5.2 Harmful Interference Definition, Spurious Emissions & Filtering
Key Takeaways
Harmful interference includes endangering a radionavigation or other safety service, or obstructing, repeatedly interrupting or seriously degrading a radiocommunication service, in the defined operating circumstances.
Section 15 uses the lesser attenuation number in its applicable frequency row. Below 30 MHz use pX. At or above 30 MHz use power P supplied to the antenna transmission line.
Harmonics occur at integer multiples of the wanted frequency. Out-of-band modulation products and spurious emissions require different diagnostic checks.
Stop the operation causing harmful interference and investigate. A dummy-load comparison can narrow down a coupling path, and direct measurement can check spectral purity.
Identify the applicable interference rule
The 2025 Interpretation Determination defines harmful interference for the Class Licence. Interference qualifies if it endangers a radionavigation or other safety service operating in accordance with the Radio Regulations or the Spectrum Plan. It also qualifies if it obstructs, repeatedly interrupts or seriously degrades a radiocommunication service operating under either of those frameworks. Compare ITU Radio Regulations No. 1.169 for the international definition.
Class Licence section 15(1) prohibits operation that causes harmful interference. For amateur apparatus licences the 2025 Amateur LCD section 11 also covers interference from off-frequency operation, Morse side effects, harmonics and emissions outside the necessary bandwidth, and requires reasonably practicable prevention measures. Act section 197 has a separate civil test for conduct that will or is likely to cause substantial interference, disruption or disturbance. During a period of emergency, the Minister may make a written restrictive order under section 222. Section 227 concerns contravention of such an order. A report about a domestic device needs investigation before assigning a cause.
Recognise the unwanted signal
The ITU groups out-of-band emissions and spurious emissions as unwanted emissions. Modulation can produce components just outside the necessary bandwidth. Spurious products include harmonics, parasitic emissions and certain intermodulation or frequency conversion products. An abrupt Morse keying envelope can spread energy to nearby frequencies and cause key clicks. Check envelope shaping and the transmitter configuration.
For example, the second harmonic of 7.100 MHz is 14.200 MHz, and the third is 21.300 MHz. A transmitter low-pass filter suited to the wanted frequency may attenuate those higher frequencies. It cannot correct nearby splatter caused by driving an amplifier into compression. Correct the drive or keying envelope where measurements point to that cause.
Calculate required attenuation
Class Licence section 15(2) compares the spurious emission with the power in the wanted emission’s necessary bandwidth supplied to the antenna transmission line. Use watts in the expression and calculate the base-10 logarithm.
| Operating frequency | Required attenuation |
|---|---|
| Below 30 MHz | Lesser of 43 + 10 log₁₀(pX) dB and 50 dB. pX is peak envelope power. |
| At or above 30 MHz | Lesser of 43 + 10 log₁₀(P) dB and 70 dB. P is power in watts supplied to the antenna transmission line. |
HF example. With 100 W pX at 7.100 MHz, the expression gives 43 + 20 = 63 dB. The lower number is 50 dB, so that is the required attenuation under this rule. VHF example. With 10 W supplied to the antenna transmission line at 146 MHz, the expression gives 43 + 10 = 53 dB. That is less than the 70 dB alternative. Meeting the spurious limit leaves the separate harmful-interference condition in place.
Check the reference power. A 1 W pX HF signal gives 43 + 10 log₁₀(1) = 43 dB, so the 50 dB branch is not the answer at that power. A high-power HF case can instead select the 50 dB branch. If the measured spurious component is 45 dB below the wanted reference in the 100 W pX example, it needs at least 5 dB more attenuation. If the suppression is 55 dB, it meets that numerical requirement. The result uses the measured component relative to the reference at the antenna transmission line, not the number printed on the front of the amplifier.
Make a separate check for energy near the band edge or interference to other services. Changing the RF envelope can improve a key-click problem without changing the calculated spurious attenuation threshold. If the measured harmonic is still too strong, inspect the filter, amplifier or antenna-system component identified by the tests. Confirm the intended emission still fits its own authorised conditions after the change.
Isolate the source before resuming operation
If the station is causing harmful interference, stop the operation producing it. Record the frequency, mode, power, times and the service affected. A comparison into a properly rated shielded load can help isolate the antenna or feedline path. If the symptom vanishes, that result alone cannot establish compliance. Use appropriate measurements to identify the unwanted product, correct the fault and confirm the correction before returning to the affected operation. Follow any ACMA direction that applies.
Amateur Class Licence, section 15 · 2025 Amateur LCD, section 11 · 2025 Interpretation Determination · ITU-R SM.329-13
Which example clearly meets the harmful-interference definition in the 2025 Interpretation Determination?
A weak station can be heard and understood above background noise.
An operator hears static while the transmitter is off.
A receiver detects an adjacent signal without any disruption to its service.
A spurious emission seriously degrades an aeronautical radionavigation service operating in accordance with the Spectrum Plan.
A 7.100 MHz class-licensed station supplies 100 W pX in the wanted necessary bandwidth to the antenna transmission line. Which attenuation does section 15(2) require for spurious emissions?
50 dB.
63 dB.
70 dB.
43 dB.
Which transmitter behaviour can cause key clicks on neighbouring frequencies during Morse operation?
A correctly matched antenna.
Using a 12 V supply within the radio’s specified limits.
Switching the RF envelope on and off too abruptly.
Reducing the receive volume.
Sections you finish are checked off in the contents.