Radar Techniques - Primer Principles
April 1945 QST Article
prospective peacetime applications of radar are beyond prediction.
Among the more obvious are those relating to navigational aids and
collision prevention. In some of these uses it will be a case of
radar replacing radio." That was 66 years ago when real-world radar
was still in its infancy that futurists were prognosticating on
potential uses for radar beyond its use for the war effort. Just
a month after the April issue of QST was published, the war in Europe
ended (V-E Day, May 8, 1945), and four months after that the war
in Japan ended (V-J Day, August 14, 1945).
April 1945 QST
articles are scanned and OCRed from old editions of the ARRL's QST magazine. Here is a list
of the QST articles I have already posted. All copyrights are hereby acknowledged.
would be utterly amazed at just how widespread radar is today. It
not only surveys the airways for commercial, military, and civilian
craft, but also for marine and land traffic, orbiting spacecraft,
and planetary science. Law enforcement uses it to add to the department
coffers, automated landing systems and security systems depend on
it. The list of applications is almost endless.
I - Primer Principles
By Clinton B.
DeSoto, W1CBD (Editor, QST)
EVERY American - adult
or adolescent - astute enough to keep up with the adventures of
Buck Rogers, Smilin' Jack, and Terry and the Pirates is well aware
of the existence of radar and probably of its operating principles,
He has been told that radar is "a radio wave
with an echo" - that a radar beam is a sharply focused radio searchlight
which searches out any object coming within range of its "owl-like
Whether it be considered as an "eye" or as an
"echo," assuredly radar is a means for projecting the range of human
senses far beyond their normal limitations. There is logic in the
thought that, as sound radio is to the ear and television to the
eye, so radar - even though it employs other sensory organs - may
be regarded as an extension of the sense of touch.
word radar, by official account, was coined from the initial letters
of the prosaic phrase "radio detection and ranging." As a military
weapon, radar is utilized both defensively and in attack. Defensively,
it performs the duty, first, of detecting a trespassing enemy and,
second, establishing his precise location. In its offensive role
radar scouts out the prey of pilots of interceptor fighters and
the commanders of naval patrol craft; it aims antiaircraft artillery
and the big guns of the battlewagons; it controls devices which
automatically align searchlights, navigate air and seacraft, and
perform many other functions.
The purpose of this
series is to discuss the techniques employed in radar, within the
limits circumscribed by military restrictions - to explain radar
systems in general, to present diagrams and simple circuits illustrating
the derivation of the generic units, and to suggest elementary methods
employed to achieve the required effects.
to the thousands of radio amateurs directly associated with this
new art (many of whom, incidentally, have made major contributions
to its development) there will be little we can say that will be
novel or useful. Even to those who, while not directly engaged in
radar work, have access to the literature on modem technical trends,
these articles will in all likelihood have 1000 50 only incidental
There remains, however, the many stay-at-home
civilian hams (and also some of those in military service) who do
not have access to such specialized technical information, and it
is for their benefit that this series is written. For them we shall
endeavor to interpret the broader aspects of the technique and evolution
of the art. Moreover, to ensure comprehension even by the neophyte,
the explanations will go back to the underlying principles. Thus
this initial discussion concerns itself only with a generalized
summary a "primer class" treatment of the subject. Details of component
units and certain aspects of the theory involved will be dealt with
in somewhat more detail in subsequent installments.
Military radar systems
must be capable of (1) searching an assigned area, which may range
from the relatively small frontal-fire arc of a night-fighter interceptor
pursuit to the entire expanse of horizon surrounding a warship or
a long-range bomber, and (2) supplying data for the accurate (and,
preferably, automatic) determination of the quantities necessary
to give an exact "fix" on enemy air or seacraft: (a) direction or
bearing (azimuth); (b) altitude (elevation), and (c) distance (range),
as shown in Fig. 1.
The prospective peacetime applications
of radar are beyond prediction. Among the more obvious are those
relating to navigational aids and collision prevention. In some
of these uses it will be a case of radar replacing radio. Radar
d/f is distinguishable from familiar radio direction-finding practice
by an invaluable quality, described thus by Dr. Smith-Rose: "An
intrinsic feature of the art is that no cooperation whatsoever is
required of the object being detected ... The latter, be it an aeroplane,
ship, building or human being, is merely required to reflect or
scatter some of the radiation which reaches it. ... The detected
object is thus merely a source of secondary radiation which results
from its being illuminated, as it were, by the incident radiation
from the primary sending station."
Fig. 1 - To establish the location of the target in space, three
quantities must be determined: distance (range), bearing (azimuth),
and altitude (elevation).
Fig. 2 - Time intervals for return of reflected signals.
Ordinary radio d/f requires that the object of the search transmit
a signal so that a bearing can be taken. If the mobile transmitter
aboard the ship or aircraft fails, or radio silence is imperative
or an enemy bomber fails to "cooperate" and does no transmitting
- well, then no radio bearing can be taken. Radar systems, however,
require of the wanderer only that he serve as a reflector - a form
of assistance which not only can be, but in wartime usually is,
from that significant difference, the two methods are essentially
similar. The procedure in taking a radar bearing may be simply that
of rotating a directive antenna for maximum response in either the
horizontal or vertical plane, and then reading the angle of azimuth
or elevation on a calibrated scale to establish direction.
The third item of information required to establish the exact location
of a target, as shown in Fig. 1, is the distance to the object.
Here radar displays another unique quality - its ability to measure
the distance to any object in the field of its beam, like a searchlight
with a coupled rangefinder, without triangulation.
This ability is predicated
on three technical factors which characterize radar: (1) radiating
energy in extremely short pulses spaced by comparatively long quiescent
intervals; (2) concentrating the radiated energy in a very sharp
(highly directive) beam; and (3) utilizing electronic devices, which
can register and measure splitmicrosecond intervals precisely,
to determine the transit time of reflected pulses or "echoes."
Because the velocity of propagation or "speed" of
a radio wave is constant in space, and very nearly so in air, the
time taken by a pulse in traveling any given distance represents
an accurate measure of that distance.
may be described as akin to sending a messenger out into space,
traveling at a known rate of speed, and therefore requiring a given
time to reach a given point and return. The radar messenger is a
pulse of r.f, energy; its speed is approximately the same as the
velocity of light; and the time. required to make the round trip
over any given distance and back is shown in Fig. 2.
arrangement for the measurement of distance or range by means of
reflected pulses is illustrated in block diagram form in Fig. 3.
Although bearing only slight resemblance to current practice, it
illustrates the mechanics of radar in readily comprehensible fashion.
Fig. 3 - Block diagram of a simple radar system.
Modulated by the pulse generator the radar transmitter radiates
short pulses of r.f. The interval between individual pulses is
made somewhat greater than the total time required for the wave
to travel to a reflecting target at maximum range and back to the
The transmitting antenna emits radiation
beamed in the approximate direction to be explored. Whenever this
radiation strikes a surface having characteristics of electrical
conductivity or dielectric constant appreciably different from those
of air, some of the energy will be reflected or scattered back towards
While the power radiated from the transmitting
antenna is concentrated principally in the beam reducing the local
field to a minimum, the direct radiation is sufficient to energize
the receiver. If the distance between transmitter and receiver is
small, transmission of this direct wave, indicated by the dash line
in Fig. 3, is practically instantaneous. The direct radiation from
the transmitter therefore establishes the starting time of the exploring
Fig. 4 - Timing (direct pulse) and target (reflected pulse) "pips"
on the cathode-ray tube indicator screen.
and reflected pulses are picked up by the receiving antenna and
generate corresponding pulses of signal voltage in the receiver
input circuit. After being amplified and rectified both signals
are applied to the vertical deflecting plates (Y axis) of the cathode-ray
tube indicator. There the pulses register on the screen as vertical
deflections of the horizontal timing trace.
of the screen is shown in Fig. 4. By comparing the distance between
the direct and the reflected pulse indications on the screen, using
a known time base, the distance traveled by the reflected wave can
be read on a calibrated scale. The horizontal (X axis) deflection
is synchronized with the transmitted pulses, giving a known horizontal
time base which is adjusted so that the direct pulse indication
coincides with 0 on the scale.
The vertical amplitude
of the pulse deflection or "pip" is, of course, proportional to
the relative amplitude of the received signal. Thus the height of
the trace tends to vary with distance, and may also serve to indicate,
to some extent, the size or composition of the target. Moreover,
if the target under observation is moving, the change in its relative
position will be indicated by a movement of the pip along the base
Timing the Radio Echo
It is evident that the accuracy of such measurement will be greatly
dependent upon the accuracy of the scale calibration - which, in
turn, is dependent upon the accuracy of the timing base.
The key to the entire system is the pulse generator, which times
each and every step in the operating sequence. For this reason the
pulse source must be capable of delivering a continuous series of
precisely identical pulses at an exact and unvarying repetition
These control pulses synchronize both the transmitter-modulator
and the receiver-indicator functions. Each pulse going in the transmitter
direction is applied to the modulator input and serves to release
r.f, power from the transmitter for a period precisely equal to
the duration of the pulse. Similarly, in the receiver direction
each pulse triggers a sawtooth sweep-voltage generator which supplies
the horizontal time base for the cathode-ray tube indicator. Since
the resulting sweep frequency is identical to the pulse repetition
rate, the cathode-ray beam makes exactly one traverse of the screen
along the X axis in the interval between each transmitted pulse.
The cathode-ray tube is comparable to a split-second
stop watch, in which the "sweep hand" makes a complete revolution
in terms of thousandths of a second and reads time in microseconds
(millionths of a second). What this means can best be appreciated
by pointing out that, if an ordinary 12-hour clock were speeded
up to a comparable rate, the hour hand would be making several
revolutions per second rather than two per day.
Obviously, only an electronic instrument can meet such an exacting
requirement. The cathoderay tube therefore is used to measure the
time interval as a function of voltage. As explained above, the
distance or range is then found by translating that quantity into
a function of time.
Cathode-Ray Tube Indicator
At the risk of emphasizing the obvious, let us
take a backward glance at some fundamentals of cathode-ray tube
A cathode-ray tube, as has been explained in
so many places at so many times, is in effect a two-dimensional
voltmeter with an essentially weightless, massless, inertialess
pointer. This pointer is a sharply focused electron beam which impresses
its transient indication on the fluorescent material of the screen,
creating a luminous spot wherever it strikes. Normally centered
on the screen, the spot from the cathode-ray beam will be deflected.
moving up or down, left or right, in instantaneous response to the
influence of an external electric or a magnetic field. In so moving
it leaves a visible line or trace. Because of the inherent retentivity
of the fluorescent screen and the persistence of vision of the human
eye, this luminous trail will remain visible for 0.1 second or longer.
That, of course, is the principle of the cathoderay oscilloscope.
By translating any dynamic quantity - electrical or mechanical -
into voltage, its characteristics can be reproduced as a visual
image on the screen of the oscilloscope. And that is just what is
done in the radar indicator; the required quantities - time, distance,
bearing, etc. - are translated into corresponding voltages which
trace characteristic patterns on the cathode-ray tube screen.
To establish the relationship between voltage and time,
the external circuits are so arranged as to apply to one pair of
deflection electrodes (usually the horizontal or X axis) a voltage
which increases linearly over a predetermined interval of time.
At the end of this interval it will "fly back" rapidly to zero,
and then repeat its relatively slow linear traverse across the screen.
This action is pictured in Fig. 5, where the vertical (Y axis) deflection
voltage depicts three received pulses.
If the linear movement
of the beam as it is visually apparent on the screen is directly
proportional to the amplitude of the deflection voltages, the
screen may be calibrated rectilinearly in terms of voltage. Thus,
with a linear time base, a rectilinear-coordinate scale can be obtained.
It must be understood that the total length of the horizontal
base line bears no relationship to the time scale; it is controlled
solely by the peak value of the sweep voltage. Nor is the amplitude
of this voltage related to the time interval; it serves only to
establish the length of the trace. Regardless of the numerical length
of the trace, its proportional parts will always bear the same relationship
to the total time interval. Thus, for a repetition rate of, say,
1000 (0.001 second), 10 percent of the trace will represent 100
microseconds, 5 percent will be 50 microseconds, etc. - whether
the trace itself be 0.5 inch or 5 inches long. Thus any scale may
be arbitrarily divided off into linear units and attached to the
cathode-ray-tube screen; the beam deflection is made to correspond
to the scale calibration simply by adjusting the sweep amplitude
to match the scale length.
Provided the time base is perfectly
linear, the possible accuracy of measurement is limited only by
the accuracy with which the scale calibration can be read - in effect,
the number of intervals into which the scale can be divided. This,
in turn, is limited by the maximum base length, which obviously
must be somewhat less than the diameter of the cathode-ray tube
Fig. 6 - Use of a polar-coordinate time base multi. plies the effective
scale length by a factor of 3 or more.
A three-fold longer
trace and consequent better accuracy can be achieved by using a
polar-coordinate scale. With such a scale the circumference of the
screen, not the diameter, determines the maximum scale length.
To obtain a polar scale, the timing-axis trace must appear
as a circle rather than as a straight line. This requires that a
circular sweep be used instead of a linear sweep. The signal deflection
voltage then is applied radially to alter the shape of the circular
trace, causing either a "tooth" or a notch to appear in the circle,
as shown in Fig. 6.
This article is Part I of a series.
Part II will appear in the May issue of QST.
(If some kind soul would donate the May 1945
QST, I will post the next installment)
Posted 2/21/ 2011