Type 243

The Type 243 is an A-band Mark III secondary surveillance radar (SSR) interrogator designed to identify aircraft as hostile or friendly. Operating at frequencies between 157 and 187 MHz with a pulse repetition frequency of 50 or 100 Hz, a pulsewidth of 10 µs, and a peak power of 1 kW, it was deployed on small ships in conjunction with air warning radars. Its non-directional AQF antenna provides range information only. The corresponding transponders, known as IFF Mark III in aircraft and Type 253 in ships (or ABK and BK in the U.S. Navy, respectively), are identical and sweep their waveband every 2.5 seconds. When interrogated by the Type 243, these transponders return a response every 2.5 seconds, typically as a narrow (6-10 µs), wide (17-28 µs), or very wide (80 µs) pulse, displayed on the parent radar's A-scope slightly beyond the target's location echo (200-500 yards depending on response strength).

radartutorial.eu: https://www.radartutorial.eu/19.kartei/14.ssr/karte016.en.html