Christchurch (New Zealand) radio inspectors were involved with a commercial interference case affecting a major microwave link in the upper South Island.
The microwave operator was able to identify that the link was only being affected when the same fishing vessel crossed its path in the Cook Strait. This led the inspectors to a number of conclusions however the only possible way to determine the interference was to position themselves on a hilltop overlooking the Strait. Using a microwave dish and sufficient gain they were able to see the interfering signals radiating from the fishing boat. By utilising the AIS vessel tracking website their position was verified.
It was obvious that the fishing boat was the cause of the interference, however what was not known was why it was emitting these transient signals. The radio frequency signature of the noise looked to be very similar to the S Band radar aboard the vessel.
The fishing company was called and the S Band radar was turned off. Immediately the noise ceased and it was obvious that this was the cause of the interference. A subsequent letter was sent to the fishing company and the S Band radar’s magnetron was replaced.