what is the expansion of RADAR?

Answer 1

The term RADAR was coined in 1941 as an acronym for Radio Detection and Ranging. This acronym of American origin replaced the previously used British abbreviation RDF (which stands for Radio Direction Finding). The term has since entered the English language as a standard word, radar, losing the capitalization in the process.

Answer 2

RADAR : RAdio Detection And Ranging.
A pulse of radio energy is emitted. If a metallic object is there to reflect part of it, it could be detected and, using the time taken for the pulse to leave the antenna, hit the object and return to the antenna, the range (distance) could be determined.

The early antennae were fixed (they did not turn like modern radars) so only the distance was given, not the direction.

Modern devices have a mobile antenna (usually a rotating one). Pulses are transmitted at very short intervals. On a screen, the line that becomes brighter whenever an echo is detected, is rotating at the same rate as the antenna: this gives the direction AND the range of the object.

RDF: Radio Direction Finder
A radio signal can be received from any direction if you use a vertical antenna. However, if you use a loop antenna, there are two directions -- relative to the shape of the antenna) from which the signal is strongest, and two where it is weakest.

By combining the two types of antenna on the same receiver, you can find a unique direction where the signal disappears completely (called the "null" signal). This allows you to know the direction towards the transmitter. The direction finder does not transmit a signal. It only needs to "hear" the signal being transmitted.

By finding directions to more than one known transmitters, a ship officer can find the ship's position.

Answer 3

I have heard both "Radio Aircraft Detection And Ranging" and "Radio Antiaircraft Detection And Ranging".