Paging transmitter interference, a 40-minute log gap and a handheld guard receiver
KDRX shares a hilltop radio site with a commercial paging operator whose 100 W transmitter on 152.6 MHz was commissioned six months ago. Since then the AFIS receiver on 118.300 emits a loud buzz and loses weak aircraft transmissions whenever the paging transmitter keys; the buzz stops when the paging antenna is disconnected during a test. The site owner says the paging operator holds a valid licence and that the AFIS should 'buy a better receiver'. Separately, a review of the station log shows a 40-minute gap on the night shift covering the handover, and the 121.5 guard is provided by a handheld scanner on a shelf in the operations office with the volume turned low. The regulator's audit is in two weeks. The watchkeeper must decide what to do about the interference now, what to correct in the logging and watchkeeping arrangements, and what evidence to assemble.
1. What immediate action preserves the safety of the service while the interference is unresolved, and what may the watchkeeper not do?
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The priority is that no aircraft loses the AFIS because of a site problem.
- Establish the interference mechanism and get it switched. The correlation is already strong: the symptom appears when, and only when, the 152.6 MHz transmitter keys, and disappears when its antenna is disconnected. That points to receiver blocking/desensitisation from a very strong out-of-band signal, possibly with an intermodulation product falling in the band. Ask the site owner to inhibit or power down the paging transmitter during AFIS hours as a written temporary measure, and test with the standby receiver and an alternative antenna to see whether the receiver or the site is at fault.
- Establish the backup. If 118.300 becomes unusable, switch to an assigned alternate or backup frequency if one exists, and tell pilots on the frequency and via the ATIS or telephone briefing before switching. Advise the parent ATS unit immediately.
- Do not transmit on an unassigned frequency to escape the noise, do not increase power in the hope of overriding it, and do not leave 121.5 unguarded while dealing with the problem.
- Do not accept the site owner's framing. Buying a 'better receiver' may help with blocking, but it does not legitimise a new signal that degrades an existing safety service.
2. Assess the 40-minute log gap and the handheld guard receiver against watchkeeping requirements, and state the correction.
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The log must be continuous and contemporaneous: it is the evidence that the watch was maintained, and it is the primary record an investigation would use to reconstruct what a station knew and when. A 40-minute gap covering a handover means the station cannot demonstrate continuous watch, cannot show what traffic was handled, and cannot prove that 121.5 was monitored. If an incident occurred in that window, the station has no defence.
The handover itself should be a face-to-face event with a signed entry recording the time, the serviceability of each receiver and transmitter, the frequencies guarded, outstanding items (including the interference episodes), and any limitations in force. A gap of this length is consistent with an informal handover where neither watchkeeper believed it was their entry to make - which is a procedural failure, not an individual one.
The handheld scanner on a shelf at low volume is not an acceptable guard. It is not a fixed installation, it is not audible at the operating position, its battery and coverage are unverified, and it cannot be shown to have been monitored. Replace it with a dedicated receiver at the operating position, audible at working volume, with a recording facility; if 121.5 cannot be guarded, that limitation must be declared to the parent unit and logged, not concealed.
3. What evidence should be assembled for an interference report, and what should the report ask for?
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An interference report is only as good as its measurements. Assemble:
- Timing: start and end of each episode in UTC, duration, and how often it recurs per shift.
- Correlation: a table showing that the disturbance begins and ends with the paging transmitter's key-ups, including the test in which the paging antenna was disconnected.
- Identification: the interfering source (152.6 MHz, 100 W, licence holder, site address, distance and bearing from the AFIS antenna), and the victim installation (receiver type, antenna, feeder, height above site ground).
- Symptom description: audio recording of the buzz, a description of the loss of weak signals, and any specific transmissions lost, with the aircraft callsigns and times from the log.
- Site data: a site plan showing antenna proximity, and any measurements taken - received signal level of the paging carrier at the AFIS receiver input, and whether the symptom persists with a dummy load or a filter in line.
- Impact statement: the operational consequence, in terms of lost or degraded communications with identified aircraft.
Submit it to the national administration in the prescribed form, copied to the parent ATS unit and the site owner, and ask for three things: technical investigation of the interference, coordination to remove it (filtering, antenna separation, power reduction, time-sharing or relocation), and a written interim measure covering AFIS hours. Keep a copy with the log extract; the audit will ask for exactly this.