The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar Technology: Main Components | Radar Technology: Side Lobe Suppression | Radar Technology: Airborne Collision Avoidance
Radar Technology: Antennas | Radar Technology: Antenna Beam Shapes | Radar Technology: Monopulse Antennas | Radar Technology: Phased Array Antennas | Radar Technology: Continuous Wave Radar | Theoretical Basics: The Radar Equation
Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
Theoretical Basics: Ambiguity And The Influence of PRFs | Theoretical Basics: Signal Processing | Civilian Radars: Police Radar | Civilian Radars: Automotive Radar | Civilian Radars: Primary and Secondary Radar
Civilian Radars: Synthetic Aperture Radar (SAR) | Military Applications: Overview | Military Radars: Over The Horizon (OTH) Radar
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife
Radar Countermeasures: Range Gate Pull-Off | Radar Countermeasures: Inverse Gain Jamming | Advanced Electronic Countermeasures
Array antennae consist of a multitude of identical radiating elements. These are fed from a single transmitter through a network of radio frequency lines or waveguides1. In order to understand how an array antenna works, the basic wave superposition principle which was outlined in the entry about Antennae must be applied twice: the radiating elements can be thought of as consisting of radiation sources whose contributions are added in order to yield the directional pattern of the device. An array is a group of such elements, and its directional pattern is determined by superposing the superposition results once again. Hence, the shape of the beam is determined by the properties of the individual elements, plus the power distribution among the elements and the geometric details of their arrangement.
Arrays and single feed antennae produce a beam which can only be moved in space by either moving the reflector, the feeder or the whole construction.
Phased Array Antennae
Phased arrays additionally contain delay elements in the feeding structure. By controlling these delay elements, mechanical movement of the structure can be simulated. Thus, phased array antennae are capable of moving the beam position in space without moving any mechanical part. The term 'phased array' originates from the fact that when considering sinusoidal signals such as electromagnetic waves, time delay can be translated as a shift of the phase of the signal. Actually, many phased arrays use phase shifting devices rather than delay lines.
|How a phased array antenna works
|Case 1: no time delay: El1 < ) ) | El2 < ) ) | El3 < ) ) | ----------> El4 < ) ) | beam direction El5 < ) ) | The wave front is in parallel to the array face.
Case 2: with time delay (Element 5 transmits first):
El5 < ) )
El4 < ) ) )
El3 <) ) ) /
El2 < ) ) /
El1 < ) ) /
. ) /
The wave front is at an angle to the array face.
Active Phased Arrays
Even more sophisticated, complicated and expensive are active phased arrays which feature as many transmitters as they have radiating elements. Their main advantage over (ordinary) phased arrays is an economical one: transporting electromagnetic waves through cables or waveguides is subject to severe attenuation losses. If each antenna element has its own transmitter attached to it then the radio frequency (RF) energy is generated in place and the heavy RF plumbing can be omitted.
Nowadays, all of the world's largest radar installations are built around phased array antennae, and they are looking more like 30-storey buildings than radar sites. A few examples are Cobra Dane, Pave Paws and Pill Box.
Beam Shape Agility / Digital Beam Forming
In general, an antenna's beam shape is determined during the development phase and remains fixed throughout its lifetime. But sophisticated phased arrays are capable of switching beam shapes during operation. The easiest way of doing this is to simply switch off a part of the radiating elements and to redirect transmitter power into the rest. As an example, assume an array of 100x100 elements which have a beamwidth of (10° azimuth) x (30° elevation) if taken individually:
In full operation, the beam is a pencil beam of, say, 2° x 2° which is achieved by combining those 100x100 elements into an array and using a proper amplitude taper. 2° x 2° is a pencil beam which is quite suitable for tracking a target whose position is known from some previous examinations.
Now switch off half of the elements along the horizontal and vertical axis. This decreases the active antenna length and width and the beam widens up to become 4° x 4°. This wider pencil beam is useful for searching a target which may have gone missing in a previous tracking phase, or if its coordinates were handed over from a less accurate surveillance radar.
Now switch off all of the rows but one. The horizontal dimension of the aperture remains the same, hence the azimuth beamwidth remains at 2°. The beamwidth in elevation is now only determined by the properties of the radiating elements and was assumed to be 30°. This 2° x 30° fan beam is advantageous for target search in a stand-alone application.
Finally, switch off all the columns apart from one. The result is a tilted fan beam of 10° x 2°. This beam shape could be used for guarding against low-flying aircraft which appear closely above the horizon. It was also used in nodding height-finding radars of the 1950s and 1960s.
By applying some more complicated manipulations along these lines, a phased array radar can adapt its beam shape to a wide variety of environments and purposes.
Last but not least, a modern phased array antenna has a feature called 'Null Steering' in its inventory. Somewhere in those complicated polynomial equations there are parameters which allow the position of an antenna diagram's nulls to be varied, with only minor impact on the position and shape of the main lobe. By playing around with these parameters, a radar can simply make itself 'blind' in directions where hostile jammers are located. After having sent a message to a long-range missile system, that is.
History: Overview | Isle of Wight Radar During WWII
Technology: Basic Principle | Main Components | Signal Processing | Antennae | Side Lobe Suppression | Phased Array Antennae | Antenna Beam Shapes | Monopulse Antennae | Continuous Wave Radar
Theoretical Basics: The Radar Equation | Ambiguous Measurements | Signals and Range Resolution | Ambiguity and PRFs
Civilian Applications: Police Radar | Automotive Radar | Primary and Secondary Radar | Airborne Collision Avoidance | Synthetic Aperture Radar
Military Applications: Overview | Over The Horizon | Low Probability of Intercept | How a Bat's Sensor Works
Electronic Combat: Overview | Electronic Combat in Wildlife | Range Gate Pull-Off | Inverse Gain Jamming | Advanced ECM | How Stealth Works | Stealth Aircraft