A recent investigation of interference to communications between pilots and the control tower for aircraft approaching Auckland airport highlights the risk of unintended signals being generated as an unwanted side effect of radio broadcasting. This investigation took several weeks because of the low level of signal and intermittent observations of the interference. It involved considerable RSM staff resource as well as requiring the hire of an aircraft to trace the source. While not causing a safety risk, because of the availability of alternative radio channels, the interference did present a significant annoyance and distraction to pilots.
When located, the problem proved to be an unwanted spurious emission from an FM broadcast station transmitter in a community north of Auckland and was quickly addressed by the broadcaster concerned. A recent study by the Australian Communications and Media Authority, checking broadcast stations in Australia, suggests that this sort of problem is relatively common. Around 28% of the transmitters they checked showed the production of unwanted emissions. Many of the emissions were radiated in the aeronautical radiocommunication band of 108MHz to 136MHz, presenting a potential risk to safe operation of aircraft.
RSM supports the use of external filters as an effective means of limiting the radiation of spurious signals, and recommends that all broadcasters should note the tips given in the CBAA news item for monitoring the health of their transmissions.
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