Radiated-emissions measurement at 3 m for a 433.92 MHz transmitter
A technician at a small Canadian test lab is making a 3 m radiated-emissions measurement on a 433.92 MHz remote keyless-entry transmitter. The setup uses a calibrated log-periodic antenna whose antenna factor at 434 MHz is 19.8 dB/m, connected by 4 m of RG-213 with 1.6 dB of loss at that frequency, plus 1.2 dB for connectors and a surge arrestor. The measuring receiver shows 41.5 dBµV with the transmitter keyed continuously, and 32.0 dBµV with the transmitter switched off. The applied standard specifies a field-strength limit of 80 dBµV/m at 3 m and requires a quasi-peak detector with a 120 kHz resolution bandwidth between 30 MHz and 1 GHz. The technician reports a pass with 'about 16 dB of margin, so ambient and uncertainty do not matter'. The lab supervisor also asks what the result would be if the measurement were repeated at 10 m.
1. Compute the measured field strength in dBµV/m and in mV/m, and correct for ambient.
Read the worked answer
Field strength = receiver reading + antenna factor + cable loss + connector/arrestor loss:
E = 41.5 + 19.8 + 1.6 + 1.2 = 64.1 dBµV/m, which is 10^(64.1/20) µV/m ≈ 1.6 mV/m.
Both the antenna factor and the losses are added because the antenna factor converts the voltage at the receiver terminals to field strength, and the cable and connector losses occur before those terminals, so the true terminal voltage is higher than the receiver displays.
Ambient correction. The 32.0 dBµV reading with the transmitter off is not negligible by inspection, but the correct combination is a power subtraction, not a simple difference:
- 41.5 dBµV = 118.9 µV; 32.0 dBµV = 39.8 µV.
- Corrected voltage = √(118.9² − 39.8²) ≈ 112.0 µV = 41.0 dBµV.
So the ambient contributes about 0.5 dB. Corrected field strength ≈ 63.6 dBµV/m, giving a margin of about 16.4 dB against the 80 dBµV/m limit.
2. Which aspects of the measurement setup still need checking before the result is reported?
Read the worked answer
A number is not a result until the setup is defensible. Check:
- Detector and bandwidth. The standard requires quasi-peak detection and 120 kHz resolution bandwidth here; a peak reading with a 1 MHz span overstates the level, and a peak reading is not what the limit applies to.
- Distance and far-field. At 434 MHz the wavelength is about 0.69 m; 3 m is comfortably beyond the far-field boundary for a small EUT, but the antenna height, EUT height and ground-plane arrangement must match the standard.
- Receiver integrity. Confirm the input attenuator and preamplifier are set so the front end is not compressing on a strong nearby signal; verify with a known signal or attenuator check.
- Calibration status of antenna, cable and receiver, and that the antenna factor is the one applicable at 434 MHz, not an interpolated value from another band.
- EUT conditions. Continuous keying is a test convenience, but the standard may require a representative duty cycle, defined orientation, and worst-case polarisation.
- Uncertainty. Measurement uncertainty must be calculated and, where the standard requires it, applied to the compliance decision rather than quoted as a bare margin.
3. What changes if the measurement is moved to 10 m, and can the 3 m limit still be used?
Read the worked answer
For a source that behaves like a point radiator in the far field, field strength falls at approximately 20 dB per decade of distance, so the free-space correction from 3 m to 10 m is:
20 log₁₀(10/3) ≈ 10.5 dB
Applying that to 63.6 dBµV/m gives about 53.1 dBµV/m at 10 m, plus the appropriate ambient correction at the new location, which may be different.
Compliance point: distance correction is an engineering estimate, not a licence to move the limit. The standard states its limit at a defined distance, often 3 m for small EUTs and 10 m or 30 m for larger ones or for particular frequency ranges. You may only compare a 10 m result against a 10 m limit, or against a 3 m limit if the standard itself permits extrapolation. If the measurement distance was changed because the EUT could not be fitted in the far field at 3 m, the standard's own table for that case must be consulted and the choice documented in the report.