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.
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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.
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