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Duxford Radio Society


Imperial War Museum, Duxford, England 



Equipment History Files


16-01-2011


A Brief History of Rebecca & Eureka


Rebecca MKIV installed in a Vickers Varsity
Rebecca MKIV transponder as Fitted to the Vickers Varsity

Summary

A  VHF (Secondary) Radar Interrogator-Beacon System

Rebecca & Eureka formed a system of portable ground-based beacons and airborne  direction finding equipment initially designed to assist the air-drop delivery  of supplies to the Allied Armies and Resistance groups in occupied Europe.

Rebecca was the airborne station, and
Eureka was the ground based beacon

The ground based beacon consisted of  a super-regenerative receiver and transmitter, originally operating in the frequency range 214 - 234 MHz, powered from a battery via a  vibrator power supply unit. A portable tripod mounted aerial was erected when  communications was required.

Introduction

Rebecca & Eureka was a system initially designed to assist in the delivery of  supplies to the British Army or Resistance Groups in occupied Europe during  WWII, and which later expanded into a blind homing and approach aid for most of  the aircraft in Allied service.  Much of the credit for the development of this  system must go to Dr.R.Hanbury-Brown and J.W.S.Pringle of T.R.E. (UK  Telecommunications Research Establishment).

Historical Development

Rebecca I was created by bolting a transmitter on to an A.S.V. (Air-to-Surface  Vessel) Mark I receiver.  An experimental version of Rebecca I and Eureka I  built at T.R.E. was demonstrated at the C-in-C, Army Co-operation Command in the  summer of 1941.  The Rebecca system interested the Special Operations Executive  (S.O.E.) and a number were built for the use of Partisans in the underground  movement in Europe. 

Encouraged by trials of Rebecca I the Airborne Forces Equipment Committee  authorised in the summer of 1942 the development at low priority of a Mark II  system suitable for tug and paratroops aircraft.  The basic principle of  multi-channel operation in the air with up to 40 aircraft per beacon was  established and remained the aim throughout the war.  It was also decided  (wisely as it turned out) that Rebecca II should be built as replaceable  sub-units within a common frame to allow for future changes in requirement. 

Both Rebecca II and Eureka II were developed by Murphy Radio with early  pre-production of Rebecca II by Dynatron Radio.  Frequency selection by remote  control was achieved by a rotating turret with pre-set tuneable capacitors in  the Transmitter and Receiver radio frequency circuits.  Due to the limited  availability of components and design effort Murphy Radio was obliged to use  available G.P.O. (General Post Office) telephone exchange selector mechanisms so  that although the cockpit control had 5x5 channels the equipment had only four  positions.  Similar but five position manually controlled turrets were available  on Eureka II.  The latter and most early marks of Eureka operated common  transmit/receive with duration determined by battery capacity, six hours with  Eureka I and II, three hours with Eureka III.  Variants of Rebecca II were  manufactured later with modified radio frequency units to also operate with  Coastal Command Beacons on 173-176 MHz.  Many Bomber, Fighter and Coastal  Command aircraft fitted with primary radar had adapted Rebecca II sub-units  boxed in smaller frames to provide Rebecca facilities via the I.F. and video  units of the main radar.


Historical Development cont.

In the meantime, preliminary trials with Eureka II had shown that it was rather  too heavy to be handled by the path-finders and a lightweight Eureka III, was  designed and developed in conjunction with A.C.Cossor using American 9000 series  miniature valves.  Early models of Rebecca II installed in Dakota aircraft were  flown to America together with Dr.R.Hanbury-Brown who was largely responsible  for stimulating American interest.  The Americans produced their own version of  Rebecca but largely copied the Eureka III design. AN/PPN-1, AN/PPN-2 (Portable)  Eureka - AN/TPN-1 (Transportable) Eureka.  AN/APN-2 (Rebecca) developed from the  SCR 729 Airborne Interrogator of which a 1,000 sets were delivered before mid  1944 for use in 'D-Day operations.  AN/CRN-4 portable beacon supplementing  Rebecca-Eureka sending a continuous wave coded signal every 30 sec. for two  hours over a 30 mile range.

The British Eureka III, including 6 Volt batteries was carried in a webbed  harness around the waist of the paratrooper who had only to unclasp the harness,  remove and erect the telescopic aerial, to operate the equipment.  Failure of  many of the gliders to reach their target even in visual conditions over Sicily  led to a rushed development of Rebecca III, a lightweight low power  super-regenerative set.  Five transmitter and receiver frequencies could be  manually selected in the air.  Operational times were limited to about thirty  minutes to avoid drain of the glider battery.  A Rebecca IIIN was developed for  strike aircraft by the Fleet Air Arm in support of the Army and Marines in the  Pacific Theatre.

Early in 1944 there was a plan to develop a range of miniature B7G valves,  miniature components, plugs, relays, etc. within the U.K.  Rebecca IV was chosen  to be the guinea pig for this exercise and development of the set placed with  E.M.I. in view of the large scale production envisaged.  The technical design  and performance of the equipment was fully detailed in T.R.E. report T2701, Part  I dealing with design, and Part 2 the data on the vast range miniature  components incorporated.  All but a small control box and the c.r.t. indicator  unit was contained and sealed within a pressurised cylinder.  Six pre-set  transmitter and receiver frequencies were selected on the control box located in  the cockpit and continuously tuneable using an ingenious motor driven bridge  balance resistor network without having to open up the pressure unit.  Ericsson  developed a lightweight aerial/display switch unit based on a T.R.E. design to  switch both radio frequency and video on the same contact so as to obviate the  "Christmas Tree" effect on earlier Marks.  The production plan was for 8,500  sets over two years with installation in future and current aircraft and as a  replacement for Rebecca II, (7 types in the F.A.A. and 14 in the R.A.F.),  because of the sets light weight 40lbs (18kg), small cockpit units and full  tropicalisation.  A meter display system developed by A.C.Cossor was later added  to some of the aircraft.  The end of the war curtailed some of Rebecca IV  activities but the system remained in service for many years with Transport  Command out in the Far East.  Later, Murphy Radio developed lighter  non-pressurised versions of Rebecca in various formats using some of the Rebecca  IV techniques for civilian aircraft as well as the higher power Eureka IV and V  beacons.

Regrettably Rebecca/Eureka in the field did not always live up to the high  promise of the trials, mainly due to poor operational planning and damage to  ground equipment.  Throughout the war the use of Eureka I and Eureka II with the  resistance movements was fairly successful.  Of special merit was the use of  Eureka I in Norway from October 1942 to April 1944.  Despite having been buried  during the whole of one summer and part of one winter the operator deployed the  beacon successfully in seven glider/supply operations.

Principles of Operation

The airborne Rebecca equipment radiates 5µSec duration interrogating pulses on a  VHF spot frequency.  On receipt of the interrogating pulses the Eureka ground  based beacon triggers its associated transmitter, causing responses to be  radiated on a different frequency, but at the same p.r.f. (pulse repetition  frequency) of the interrogating transmitter.  The returned signals received in  the aircraft by both the right-and-left Rebecca receiver aerials are displayed  on a cathode ray tube indicator unit,  the time-base of which is  synchronised with the original pulse from the Rebecca transmitter, and applied  to the Y-plates of the c.r.t.  The received signals are switched into the  receiver in synchronism with the switching of the video-frequency output signals  to the right-and-left-hand X-plates of the c.r.t.  In this way, if the beacon is  to the right of the aircraft, the signal to the right of the timebase will have  the greater amplitude, in which case a right turn will be necessary to make the  signals on either side of the timebase equal in amplitude.  This would indicate  that the aircraft was then flying directly towards the beacon.

The ground based Eureka beacon complete with its aerial is air-dropped and  assembled on the dropping zone.  The Eureka transponder was based on  a super-regenerative receiver with a separate transmitter valve and powered by a  battery operated vibrator.  A coding unit which is part of the Eureka beacon  periodically causes the width of the beacon response pulses to vary at Morse  Code intervals for identification purposes.  This function may also be manually  controlled for transmission of simple messages.

Article Source: G.E. Rawlings G8CUN, (Restoration Adviser, Duxford Radio Society), a member of the engineering staff in the Aeronautical Division of MWTC 1959-1969).  Illustrations by Max Westley G4WEZ.

References:  E.K.Williams  "Radar Development to 1945. Published by P.Peregrinus  Ltd. on behalf of the I.E.E.
Copyright notice: This article was first published in News Letter No. 20 of the Duxford Radio Society.  Copyright reserved DRS.  
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V2.00  created 06-10-2006 updated 16-01-2011