Question

Explain the principle and working of RADAR with neat block diagram.

Answer 1

$RADARworkingandprinciple$

Most radar sets employ pulse radar wherein the transmitter sends out short intense bursts or pulses of energy of about one microsecond duration with a relatively long interval of several hundred micro-seconds between the successive pulses. When sufficient time has elapsed to permit the reception of the echoes from the most distant desired objects, the transmitter sends another short pulse and the cycle repeats.

The transmitting antenna also functions as the receiving antenna. The reflected signal after amplification of several million times is used to deflect the electron beam in a cathode ray tube. The echo picked up by the receiver strengthens or deflects the flow of electrons in the tube, causing a point of light to appear on the screen which remains visible by phosphorescent after-glow until fresh echoes are picked up on the next revolution of the scanning antenna.

The angular bearing to the target is determined by measuring the direction from which the reflected energy is received. It is expressed in two components; azimuth angle measured in the horizontal plane and elevation angle measured in a vertical plane. The indicator tube is arranged to directly display the target co-ordinates and distance.

$Blockdiagram$

$Masterclock\left(Timer\right)$

The timer device is used for coordinating the action of the transmitter, receiver, and indicator, to ensure synchronized operation.

$Modulator$

The modulator is used to amplify pulses supplied by the timer to sufficient amplitude to operate the transmitter.

$RFPulseGenerator\left(Transmitter\right)$

The transmitter is used to convert modulator pulses into RF pulses.

$Antenna$

The antenna is used to radiate the transmitted pulse and receives the reflected signal over a sharply defined beam.

$Scanner$

The scanner, an electro-mechanical system for moving the radiator or antenna to scan the space for targets scanner also generates signals (voltage or current) proportional to the azimuth and elevation components of the angular bearing of the beam for transmission to the indicator.

$Receiver$

The receiver amplifies and demodulates the received signal. The receiver used in the radar systems is generally of the superheterodyne type. The local oscillator of the receiver is ultra high-frequency triode for frequencies below $1000\mu$, and for higher frequencies, ‘reflex klystron’ is used as a local oscillator. The I.F. is generally in the range of $30\mu$ which is further amplified in I.F. amplifiers.

$CathodeRayIndicator\left(Display\right)$

The cathode ray indicators for displaying the echo signal and the target coordinates.

$DuplexSwitchorTransmit-ReceiveSwitch$

This may consist of an open spark gap across the transmission line or as is generally the case, an enclosed low-pressure T/R tube is used. This is placed in a cavity that steps up the voltage existing across the transmission line so that the gap will break down when the transmitted pulse appears across it. When the transmitted pulse is concluded, the ionization within the T/R tube clears up in about a micro-second, freeing the transmission system to carry the received signals to the receiver.

$RADARTransmitter$

It is essentially a device for converting modular pulse into RF energy. It is a self-excited oscillator, which for carrier frequencies below 500 Mc employs UHF triode, and for higher frequencies of the order of 1500 – 3,00,000 Mc/s special ultra-high-frequency tubes like magnetron and klystron are used.

The transmitter must generate pulses at the highest possible power for the pulse duration. The cathode of the transmitting tube is designed to be suitable for heavy emissions and the plate voltage is of the order of 10 – 25 KV. The R.F. energy is conveyed from the transmitter to the antenna and from the antenna to the receiver over co-axial cables or waveguides.

The co-axial cable consists of a hollow outer conductor within which a small conductor is supported on low-loss insulating beads or flexible solid dielectric (Polyethylene). With properly matched terminating impedances, such cables can carry peak power of the order of hundreds of kilowatts without developing an excessive voltage across the line.

$Waveguide$

The waveguide transmission line is a hollow conductor of rectangular or circular cross-section.

$Indicators$

The cathode-ray tubes used as indicators in radar systems take various forms depending upon the required presentation; the most widely used indicator being the plan-position indicator on a polar diagram. In this device, the cathode-ray spot is deflected radially outward from the center of the tube at a constant rate and the beam is intensified whenever a reflected signal appears. Consequently, the range of the target is indicated by the distance from the center at which the spot appears. The polar angle on the indicator screen indicates the azimuth angle.

$Auxiliarycircuits$

The cathode-ray tube includes a high-voltage power supply and controls for adjusting the brightness and focus of the cathode-ray spot. The echo signal from the receiver is applied to the control grid (if positive) or cathode (if negative) of the electron gun of the indicator tube. The deflection of the cathode-ray beam to indicate the target coordinates is produced in sweep circuits. The range sweep creates a voltage or current which increases linearly with time and then suddenly reverts to its initial value, i.e. it has saw-tooth shape when plotted against time. Each tooth of the wave starts simultaneously with the transmitted pulse and ends after sufficient time has elapsed to allow the echo to arrive from the farthest target. The direction sweeps are similar, linear saw-tooth waves of current or voltage created by the motion of the scanner in azimuth and elevation.