Licence-exempt 900 MHz link, ERP budgeting and a neighbour's cordless phones
A rural grain co-operative plans a 902–928 MHz frequency-hopping data link to carry scale readings from an elevator to an office 6 km away. The radio is certified licence-exempt equipment rated 1.0 W conducted output. The antenna is a 9 dBi yagi fed by 25 m of 12 mm foam-dielectric coax; the installer's sweep sheet shows 3.4 dB of cable loss plus 0.5 dB for connectors and the lightning arrestor at 915 MHz. Since the link went live, a neighbour reports that her 900 MHz cordless phones have become unusable, and she has complained to the co-operative rather than to ISED. The installer says 'the radio is only 1 W, so it cannot be the cause' and proposes adding three metres of mast height and substituting a 12 dBi yagi. The co-operative also asks whether the 902–928 MHz band is protected, whether a spectrum licence is needed, and what would happen if the taller, higher-gain installation made the neighbour's problem worse.
1. Calculate conducted power, EIRP and ERP in dBm and watts, and explain which figure matters when assessing the neighbour's complaint.
Read the worked answer
Conducted power: 1.0 W = 30 dBm.
Feedline and connector loss: 3.4 + 0.5 = 3.9 dB.
EIRP = 30 + 9 − 3.9 = 35.1 dBm ≈ 10^3.51 mW ≈ 3.2 W.
ERP = EIRP − 2.15 dB = 32.95 dBm ≈ 2.0 W, because 0 dBd = 2.15 dBi and ERP is referenced to a half-wave dipole.
Why it matters: the installer's figure describes power at the radio connector, before the antenna. What leaves the antenna toward the neighbour is roughly 5 dB higher here (a factor of ~3), and the yagi concentrates that energy in a narrow forward lobe rather than spreading it uniformly. Complaint analysis, RF-exposure screening and certification-limit checks must therefore be done in EIRP/ERP plus antenna pattern terms, not conducted power. Note also that the certified maximum is usually stated as a combination of conducted power and antenna gain, so both numbers must be tracked together; swapping either one moves the system away from the configuration that was tested.
2. Is the 902–928 MHz band protected? What should the co-operative do about the complaint?
Read the worked answer
The band in Canada is licence-exempt and shared with industrial, scientific and medical equipment and many other users. No spectrum licence is required, but operation is conditional: equipment must be certified, must accept interference received, and must not cause harmful interference. So 'it is only 1 W' is not a defence, and the co-operative cannot ignore the report.
Investigate systematically:
- Is the neighbour inside the yagi's main lobe, or only in a sidelobe?
- Is her phone suffering in-band contention (a 900 MHz cordless phone hops through the same spectrum the FHSS link uses) or receiver blocking from a very strong nearby signal?
- Test by temporarily reducing conducted power 10 dB, re-aiming away, or terminating into a dummy load; if the phones recover, the link is implicated.
- Log times, frequencies and weather to separate intermittent from continuous effects.
Likely remedies: lower EIRP, re-aim or lower the antenna, add filtering, shorten the path with a relay, or advise a different cordless technology such as DECT at 1.9 GHz. Document everything, because an unresolved harmful-interference case can be escalated to ISED.
3. Evaluate the proposed taller mast and 12 dBi yagi. What compliance issues arise?
Read the worked answer
New EIRP = 30 + 12 − 3.9 = 38.1 dBm ≈ 6.4 W, i.e. about +3 dB over the present 35.1 dBm - roughly double the power in the wanted direction - and the extra three metres of height gives a clearer path and a larger radio horizon, so the neighbour's exposure to the signal is likely to get worse, not better.
Compliance issues:
- Certification covers a defined transmitter-plus-antenna configuration. Fitting a higher-gain antenna generally requires an equal reduction in conducted power (for example, dropping to about 0.5 W to hold EIRP constant); otherwise the installation no longer matches the certified configuration and the equipment is being operated outside its approval.
- Greater height and gain can push the installation past the RF-exposure assessment limits for accessible areas, and can extend the interference contour.
Recommendation: fix the interference with the existing hardware first - power reduction, re-aiming, relocation, filtering - and document the result. Only then consider a gain change, and if it is made, apply the compensating power reduction stated in the manufacturer's instructions and re-check the exposure and interference picture.