Part 2 advanced Air-Sea rescue systems
The emergency transmitter was a BC-778A Gibson Girl (fixed mounted on a special mounting plate), and the receiver an R-1545 providing two way telegraphy communication. Normally the kite aerial was employed but alternatively a fixed emergency aerial running through the mast could be used. No balloon aerial was used with this type lifeboat radio station. See the typical installation diagram and installation schedule. Note that by means of a switch the aerial in use was connected either to the receiver or transmitter. A separate aerial winch type 10 (thus not the one fitted on the Gibson Girl) was used for paying out the aerial wire attached to the kite. It was normally placed in position into a forward rimlock socket on either side of the boat, the choice depending on the wind direction.
On Mk. 1 and Mk. 1A lifeboats mountings for the receiver and associated Gibson Girl transmitter were provided on the port side of the lifeboat forward of the port engine hatch. The receiver was stowed separately in a hatch and should be placed in position before communication is commenced. In the later Mk. 2 airborne lifeboats the receiver and transmitter were permanently attached to a W/T shelf in a forward position of the boat. The interconnections between the units are similar to the earlier models.
The R-1545 was a self contained conventional TRF receiver with one RF stage principally for 500kHz operations, but tuneable from 470 to 530 kHz. The receiver is mounted in a resin-bounded plywood case and a detachable lid. It used type ARP12 or VP23 valves throughout, and operated on internal dry 45 V HT battery and an 2V LT accumulator. An interesting feature was the provision of a socket type 359 (8 in the picture left) for connection of the operator's own flying helmets phones should the provided headphones fail.
In the course of the war, Air-Sea Rescue systems were developed operating on other frequencies allowing the use of other means and avoiding the awkward long aerial wire, the latter absolutely necessary to obtain any range. These sets fall, however, still in the same category as the previously discussed equipment. New developments and employment of VHF made it possible to miniaturise the emergency radios, giving an opportunity to provide this equipment in dinghy-packs in single seater aircraft. Both the Allies and the Germans developed Air-Sea Rescue sets operating on VHF. However, technically and operationally they differed considerably.
Transmitter Type T-3180 or 'Walter' was a British Air-Sea Rescue homing transmitter beacon. Owing to its compactness and very light weight it was used in virtually any type of life-saving craft including single-seater dinghies type K, Q and S. It operated on the Air to Surface Vessels (ASV) radar beacon frequency of 177 MHz. It signals could be received by airborne ASV Mk. II radar sets or Rebecca Mk. IIb equipment in Coastal Command and Fleet Air Arm aircraft.
The T-3180 was a self-contained unit comprising a battery in waterproof case, aerial mast and a small oscillator/aerial unit. The power supply was fed from the battery case on the bottom to the oscillator unit by a length of cable running alongside the mast, An on-off switch was located on the battery case. The horizontally dipole aerial was part of the oscillator unit. The two dipole arms are hinged and spring loaded, in non-operational position lying parallel with the case assembly. When operational, the light weight telescopic mast was extended to a height of about 6 ft. The mast was fitted with three guy cords, normally secured to the sides of the dinghy. A felt pad fitted on the bottom of the set prevented slipping.
'Walter' operated from dry batteries and, depending on the circumstances, could operate continuously for 20 hours. However, if switched on and off at periodic intervals, the life was extended by four to six times compared to continuous operation. When not in use, the set packed into a very small space, being normally supplied in a felt bag enclosed in a waterproof rubber bag, weighing only 2 lb.
The 'Walter' transmitter consisted of a simple self-quenching or 'squegging' type oscillator, using only one valve, sending out approximately 40 000
pulses per second.
Due to this very high pulse repetition frequency (PRF), its signals appeared as a number of very characteristic, continuously moving blips on the radar screen.
It is not a response beacon and cannot be triggered by the airborne equipment, hence no indication of range is provided.
However, it was possible to use the normal indications of relative bearing to home on the 'Walter' by turning the aircraft to equalise the blips on each side of the timebase. A gradual increase in the amplitude of the blips indicated the approach of a 'Walter' while a series of fades occurred when the aircraft was actually over the beacon. An easy way of finding whether the aircraft was approaching the Walter beacon is to swing the aircraft, for example to starboard. If 'Walter' is ahead, the left hand blips will increase in amplitude; if behind, the right hand blips will increase. From an aircraft flying at an altitude of 4000ft, the range was approximately 50 miles.
Circuit diagram and part sections of the T-3180 'Walter' Mk. 1 beacon transmitter. The PRF depends upon the value of C2 and R1 and the average amount of grid current, set to a suitable value (between 25 and 60 kHz) by R1 during manufacture. The type CV-93 valve is soldered directly on the Paxolin circuit board. The dipole aerial is connected to the two tags on the left hand side of the board.
There was an American version of the 'Walter' beacon, the AN/CPT-2 which was functionally similar and rated at a battery life of 12 to 20 hours. Since the USA had Air to Surface Vessels (ASV) radar adapted from the British design (e.g. Army Air Forces SCR-521 and Navy ASC), it was principally possible to find each other's ditched pilots.
Late in 1941 development in Germany was directed towards an Air-Sea Rescue transmitter especially for single seater aircraft. This set, Notsende-Gerät 4 (NS4), code-name 'Jäger' (Hunter), operated on VHF and was manufactured by the German firm Löwe Opta. It consisted of a very small, self contained, single unit, when in use attached to a separate chest plate, together weighing 4 lb. The transmitter operated automatically when the 4ft long steel tape aerial was completely unrolled from the set, sending a continuous tone modulated signal.
Using a specially developed airborne Air-Sea Rescue homing set FüG 141, a range of 90 miles was obtained at a flying level of 3000ft. Initially the NS4 operated around 58.4-58.8 MHz. Later NS4c versions operated around 42.0-42.5 MHz and could not only be homed with the FüG 141 but also with the standard VHF communication sets FüG 17Z* and FüG 17Z.
The NS4 and NS4c sets were painted the usual bright yellow and are hermetically sealed. When not in use, it was normally detached from its baseplate and worn in the left pocket of the flying suit trousers (affectionately named 'Kanalhose'). The baseplate is worn on the chest under the life jacket and dinghy. After ditching, the NS4 is taken off the pocket and attached to the baseplate. Releasing the steel tape aerial, which is spring-wound around the set case, automatically switches the set to operation. It was supposed to keep the aerial in a vertical position as much as possible. A 1 ft long rubber sleeve fitted on the bottom part protected the aerial from coming into contact with sea water.
The circuit diagram reveals that the NS4 has two stages: a master oscillator valve type LS1, driving a push-pull RF amplifier valve type LS2. The RF output is approximately 0.3 watt, decreasing to 0.08 watt after a prolonged time of use. Power for the NS4 is derived from 11 tiny accumulators, giving about three hours continuous service. HT (alternating current of 200Hz) is obtained from a non-synchronous vibrator, this arrangement resulted in 100% modulation.
The design of the German NS4 'Jäger' was definitely a much better effort than the British 'Walter'. It was built in a strong, pressurised and hermetically sealed metal container, having a nearly unbreakable aerial and its internal construction was a piece of contemporary 'state of the art' achievement. The 'Walter' was a straightforward design, with fragile Bakelite parts and a rather unstable aerial construction. However, it was much quicker and cheaper to produce, its working life three to seven times longer than the NS4 with only half the weight!