May 1945 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
This second part of the "Radar Principles" article by British engineer
Dr. R.L. Smith-Rose provides a historical perspective of the
very beginnings of radar systems. Although alluded to by technical
visionaries like Hugo Gernsback, George Orwell, Jules Verne, H.G.
Wells, et al, for use in target detection and ranging
(but rarely speed, oddly enough).
According to Dr. Smith-Rose, the first use of radio waves for detection
and distance measurement was in atmospheric studies to characterize
the ionosphere, in a bistatic configuration. It is an interesting
and quick read, and you might even be introduced to the concept
of a "squegging"
Principles - Part I" appeared in the April 1945 edition of
Radar Principles - Part II
Part II - Historical Development of Radiolocation
By R. L. Smith-Rose, D.Sc., Ph.D., M.I.E.E., F.I.R.E. *
Fig. 7 - How height of ionosphere is checked.
The first applications of radio waves for determining the distance
of a reflecting surface were devoted to demonstrating the existence
of the Heaviside layer as a portion of the upper atmosphere, now
known as the ionosphere, which is responsible for the transmission
of waves around the earth. After many years of speculation with
a variety of indirect experimental evidence, the first direct demonstration
of the existence of the ionosphere as a reflecting region was provided
by Dr. (now Sir Edward) Appleton and M. A. F. Barnett at the end
of 1924 and during 1925.
With the cooperation of the British Broadcasting Corporation
the wave length of the Bournemouth broadcasting station was varied
over the range 385 to 395 meters over a period of from 10 to 30
seconds, and the strength of the resulting signals at Oxford, about
100 miles distant, was measured. It was found that as the wave length
was varied, the received signal passed through a series of interference
maxima and minima, indicating that the signal was the result of
two sets of waves arriving by different paths; one set of waves
was transmitted along the ground, while the other arrived by an
indirect path after reflection from a layer. After verifying that
the paths were in the same vertical plane, a measurement of the
number of interference fringes caused by a known change in wave
length gave a measure of the height of the reflecting layer in the
region, which later became known as the ionosphere.
This was the first classical example of the use of frequency-modulated
radio waves for determining the existence and location of a reflecting
layer which had hitherto remained undetected by any direct experiment.
We may therefore say that the Heaviside layer was the first object
to be detected by radiolocation experiments.
Shortly after the first of the above measurements were made,
G. Breit and M. A. Tuve began some tests in the United States of
America, using interrupted continuous waves which were the equivalent
of pulses of continuous waves about 1 millisecond in duration and
with a recurrence frequency of 500 per second. At the receiving
station a high-speed oscillograph was used to record the incoming
signals and permit the examination of their wave-form. In July,
1925, experiments were made over a distance of 7 miles using wave
lengths of 71 and 42 metres, and it was observed that the received
pulses nominally of square wave-form, were distorted by the attachment
of humps, sometimes in duplicate.
These humps clearly indicated the arrival of a second wave-train,
or echo, by an indirect path; and from a measurement of its time
retardation in relation to the original hump due to the direct or
ground wave, the path difference of the two sets of waves could
be determined. (See Fig. 7.)
In one of their publications, Breit and Tuve remark that their
experiments on the above lines arose out of some work being carried
out at the time on another method proposed by W. E. G. Swann and
J G. Frayne. It is also of interest to remark here that a United
States patent was issued to H. Löwy on an application filed
in July, 1923, for a radio-frequency counterpart of Fizeau's method
of determining the distance of a reflector, to which reference has
already been made. In this patent Löwy describes an electronic
switch used for alternately keying a transmitter and receiver, so
that the latter is only in a sensitive condition after the pulse
or train of waves has been emitted by the transmitter. It is not
known whether this device was put to any practical use.
Fig. 8 (a) - Records of ground pulses and timing oscillations.
(b) - Same, with returned echoes. (c) - Same, photo of C-R
In the years following the dates mentioned above, a considerable
amount of research work was devoted to the development and use of
methods of determining the height of the reflecting layers of the
ionosphere, using both the frequency-change and pulse-modulation
methods. A direct comparison of the two methods showed that they
gave substantially the same result in height determination; and
in a paper published in 1931, E. V. Appleton and G. Builder described
certain important improvements in sending and recording technique
which demonstrated the advantages and illustrated the possibilities
of the pulse method, in so far as the signals arriving at the receiver
due to the ground wave and successive reflected echoes could be
separately received and recorded. At the sending station a tube
oscillator arrangement was used in the well-known "squegging" condition
to produce trains of oscillations or pulses of a duration of about
100 microseconds, spaced in time 0.02 second apart; i.e., at a recurrence
frequency of 50 per second. This type of oscillator had been used
previously to give a linear time base for cathode-ray oscillographic
delineation of waveform by E. V. Appleton, R. A. Watson Watt, and
J. F. Herd, and its application to ionospheric recording had been
suggested by Appleton in 1928.
Since the time of transit of the waves to the E region of the
ionosphere and back again is of the order of 0.002 second, it is
clear that using pulses of the type just described, first the ground-wave
pulse will be all over before the arrival of the first echo, and
secondly, that there is ample interval between successive ground-wave
pulses to receive and record one or more echoes. For visually observing,
and subsequently photographing, the nature of the received signals,
a cathode-ray oscillograph was used, with a time-base provided from
a similar basic circuit using a squegging oscillator, the stroke-frequency
of the time-base being synchronized with the pulse recurrence frequency
of the sender, so that a stationary image on the oscillograph screen
was produced showing the ground wave and any echo waves received.
The type of result obtained is shown in Fig. 8 (a), (b) and (c)
which are reproduced from the paper referred to above, and are specimens
of the actual records obtained by Appleton and Builder in 1931.
Fig. 8 (a) shows the ground-wave pulses received without echoes,
while Fig. 8 (b) shows the presence of a single echo signal after
reflection from the F1 layer. In this case the time interval
can be measured in terms of the trace of an alternating current
of frequency 1115 cycles per second shown below the signal record.
Fig. 8 (c) is a snap photogrgph of the echo pattern on the cathode-ray
tube, showing the ground wave G and the F region echo delineated
on a time-base, which in this case corresponds to a period of about
12 milliseconds. This was probably the first published picture of
what is seen on the screen of the cathode-ray tube of a sending
and receiving system used for determining range by measuring the
time delay of the echo signal relative to that of the ground or
direct path signal.
The pulse-generating oscillator, and the cathode-ray tube and
linear time-base combination so described by Appleton and Builder
in 1931, formed the basis of the technique used some four years
later in the first Radar experiments on aircraft detection conducted
in this country.
Aircraft Height Indicators
While scientific research on methods of exploring the ionosphere
was being conducted on the lines described above, a corresponding
technique was being developed concurrently and on very similar lines
for the purpose of producing an instrument for indicating the height
of an aircraft in flight above the ground. For example, in 1928
J. O. Bentley described a method in which frequency-modulated waves
are radiated towards the earth from a transmitter on the aircraft.
A receiver, also on the aircraft, receives the waves after reflection
from the ground and combines them with those received direct from
the transmitter, the latter waves differing slightly in frequency
due to the time of travel of the waves to the ground and back again.
The frequency of the beats in the receiver resulting from the two
sets of waves is thus a measure of the height of the aircraft above
the ground beneath, as distinct from its altitude above sea-level,
which is what is indicated by the type of altimeter dependent upon
This instrumental technique was later improved by L. Espenschied
in 1930, and culminated in a commercial pattern of "terrain clearance
indicator" produced by the Bell Telephone Laboratories in 1938.
The apparent delay in the successful production of this instrument
was due to the fact that the heights in question are much smaller
than those involved in ionospheric research, and that therefore
the echo-time intervals to be measured are correspondingly less;
e.g., 10 microseconds for about 5,000 ft. An illustrated description
of this method of echo sounding for aircraft was given in Radio-Craft
for January, 1939.
The pulse modulation method, of altitude determination in aircraft
is clearly applicable, provided that the pulse lengths are reduced
sufficiently to discriminate the echoes arriving at a much shorter
time delay than is the case of the ionospheric work. Such a system
was, in fact, described by the Submarine Signal Company in June,
1933. Here the scheme proposed, for measuring distances used pulses
of electric waves, in association with a means of receiving the
reflected echoes, and determining the time interval between the
emitted and received pulses with the aid of a cathode-ray tube and
In December, 1931, the British Post Office observed the effects
of reflection of waves from aircraft in the course of some radio
communication tests being conducted on a wave length of 5 metres
over a path 12 miles long. Extracts from the station log show that
on various occasions the received signal was subject to a beat type
of variation, which was not only audible but was detectable on the
volume indicator of the receiver. The amplitude of the beat varied
from about ± 1/2 db. up to 10 db. on some occasions, and at all
times when this occurred an aircraft was found to be flying in the
neighborhood at various distances up to 2 1/2, miles and at heights
up to 500 feet. The period of the beats varied from 5 to 15 per
second; and this is to be compared with the calculated value of
11 per second for an aircraft flying directly towards the receiving
aerial at a speed of 60 m.p.h.
This experience was confirmed by further observations made in
America in 1932 by engineers of the Bell Telephone Laboratories
in the course of an investigation of the mode of propagation of
radio waves in the range of wave lengths between 4.7 and 5.7 metres.
In a paper describing this work by Messrs. C. R. Englund, A. B.
Crawford and W. W. Mumford, and published in March, 1933, it is
stated that an aircraft flying about 1,500 ft. overhead and approximately
along the line joining transmitter and receiver, a noticeable flutter
of about four cycles per second was produced in the low-frequency
detector meter of the receiver. When observations were carried out
in the neighborhood of an airport, it was noticed that nearby aircraft
produced field strength variations up to 2 db. in amplitude. Similar
re-radiation was noticed at various subsequent times, occasionally
when the aircraft was invisible.
It was thus clearly established, over ten years ago, that radio
waves reflected from aircraft in flight could be detected with suitable
receiving equipment on the ground; and it now remained to be seen
whether this principle could be applied to the development of a
technique for the detection and location of aircraft at ranges and
under conditions of practical utility as an aid to navigation in
peacetime and as a defensive weapon in war. This important, and
by no means easy, step was accomplished by a small group of scientists
working under the direction of Mr. (now Sir Robert) Watson Watt,
who was at the time Superintendent of the Radio Department of the
National Physical Laboratory, incorporating the Radio Research Station
at Slough where the initial experiments in the radio location of
artificial objects in this country were conducted.
Watson Watt, in association with the late J. F. Herd, had also
devised the original form of visual direction finder, using twin
balanced amplifiers and a cathode-ray indicator.
After some preliminary experiments, members of the staff under
Watson Watt's supervision established a new "ionospheric" exploring
station on the East Coast of England, at which were installed the,
for those days, high-power pulse transmitters made at Slough, together
with suiitable receivers and appropriate aerial systems and goniometers
for determining the direction of arrival of the echo waves, both
in azimuth and elevation, scattered back to the receiver from the
aircraft which was illuminated, as it were, by the flood-lighting
effect of the radiation from the transmitter.
The members of that small band of scientists and techniccal assistants
will well remember the thrill of seeing for the first time a clear
image on the cathode-ray tube due to an aircraft which was so far
away as to be invisible to the naked eye; the distance of the pip
along the base line gave the range of the aircraft while its bearing
and elevation were obtainable by turning the knobs of the goniometers.
Much hard work and not a little ingenuity were still required
to convert the technique from an experiment in the hands of scientists
to a working system which could be used and maintained by this miscellaneous
type of personnel which was at that time provided by the Service
departments for this new "side-line" of radio communication or signaling.
It was not long, however, and well before war was declared, before
more than one Service station was in operation, and the plotting
of the tracks of various aircraft, some on their legitimate civil
or military duties, and others whose business was perhaps less innocent,
was a matter of daily routine.
Work in Other Countries
An indication of the trend of thought and activities in other
countries in the years before the outbreak of the present war can
be gained from a perusal of one or two publications which are available.
Reference has already been made to the patent taken out in U.S.A.
by H. Löwy; but the main development in America seems to have
taken place partly in the Service research institutions, and partly
at the Bell Telephone Laboratories. The latter organization, after
developing the aircraft altimeter, demonstrated the use of this
instrument in a modified form to the detection of ships over short
distances. With regard to the Continent, it is to be noted that
the Telefunken Company filed a patent in 1935, disclosing an arrangement
similar to the frequency-change method used by Appleton, with the
modification also suggested by Appleton that, while the carrier
frequency remained unaltered, the frequency of the modulation was
varied, while the number of interference fringes was counted at
the receiver. The American journal Electronics published in September,
1935, a two-page set of illustrations descriptive of the aircraft
detection arrangements alleged to be under development by the Telefunken
Company. An interesting feature of this pictorial display was the
reference to the use of wave lengths in the band 5 to 15 cm. and
of magnetron valves with permanent magnets developed specially for
wave lengths of about 10 cm. An alternative scheme was also described
by the Telefunken Company in 1937, which utilized two beams of transmitted
waves to produce a stationary interference pattern, the disturbance
of which by an object moving across it was detected at the receiver.
French and Italian Apparatus
In Italy, E. Montu described a twin rotating aerial arrangement
for locating aircraft in bearing and elevation, and the patent specification
of this arrangement was published in this country in December, 1936.
About three years later U. Tiberio published the first part of a
comprehensive paper, discussing various aspects of the radiolocation
of ships and aircraft, in the Italian periodical Alta Frequenza:
the later parts of the paper were apparently withheld from publication
after the outbreak of the war. An interesting development in France
was the fitting of the steamship Normandie with an iceberg detector,
which was described and illustrated in Wireless World for June 26,
1936. This equipment comprised a transmitter and receiver operating
on a wave length of 16 cm. and mounted in the fore part of the ship.
The transmitting and receiving aerials were of the dipole type and
mounted in parabolic reflectors, 75 cm. in diameter and installed
at a distance of 6 metres apart; this arrangement provided a beam
having a width of ± 10 deg. at half amplitude, and the reflectors
could be rotated automatically through an arc of 40 deg. When the
receiver indicated the arrival of a signal from the transmitter
after reflection from a distant object, the two parts of the equipment
could be manually and accurately trained on this object, the distance
of which could then be calculated from the directions of the transmitted
and arriving waves. In this manner it was claimed that a coastline
could be located at a distance of 20 km., and large ships were detected
at ranges up to about 7 km.
Such was the state of affairs abroad as judged by the sparse
published information available. As to what was the actual state
of affairs at the outbreak of the war in Europe must remain a matter
of speculation at the present time; but many readers will look forward
with interest to the time when more facts may be disclosed, and
the progress of the Radar technique conducted by the various belligerent
nations may be described and compared.
(The above article was reprinted by special permission of Wireless
World, London, England.)
Our Cover Feature "Japanese Radar"
Japanese radar, which appears on our front cover this month, is
a sequence from the Warner Brothers' film Objective Burma.
This elaborate radar installation was located in the Northern part
of Burma and a United States Task Force was charged with eliminating
it. In the movie this mission was successfully completed and the
installation blown up.
This fanciful radar, cooked up by the Hollywood technicians,
looks most impressive in the motion picture and is supposed to let
the public in on the sacrosanct wonders of radar - still suppressed
by the Allied military authorities.
This particular Japanese radar installation was manned by two operators
and was a revolving affair, the entire framework, transmitter, receiver,
and operators rotating continuously.
Spectacular as it appears in the motion picture, modern radar
installations do not look anything like this. Indeed, most modern
installations are quite compact, probably not too many of the cumbersome
revolving types being in existence today.
Nevertheless, the radar principle of transmitting microwaves,
which are then reflected back to the operators, is correctly pictured
for a not too technical public consumption.
Needless to say, the Hollywood technicians could reasonably well
have shown a modern radar installation as it really appears, but
in this they were prevented by military censorship.
*National Physical Laboratory.
Posted August 22, 2014