Radiated Emissions Measurement

Radiated Emissions Measurement How To

Ambient Field Strength values are measured as follows.


  1. Place the antenna at the proper distance from the source of the emissions, or the product being tested. This will be one, three or ten meters depending on the exact specification being tested.

  2. Aim the antenna at the product being tested. This includes bore-site direction, and polarization (polarity).

  3. Connect the output of the receiving antenna to the input of the receiving system (spectrum analyzer used to measure field), using a low VSWR, low loss coaxial cable. The input impedance for the receiving system, and the receiving antenna should all be matched to 50W . Most antennas are already matched to this impedance, if they are EMI/EMC antennas, as should most receiving systems. If this is not the case, the readings will require correction for the impedance mismatch.

  4. Select the frequency of interest on the receiving system. Ensure that the receiving antenna is capable of providing accurate measurements at that frequency. This includes ensuring the antenna can receive a signal at the frequency of interest by its designed frequency range, provided by the manufacturer, and that the antenna is properly calibrated at that frequency.

  5. Measure the RF Voltage, Va, referenced to the receiving system. The units should be in decibels referenced to 1 micro-volt, dBm V. If the units are in absolute micro-volts (dependent on the receiving system used), they must be converted. The conversion factors are as follows, however, more information about the conversion can be found in the Conversion Technical Note.

  6. Va(dBm V) = 20Log10[Va(mV)] 

  7. Once the reading is converted to dB micro-volts (dBm V), it can be added to the antenna factor to determine absolute field strength. The antenna factor is provided with an antenna when it is calibrated or newly purchased. If the antenna factor for an antenna is not known, typical values can be used to approximate field strength, in development, however, this is not acceptable for compliance.

  8. Field Strength (dBm V/m) = RF Voltage (dBm V) + Antenna Factor (dB/m)

    Ea (dBm V/m) = Va (dBm V) + AF (dB/m) 

    However, the losses due to the cable between the receiving antenna and the receiving system should also be included. The cable losses can be determined easily, by connecting the cable in question between a transmitter (such as a tracking generator) and the receiver (the spectrum analyzer) at the frequency of interest, and examining how much lower the signal is than the output source is set to provide. (for example, if the tracking generator is transmitting at a given strength, then this strength minus the strength measured at the spectrum analyzer is the cable loss). The above formula, including cable losses, now becomes...

    Ea (dBm V/m) = Va (dBm V) + Ac (dB) + AF (dB/m)

Contact Details

A.H. Systems, inc.
9710 Cozycroft Ave.
Chatsworth, CA 91311

P: (818) 998-0223

F: (818) 998-6892

E: Email