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How to measure radiation above 30MHz? Explain the harassment power test thoroughly once

Author.

LCS

Source:

Post time:

2026-06-30

Industry pain point: After transmission, why did radiation fail?


 

Most R&D engineers who work on AV products, household appliances, and power tools have experienced this scenario:


 

Conducting emission (CE) once;

Radiation emission (RE) below 30MHz is also not a problem;

When the frequency exceeds 30MHz, the curve suddenly surges and is deemed unqualified.


 

Where is the root cause? Cable.


 

Experimental experience has shown that when the frequency exceeds 30MHz, most of the disturbance energy generated by the equipment does not leak out from the casing, but is transmitted along the power lines, signal lines, and control lines, and then radiated outward through the surface of the cable. These cables have become 'passive antennas', broadcasting the noise inside the device to the surrounding space.


 

This is the underlying problem that Disturbance Power Measurement aims to address.

2


 

Core principle: How to "grasp" interference with absorption pliers?


 

The basic logic of harassment power testing can be summarized in one sentence:


 

Define the disturbance level of the device using the disturbance power on the cable close to the device.


 

In the frequency range of 30MHz-300MHz, it can be considered as an alternative method for testing radiated disturbance field strength - especially suitable for products that are difficult to standardize in open or dark environments.


 

📍  Testing environment and layout

① Venue requirements: It must be conducted in a shielded room to isolate external electromagnetic interference.


 

② Placement of the tested equipment (EUT):


 

Placed on a non-metallic tabletop with a height of 0.8m

At least 0.8m away from any metal object or human body


 

③ Cable handling:


 

The tested feeder line is unfolded flat on the table

The length must be sufficient for the absorption clamp to slide freely to find the maximum radiation position


 

④ Absorption clamp direction: One end of the current converter faces the device under test


 

📡  Signal path

Absorption clamp captures interference → converted into current signal → sent to measurement receiver or spectrum analyzer through coaxial cable


 

During testing, the absorption clamp needs to slide repeatedly along each cable to find the position with the highest absorption power. The reading at this position is the final judgment value of this cable - the maximum disturbance level.


 

⚠️  Current Status of Frequency and Limit Values

At present, CISPR only provides disturbance power limits for the 30MHz-300MHz frequency band; The limit for 300MHz to 1000MHz has not yet been introduced.


 

4


 

Unqualified reasons: Six typical failure modes


 

Harassment power failure and radiation emission failure share the same root and origin - both are caused by high-frequency noise escaping to the outside through unexpected paths.


 

① Metal terminal suspension: The terminal is not connected to the ground of the entire machine and is at a floating potential. When interference accumulates on the ground of the terminal, the external cable instantly becomes a "transmitting antenna".


 

② Poor terminal casing overlap: High impedance overlap occurs between the metal terminal and the metal casing. This impedance drives the common mode interference current to radiate outward - it is the most typical common mode interference model.


 

③ PCB layout defect: High frequency signals cannot be reflow inside the circuit board, forcing the search for "external paths" - cables are the first to be affected.


 

④ Interference between interface cables: Improper internal wiring layout leads to coupling interference between signal lines, ultimately resulting in leakage through terminal wires.


 

⑤ Missing or insufficient signal cable filtering: There is no filtering circuit on the line, or the filtering parameters (cut-off frequency, insertion loss) cannot cover the problematic frequency band.


 

⑥ Excessive brush noise: In electric tools, the high-frequency spark noise generated by the motor brush is directly conducted and radiated along the power line.


 

5


 

Rectification measures: Five step closed-loop from design to remediation


 

Source control: Strictly follow EMC design specifications during the PCB stage - shorten high-frequency wiring, reduce signal loop area, and reduce interference from the source.


 

Grounding treatment: The metal terminal should be reliably connected to the ground of the machine casing, and the grounding impedance should be as low as possible to prevent floating state.


 

Internal interconnection optimization: Pay attention to shielding and path planning of terminal board connection wires to prevent crosstalk. For ports with excessive disturbance power, a ferrite magnetic ring is added to the internal wiring to effectively suppress high-frequency components.


 

Filter reinforcement: Adding magnetic beads/resistors/inductors capacitors to the signal line to construct a filtering network and filter out unwanted signals.


 

Motor Special: The high-frequency interference of electric brushes is filtered using a combination of capacitors and inductors to block noise from being transmitted to the outside through power lines.


 

💡  Troubleshooting technique: Use the "cable plugging and unplugging method" - remove the external cables one by one, observe which one significantly improves the test curve after unplugging, and that is the problematic port.

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