How Good is Radar Jamming?
November 1962 Radio-Electronics

November 1962 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Radar jamming, as with radio communications jamming, has been a critical piece in military and intelligence realms since the advent of radar and radio. Early methods involved a brute force transmission of RF energy in the known band of operation, effectively overwhelming the receiver input. This is far from the preferred option due to large, heavy, mobile systems which need to be privy to the exact (or nearly exact) frequency being jammed. Unless the receiver operates in a very narrow band and/or has some degree of anti-jamming features, blanking out a signal is pretty easy to do. I've written before how my turning on a 100 mW radio control transmitter in the 27 MHz band (as a kid in the 1970s) would wipe out TV channels 2 and 5, causing mothers on my neighborhood street to stick their heads out their front doors and scream at me because I was interrupting their soap operas and game shows. As techniques were developed to thwart jamming of one sort, another was invented. Frequency hopping, as first suggested by actress Hedy Lamarr and her pianist, was one of the first successful means for secure communications that was hard to jam. Incredibly sophisticated jamming and, more intensively, anti-jamming schemes are used in modern communications for everything from Wi-Fi and cellphone to military radar and radio to critical medical equipment.

How Good is Radar Jamming? Can we block enemy radar?

By Jordan McQuay

Modern radar is used widely in both military and industrial applications to detect, and locate aircraft, ships and other objects in the air, on land and on the sea. Radar is a basic military weapon - useful in wartime as an aid to combat, in peacetime for defense.

The development of every new military weapon has produced, in turn, a counter-weapon or defensive measure. Just as the use of gas resulted in the gas mask, just as the heavy bomber established the need for radar, radar countermeasures evolved against radar itself.

The accuracy, sensitivity and other unique attributes of radar were described in the June and July 1960 issues of Radio-Electronics. And by the very nature of its operation, radar is extremely vulnerable to interference and jamming, The extreme sensitivity and other characteristics of modern radar can be turned against it by adroit radar countermeasures. We can expect these countermeasures to be used against our own radar installations by an enemy. Similarly, the United States is ready to administer the same treatment to any potential enemy.

The "treatment" may include any of several kinds of countermeasures. Most frequently used is radar jamming - the deliberate transmission of signals intended to interfere with the operation of enemy radar. The purpose is to nullify or at least minimize their effectiveness by obscuring or confusing radar scope displays and thus eliminating or distorting the reception of intelligence.

The first classic example of large-scale radar jamming took place in early 1942 when three German warships escaped from Brest, moved through the English Channel and reached the safety of northern ports. Nearly a hundred German jamming stations in France so effectively blinded British coastal defense radars that they could not detect the warships passing through waters under British surveillance.

There was limited jamming during the Korean war. And this specialized electronic warfare can be expected during any future limited or global war. Although not always completely successful, it is a potent counter-weapon.

Weaknesses and Antidotes

Deliberate radar jamming and countermeasures are potentially successful because of certain characteristics and inherent weaknesses of radar.

Chiefly these are its extreme sensitivity to returning rf signals, the visual nature of. these results on radar display scopes, and the inability of radar to distinguish the precise nature or number of relatively small targets.

A radar transmits recurring pulses of tremendous magnitude - often several megawatts of peak power. These pulses travel long distances before they impinge on a target or other object and are reflected to the radar. Often they return with only a few millivolts, or even microvolts, of input power. Very sensitive rf receivers detect these weak "echoes." They also pick up interfering, jamming, noise and other signals (at the frequency of operation) in the path of the radar antenna. Thus, we must carefully discriminate between the wanted signal from a distant target and extraneous and jamming signals from a multitude of other sources.

An operating radar is its own best advertisement, continually blasting the air with its operational presence. It cannot function secretively, and thus betrays its existence as well as its frequency and other characteristics (by electronic surveillance and analysis) , its direction (by radio direction finding) and its location (by triangulation with two or more RDF stations). If the radar is not driven off the air by enemy jamming, it may be blasted off the earth by enemy bombers.

To minimize some of these inherent weaknesses, modern radars incorporate a variety of advanced-design electronics stages and circuitry.

Some radars have provisions for varying or slightly changing the operating frequency of the transmitter. This is invariably used with cavity magnetrons. The frequency of received signals can be varied by a change in the tuning of the local oscillator of the super-heterodyne radar receiver. High- and low-pass filters are used in the rf and video stages of most receivers to screen and remove unwanted or interfering signals. Both the duration and the frequency of transmitted pulses can be varied. A change in prf (pulse recurrence frequency) is being widely used to "lose" jamming signals that are synchronized with the radar. Most modern installations are also equipped with "black-box" AJ (anti-jamming) circuits. When they are operating, the normal display on the radar scope is divided into a number of magnified segments to permit better visual discrimination between wanted and unwanted signals.

But despite these technological advances, all types of modern radar are susceptible, in greater or lesser degree, to many types of jamming and other countermeasures.

Principal types of jamming may be described in terms of the visual appearance of the two basic types of radar displays-the A-scope and the PPI-scope. There are countless varieties of these principal patterns, the nature of variety depending upon the jamming. Through many of these jamming patterns, however, the target signal can still be observed by operators with patience.

A-Scope Effects

Visual effects of several types of jamming viewed on a radar A-scope are shown in Fig. 1. All synthesized effects are obtained with the same radar and scope, and the same target.

A normal A-scope presentation, without jamming, is shown in Fig. 1-a. At the left is the radar transmitter pulse marking the start of measurement of distance to a target, at the right, the target at a distance along the base line. Minor deflections along the base line-known as "grass" or "clutter" - are caused by atmospheric noise and other unintentional electronic interference of an external nature.

Occasional pulses which may sometimes move quickly along the base line are known as "rabbits" and are frequently too swift to photograph. This type of intermittent interference is caused by a jammer or by any rf transmitter that is not synchronized with the radar.

When a jammer is using CW pulsed modulation, the A-scope effect (Fig. 1-b) is known as "railings." If these appear stationary, the jammer and the radar are synchronized. Any movement of the "railings" along the base line indicates a difference in prf between jammer and radar. With a little practice, a radar operator can easily read through the "railings" and detect and locate the target.

An AM jamming signal usually produces steep-sided visual effects on an A-scope. An FM signal usually produces sloping waves or "humps."

In a typical instance of FM jamming (Fig. 1-c] the resulting "hump" distorts a portion of the base line. This distortion is caused by frequency sweeps greater than the response curve of the receiver. The undistorted part of the base line is linear and unaffected. When all of the base line is disturbed and distorted, the frequencies of the FM jamming signal are within the response curve of the radar receiver. If the FM jammer is synchronized with the radar, the "hump" effect will be stationary and the target difficult to detect. If the jammer and radar are not synchronized, the "hump" effect will move along the base line, permitting an occasional glimpse of the target.

The most effective jamming signals are combinations of FM with noise (Fig. 1-d) or pulsed AM with noise (Fig. 1-e). The addition of electronic noise produces a highly complex signal, which almost completely masks the target signal. With much patience, a radar operator can detect and locate a target signal in the confusion on his scope. "But usually this type of jamming is almost 100% effective.

PPI-Scope Effects

The effect of radar jamming signals on a PPI-scope has a different appearance, mainly because a PPI-scope presents target azimuth or direction as well as target distance. The rotating base line is synchronized with the radar antenna, and both revolve several times a minute. As a result, PPI-scope effects of radar jamming are frequently multi-circular, often very complex and usually symmetrical. Fixed patterns indicate the jammer is synchronized with the radar; moving patterns, a difference between their prf's. In fast-sweep PPI-scopes, the target is frequently obscured by the circular maze of confusion.

A normal PPI-scope presentation, without jamming, is shown in Fig. 2. The dots and splotches indicate recurring signals from targets and objects within range of the radar, represented at the center of the PPI-scope.

Four examples of the effect of CW pulsed jamming are shown in Fig. 3 - the CW pulses vary in width from extremely narrow to very broad. A broad CW pulse means that the jammer is transmitting an excessive amount of average power, producing more interference on the PPI-scope, but at the expense of greater output power at the jammer.

Four effects of CW pulsed jamming of fixed pulse duration, but with four different prf's, are shown in Fig. 4.

Examples of the effects of FM jamming, at four carrier frequencies, are shown in Fig. 5. Harmonic relations are responsible for the exceedingly complex patterns viewed on a PPI-scope. By further varying the prf, even more complicated designs appear on the scope.

Jamming Equipment

Jammers are of three broad types: ground based, shipborne or airborne.

All modern ones are capable of generating and transmitting a variety of jamming signals at any specified operating frequency.

While technical details of current jammers are classified as military information, some general data on several typical jammers can be revealed. Newer models constitute improvements in sophistication - primarily the greater variety of intermixed AM/noise and FM/noise signals that can be generated and broadcast.

A typical ground-based jammer (Fig. 6), the TDY-2, has been used extensively by both the Army and Navy. The transmitter's final stage uses a CW cavity magnetron.

An airborne jammer (Fig. 7), the AN/APT-4, also uses a cavity magnetron. The omnidirectional wide-band antenna is characteristic of many types of jammers.

Special-purpose jammers include one that is essentially a miniaturized transmitter, which can be dropped by parachute, suspended from a balloon, or launched in the vicinity of an enemy radar by a rocket or artillery. Battery-powered, it is small in size and weight, and equipped with a self-destruction mechanism to prevent its falling into enemy hands.

Passive Devices

All the jamming equipment and techniques so far described are known as active countermeasures. When used against enemy radar, they are easily controlled and involve electronic components and circuitry.

Another important category, the·passive countermeasures, require no equipment or circuitry and use the transmitted pulses of an enemy radar to counteract it. This is done with rf reflectors, especially designed to produce maximum "echoes" at the enemy radar.

There are two types of passive countermeasures: chaff and rope.

Chaff, also known as window, consists of literally thousands of thin strips of lightweight reflecting material - tin foil, aluminum foil or metallic-coated paper - about 3/8 inch wide. The length of each strip depends upon the operating frequency of the enemy radar to be jammed. A microwave radar requires strips about 1 inch long; lower radar frequencies require longer strips.

Bundles of precut chaff are dropped by aircraft at high altitudes, or they may be fired into space by small rockets (Fig. 8). The bundles quickly separate and disperse the many reflective strips - which then float gently down through a predetermined air-space area.

When pulses from an enemy radar strike the moving mass of chaff, "echoes" returning to the radar indicate hundreds of reflections. Since a radar cannot distinguish differences in the size of small objects, the effect of chaff on a PPI-scope (Fig. 9) is that of hundreds of aircraft - a massive deception.

While the effect may last for only 30 minutes, this is often enough time to confuse the enemy or to synchronize some diversionary tactic.

Chaff is cut to about one-half the wavelength of the radar to be jammed; and has an effective bandwidth approximately 15% of center frequency.

Rope consists of bundles of long pieces of metallic tape, often as long as a hundred feet. Used against low-frequency radars, it produces a deceptive effect similar to chaff. Rope is essentially an untuned reflector and is used best against radars operating at frequencies below 300 mc.

The principal disadvantages of all passive countermeasures are that they fall rapidly, drift with prevailing wind and quickly disperse due to falling and drifting in space.

Using Countermeasures

Tactical use of radar countermeasures requires a good deal of military preplanning and coordination. Long before a ground-based or ship-borne jammer goes into action or before chaff is dropped by aircraft, there is electronics activity by technical intelligence teams and other groups concerned with the success of the operation.

The search phase of radar countermeasures involves the location and continuous monitoring of enemy radar. Established as near the enemy as possible, intelligence teams maintain an electronic surveillance of all enemy transmissions. Results - operating frequency, prf', pulse duration, other technical characteristics - are carefully measured, recorded and analyzed. Geographical location of all enemy radars is determined by precision rdf equipment. Supplementary data are collected by special aircraft - known as ferrets or electronic snoopers - equipped with receiving and recording gear for close contacts with enemy radars.

Based on all technical data collected during the search phase, plans are completed for the most effective type of countermeasures to be used against the enemy sites. Appropriate jamming equipment is set up at key sites. But the jamming transmitters are not fired up, not even tested with dummy antennas.

There is a period of waiting - until the countermeasures operation can be coordinated with a major military operation against the enemy. Then, at H-hour, the jamming transmitters open up with a barrage of composite jamming signals directed against the enemy radars. Perhaps at the same time, the Air Force is dropping chaff in the skies above the invasion area. Surprise is an important factor in the success of radar countermeasures. Confusion is introduced suddenly and unexpectedly to assure maximum effect.