Radar and Antennas

The word “RADAR” is an acronym for Radio Detection And Ranging. It uses electromagnetic (Radio) waves to detect the presence of a target and to determine its distance or range. RADAR operates similarly to the principle of sound-wave reflection. If you shout in the direction of a sound-reflecting object (i.e. hill or cave), you will hear an echo. As the speed of sound in the air is known, you can estimate the distance and general direction of the object. Similarly, RADAR illuminates the target with a microwave RF signal, which gets reflected and picked up by a receiving device. The received electrical signal is called an echo or a return signal. If the echo is from undesired targets (such as the earth), the echo is referred to as clutter. Based on the echo signal, RADAR can estimate the distance, direction, and relative velocity of the target.

There are various categories of RADAR. Based on its operation, it can be designated as surveillance RADAR or tracking RADAR. Surveillance RADAR is designed to provide attack warnings and is characterized by high power output, large antennas, and low frequencies. Tracking RADAR is designed to provide target engagement information and generally has shorter range capability and higher target range / azimuth data accuracy than surveillance RADAR and it operates at higher frequencies.

Based on RF signal used, RADAR can be classified as continuous wave (CW) RADAR and pulse RADAR. Simple CW RADAR cannot indicate the target range but gives better performance in case of stationary target. Compared to CW RADAR, pulse RADAR uses more power but is more accurate in multiple target scenarios.

Basic building blocks for any RADAR system are a transmitter, receiver, antenna, synchronizer, and indicator. The transmitter is responsible for producing the high-power & short duration RF signals which are transmitted by antenna. Returned RF signals from the target (Echo) are amplified and demodulated by the receiver. The indicator is the interface between a complex RADAR system and its user as it displays continuous, easily understandable, graphic symbols of the relative position of targets. The synchronizer is the master timer which coordinates the operation of the entire RADAR system. It triggers the transmitter to initiate transmission of an electromagnetic wave. Simultaneously, it sends a signal to initialize the indicator to display range and azimuth information.

An antenna is a transducer which converts electromagnetic waves into high-frequency electrical voltages and vice versa. Mechanical size of the antenna, inverse to the function of the RF signal frequency, means higher frequency requires smaller mechanical size of antennas and vice versa. Electrical characteristics of any antenna are defined in terms of antenna pattern, gain, beam width, aperture, major/minor lobes, front-to-back lobe ratio, polarization and directivity. These characteristics vary based on the type of antenna used.

In any RADAR System, the antenna plays a major role as it either receives echo signals from the target in the form of electromagnetic energy, or radiates RF signals produced by a transmitter. Usually two types of antennas are used in RADAR systems: parabolic dish antenna and phased array antenna. Phased array antennas are typically used in tracking RADAR, and parabolic antennas are used with surveillance RADAR.

In parabolic dish antenna, there is one circular parabolic reflector and a point source (feed) situated in the focal point of this reflector to produce a pencil beam. An elliptical shape reflector is used to produce a fan beam. To achieve the required pencil beam in azimuth and the squared fan beam in elevation, surveillance radar generally uses two different curvatures - parabolic & elliptical. Phased array antenna contains many radiating elements and each element has its own phase shifter. By shifting the phase of the signal emitted from each radiating element, beams are formed in the desired direction.

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