It's a long time ago, but if I recall correctly from researching the Flight article, because of volume constraints inside the Nimrod they had to split the transmitter into two, one for the forward antenna and one for the rear. Because this type of radar relies on a very stable reference frequency to detect Doppler shifts, that led to challenges correlating returns from the forward and aft radars to produce a single coherent track. Not the Nimrod AEW's sole problem, but one of them - again if I recall correctly