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,
April 1945 QST
of Contents]These 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 (if any) are hereby acknowledged.
Editor DeSoto 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.
See all available
vintage QST articles.
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, as well.
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 eye."
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.
The 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 interest.
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.
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
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, rendered involuntarily.
Apart 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.
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,
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.
The process 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.
A typical 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 receiver.
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 the receiver.
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.
Both direct 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
The appearance 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.
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 line.
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 rate.
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
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
Cathode-Ray Tube Indicator
At the risk of emphasizing the obvious, let us take a backward glance
at some fundamentals of cathode-ray tube operation.
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 screen.
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.
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