Friday, June 10, 2011

The AN/SPQ-2D was a surface search radar that could transmit both short and long pulses at the same time. It was designed and manufactured by the Italian firm Signalamento Marittemo eo Aero of Florence Italy. The AN/SPQ-2D alternated the long and short pulses, receiving the transmitted RF returns in the (PRF limited) range prior to transmitting the next alternate pulse. Short pulse was used for close-in targets (giving minimum surface ranges), while LP was for longer surface distances and the "air coverage". The SPQ-2D's antenna was a Cosecant-squared parabolic mesh. Sometimes people associate the SPQ-2D with having two antennas. The "second antenna" in the AN/SPQ-2D radar is actually the Mk XII IFF interrogator antenna associated with that radar. It was fitted to the 280 class ships in their original configuration.

Impulsive noise, which could shock-excite the narrow-band radar receiver and cause it to ring, could be reduced with the Lamb noise-silencing circuit, or Dicke fix." This consisted of a wideband IF filter in cascade with a limiter, followed by the normal IF matched filter. The wideband filter was designed to include most of the spectrum of the interfering signal. Its  purpose was to preserve the short duration of the narrow impulsive spikes. These spikes were then clipped by the limiter to remove a considerable portion of their energy. If the large noise spikes were not limited and were allowed to pass they would shock-excite the narrowband IF amplifier and  produce an output pulse much wider in duration than the input pulse. Therefore the interference would be in the receiver for a much longer time and at a higher energy level than when limited before narrowbanding. Desired signals which appeared simultaneously with the noise spike might not be detected, but the circuit did not allow the noise to influence the receiver for a time longer than the duration of a noise spike.