Enforcement Report on a 460 MHz PMR Installation
A regulator's monitoring team investigates interference reported by a taxi operator whose channel is the adjacent 12.5 kHz allocation at 460.7625 MHz. The suspect is a licensed PMR base on 460.7500 MHz. From a fixed monitoring station 30 m from the base antenna, in clear line of sight, the team uses a calibrated antenna (factor 19.5 dB/m at 460.75 MHz, 24.0 dB/m at 921.5 MHz), 2.5 dB of cable loss and a spectrum analyser. On the carrier, with 10 kHz resolution bandwidth, the analyser reads -21 dBm; at 921.5000 MHz, with 100 kHz resolution bandwidth, it reads -30 dBm. A frequency counter gives 460.7515 MHz, and a channel-power measurement gives a 99% occupied bandwidth of 16 kHz. The licence permits +/-1.0 kHz frequency tolerance and 25 W ERP, and requires all emissions outside the authorised bandwidth to be at least 60 dB below the mean carrier power. The complainant's receiver is 1.5 km away and its noise floor drops to normal when the suspect base is switched off.
1. Convert the measurements to field strength and ERP, and list which licence conditions are breached.
Read the worked answer
Conversions. E(dBµV/m) = P(dBm) + 107 + antenna factor + cable loss; ERP from E at distance d: E = 114.8 + 10log P(W) - 20log(d/10).
Carrier. E = -21 + 107 + 19.5 + 2.5 = 108.0 dBµV/m at 30 m. 108.0 = 114.8 + 10log P - 9.54, so 10log P = 2.74 and P = 1.9 W ERP against a licensed 25 W.
Harmonic at 921.5 MHz. E = -30 + 107 + 24.0 + 2.5 = 103.5 dBµV/m at 30 m. 103.5 = 114.8 + 10log P - 9.54, so 10log P = -1.76 and P = 0.67 W.
Breach list.
- Spurious emission. The harmonic is only 108.0 - 103.5 = 4.5 dB below the carrier, against a required 60 dB, i.e. roughly 55 dB out of compliance. This is the most serious finding.
- Frequency tolerance. 460.7515 - 460.7500 = +1.5 kHz (3.3 ppm), against a permitted +/-1.0 kHz. Non-compliant.
- Occupied bandwidth. 99% power in 16 kHz on a 12.5 kHz channel plan exceeds the authorised emission bandwidth and is the likely cause of the neighbour-channel complaint.
- Power. Measured ERP is inside the licensed 25 W, but it is about 11 dB low, which is itself a symptom worth investigating rather than a breach.
2. Assess whether the measured non-compliances explain the taxi operator's complaint, and say what further measurement you need.
Read the worked answer
The harmonic is not the cause. A 921.5 MHz emission cannot raise the noise floor of a receiver tuned to 460.7625 MHz; the victim's front end will not respond there. It is a separate, and serious, compliance failure found during the inspection, and it must be reported as such rather than as the explanation of the complaint.
The likely cause is adjacent-channel spill. A 16 kHz occupied bandwidth against a 12.5 kHz channel plan puts energy directly into 460.7625 MHz, and the complaint clears when the base is switched off, which fits an adjacent-channel mechanism. Two further candidates should be tested rather than assumed: front-end blocking or desensitisation of the taxi receiver from the strong local carrier, and broadband noise from a degraded output stage.
Measurements to obtain.
- Adjacent-channel power ratio at the transmitted channel edges, measured as channel power rather than as a spot reading, with the base keyed on a test tone as well as on speech.
- Peak deviation and modulation limiting, since excessive deviation or a mis-set audio limiter is the usual reason a 12.5 kHz design spreads to 16 kHz.
- Noise floor at the taxi receiver with the base keyed and unkeyed, and with a notch filter on 460.75 MHz, to separate adjacent-channel spill from blocking.
- A conducted measurement at the transmitter output through a calibrated attenuator and a 460 MHz notch filter, to confirm both the harmonic level and the carrier power without exposing the analyser to 25 W.
Diagnostic to keep in mind. The combination of a carrier 11 dB below licensed power and a harmonic only 4.5 dB down points at a degraded output stage or a damaged output filter, not at an adjustment error. That single fault could explain the low carrier, the poor harmonic suppression and, through degraded linearity, the wider emission.
3. State the measurement uncertainties that could be challenged and set out the enforcement sequence.
Read the worked answer
Uncertainty sources to declare. Antenna factor calibration uncertainty, typically about +/-1 dB; cable and connector uncertainty; site multipath, which on field strength at 30 m can be several dB even in clear line of sight; alignment of the antenna to the direction of maximum field; analyser amplitude accuracy; and the resolution-bandwidth correction, since a 10 kHz RBW on a 12.5 kHz signal is not a channel-power measurement and a 16 kHz occupied bandwidth should be quoted with the method used. The assumed 30 m is a distance to a point on the structure, not necessarily to the antenna phase centre. None of these comes close to covering a 55 dB spurious shortfall or a 50% bandwidth excess, but they matter for the power figure and should be stated.
Enforcement sequence.
- Issue the measurement report with uncertainties and the licence clauses cited, and give the licensee the opportunity to witness a retest.
- Require corrective action: repair or replace the output stage and output filter, re-adjust deviation and audio limiting to fit 12.5 kHz, recalibrate the reference oscillator, and re-coordinate the 921.5 MHz spurious so that it meets the 60 dB requirement.
- Impose a retest date and a monitoring period, with the adjacent-channel power and carrier frequency checked again at the same site and distance.
- Consider the interference to the complainant separately: the licensee is responsible for emissions outside the authorised bandwidth, and remediation should be confirmed at the victim receiver, not only at the transmitter.
- If the spurious radiation persists at a level that can affect other services, escalate to restriction of the authorisation, since a spurious emission at 0.67 W ERP on 921.5 MHz is capable of affecting services well beyond the local area.