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Screening
There are two possible problems here ,you could be picking up RI from the plugs,have you come across resistor plugs or resistor plug caps, dont go fitting them unless it is okayed in the engine manuel there may or may not be equalivent plugs for your motor not all ignition systems can cope with them.The problem is probably caused not by the introduction of the plotter,but the cables running in close proximity to the radio areial cable.If you ran a live wire beside the areial lead a voltage can be induced into it from the live wire, (positive).Without doing anything, try to seperate if it is done the new cables from the vhf cables, see if this cures the problem, try turning off plotter from its source, see if this cures the problem, finally with the VHF switched off disconnect the aerial connection, check if the outside braid of the coax is making a good connection to the outer part of the gland, this screen should only be connected at one end, and not to anything else which represents earth at the other end, ie A frame metalwork, or a metal mount which has a connection to earth.Check to see if the VHF has negative earth, this will be evident from the manuel ,it more than likely will have, take a jump lead from the battery negative and connect it to the outside of the VHF radio aerial gland and see if this provision of a new negative removes induced noise.
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