireground radios. However there remained exceptions to this situation in large sewers, foot tunnels and basements etc where no radiating cable was present rescues might be required. Also, radiating feeders can be severed by accidents and thus communications might be denied to the very spaces where it is most needed.
Following the privatisation of D/Tels the responsibility for sub-surface communications became vested in Home Office F7 Division, later to be known as RFCPU. Mike Phillips was to look at new developments in technology, at first supported by Brian Murgatroyd and later by myself. By 1992 it was evident that there was little available off the shelf. However, it was known that LFB had a temporary leaky feeder system based on 80 ohm TV feeder and a portable UHF repeater. Whilst this system was functional there were limitations as to the length of coax which could be carried, mainly due to weight and cable attenuation.
It was thought than an alternative cost effective idea would be to use a simple twin paired line or audio bus into which simulcast (QSFM) two frequency radio repeaters could be jacked in. There being no project finance to develop the idea, Dave Booker (NTL seconded to RFCPU) began the task of making four prototype UHF FM repeaters which were fabricated from surplus Phillips PFX radios in diecast boxes. Dave did a sterling job and the prototype system was successfully tried at the old RSG’s at Kelvedon Hatch and Cowley, Chislehurst Caves and Aldwych. We found that intercarrier beats were not usually a problem below ground, especially if the repeaters were located at range limits.
A production system was needed to take the concept forward to operational trials and following a competition an order was placed upon Cytron Ltd of Crawley. A production prototype was made and tried out by several brigades at many location, this time including mines .. again with much success.
With the changing times the Home Office RFCPU formalised its radio type approval procedures and required compliance testing for all equipment. Unfortunately Cytron had chosen radio modules which although had an FCC approval did not have UK type approval. The cost to finance compliance testing for our approval could not be met by the company unless an order was promised. There being no orders forthcoming the system could not be marketed and in about 1996 the only working system available was passed to LFB for safe keeping and emergency deployment below ground at their discretion. It is believed that the equipment has now been stood down.
It is ironic that just a few years later that the RTTE Directive came into force whereby manufacturers and suppliers could take more type approval responsibility upon themselves.
(1) Photographs of the Harrow basements with some interesting history of the site can be found at: