External cross-modulation in the 100-Mc/s band

The introduction of very-high-frequency equipment to the Fleet Air Arm involved the fitting of numerous v.h.f. transmitters and receivers in ships, and the number was further increased by the discovery that these frequencies were also convenient for ship-to-ship working. It was found that when a number of transmitters and receivers were fitted in the same ship, the number of workable channels available in the band was much smaller than was the case ashore, where grouped transmitters and receivers are set up on well separated sites. Experiments were carried out in 1944 by members of the Communications Department, Admiralty Signal Establishment, which proved that the effect was due to external cross-modulation arising in parts of the ship´s structure, which act as non-linear conductors. A description is given of an apparatus which has been recently developed to assist in locating accurately the parts of the ship´s structure responsible for the interference, as part of an investigation into the possibility of reducing the effect.