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Radar Techniques - Primer Principles
April 1945 QST Article
"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." 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
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. As time permits, I will
be glad to scan articles for you. 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.
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 eye."
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.
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.
Admittedly, 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. 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, 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.
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.
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.
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 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 pulse.
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 trace.
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.
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 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 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 operation.
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 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.
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