What is RADAR and its principle in detail?

• Waveguide

The pulses are transmitted to the scanner unit by the waveguide. A waveguide is hollow copper tubing, usually rectangular in cross section, having dimensions according to the wavelength of the carrier frequency. An electronic switch in the waveguide, called the transmit/receive cell (T/R) isolates the receiver during transmission to protect it from the high power of the transmission. In modern radars the waveguide and the T/R switch are usually located within the scanner unit.

•  Scanner

The most common type of radar aerial, the open array scanner is shown below.

Scanner can be housed within a radome as below. This particularly suits yachts as the rotating scanner can turn without fouling sails and rigging.

The scanner unit radiates the radar pulses and passes returning target echoes to the receiver. The scanner also focuses the outgoing microwaves into a tight beam in much the same way as a torch reflector focuses the light from a bulb. This is done in most marine radars by feeding the microwaves into a hollow tube inside the rotating scanner, which is also called a waveguide.
The waveguide is sealed at both ends but has a series of small slots in one side. Each slot acts like a small aerial but their combined effect is to focus the microwaves into a narrow beam. This type of scanner is called a horizontal slotted waveguide. The length of the scanner will determine the radar’s horizontal beamwidth (HBW). A wider scanner will result in a narrower beamwidth which will produce better bearing discrimination. The inside of a horizontal slotted radar scanner is shown below.

In every revolution of the scanner, radar pulses hit the target not once but many times during the time it is aimed at the target. The effect on the screen is cumulative, the more hits the brighter the target appears on the screen. Should insufficient pulses hit the target only a weak echo will be displayed and it will disappear quickly. The number of scanner revolutions should be between 20 and 30 revolutions per minute in order to both display the target brightly and prevent the disappearance of the picture between scanner revolutions. Most scanners operate at 24 rpm.
A more recent innovation is the patch aerial. This aerial uses a printed circuit consisting of an array of copper pads, to focus the beam.