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[Official FAQ explainer: short answers with the numbers you can check]
Q1. How do I convert dBm to dBuV?
In a 50 ohm system, dBuV = dBm + 107 (more precisely 106.99). 0 dBm is 0.2236 V rms, which is 107 dBuV. -60 dBm is 47 dBuV. In a 75 ohm system (CATV), add 108.75 instead.
Q2. What is antenna factor (AF)?
AF converts the voltage at the antenna terminals to the incident field: E (dBuV/m) = V (dBuV) + AF (dB/m) + cable loss (dB). AF is what EMC receivers and test reports use. It depends on frequency and on the antenna gain.
Q3. How is AF related to gain?
For a 50 ohm antenna: AF (dB/m) = 20 log10 f(MHz) - G(dBi) - 29.79.
Example: at 100 MHz with G = 6 dBi, AF = 40 - 6 - 29.79 = 4.2 dB/m. At 1 GHz with the same gain, AF = 24.2 dB/m. A constant-gain antenna therefore has an AF that rises 20 dB per decade. This is why reading noise floors in dBuV/m at high frequency looks worse even when the receiver is unchanged.
Q4. What field strength does a transmitter produce?
In the far field, E (V/m) = sqrt(30 x EIRP in W) / d (m).
Example: 1 W EIRP at 3 m gives 1.83 V/m, which is 125.2 dBuV/m. At 10 m it is 0.55 V/m (114.8 dBuV/m). The power density is E^2/377, about 8.9 mW per square metre at 3 m.
Q5. Why do my 3 m and 10 m measurements not differ by exactly 10.5 dB?
20 log10(10/3) = 10.5 dB holds only in the far field over free space. On an open-area test site, the ground reflection, receive antenna height scan and near-field effects at low frequencies (below about 100 MHz at 3 m) all break the simple 1/d law. That is why EMC standards specify site validation (NSA or sVSWR) instead of relying on the formula.
Q6. I have a received power in dBm. What is the field?
With AF in hand: E (dBuV/m) = P (dBm) + 107 + AF + cable loss. Example: -40 dBm at the receiver, AF = 18 dB/m, cable loss 2 dB gives 87 dBuV/m.
Q7. What about dBpT or A/m for magnetic fields?
In the far field, H = E / 377. In dB terms, dBuA/m = dBuV/m - 51.5. Near the source (loop antennas, below 30 MHz) this ratio does not hold and the field must be measured with a calibrated loop.
Checklist: always state the impedance (50 or 75 ohm), whether the value is peak, quasi-peak or average, and the measurement bandwidth. A level without its detector and RBW is not a complete number.
Questions or corrections? Reply below and we will fold good ones into this FAQ.
— rfforum 编辑部 / Editorial Team
Q1. How do I convert dBm to dBuV?
In a 50 ohm system, dBuV = dBm + 107 (more precisely 106.99). 0 dBm is 0.2236 V rms, which is 107 dBuV. -60 dBm is 47 dBuV. In a 75 ohm system (CATV), add 108.75 instead.
Q2. What is antenna factor (AF)?
AF converts the voltage at the antenna terminals to the incident field: E (dBuV/m) = V (dBuV) + AF (dB/m) + cable loss (dB). AF is what EMC receivers and test reports use. It depends on frequency and on the antenna gain.
Q3. How is AF related to gain?
For a 50 ohm antenna: AF (dB/m) = 20 log10 f(MHz) - G(dBi) - 29.79.
Example: at 100 MHz with G = 6 dBi, AF = 40 - 6 - 29.79 = 4.2 dB/m. At 1 GHz with the same gain, AF = 24.2 dB/m. A constant-gain antenna therefore has an AF that rises 20 dB per decade. This is why reading noise floors in dBuV/m at high frequency looks worse even when the receiver is unchanged.
Q4. What field strength does a transmitter produce?
In the far field, E (V/m) = sqrt(30 x EIRP in W) / d (m).
Example: 1 W EIRP at 3 m gives 1.83 V/m, which is 125.2 dBuV/m. At 10 m it is 0.55 V/m (114.8 dBuV/m). The power density is E^2/377, about 8.9 mW per square metre at 3 m.
Q5. Why do my 3 m and 10 m measurements not differ by exactly 10.5 dB?
20 log10(10/3) = 10.5 dB holds only in the far field over free space. On an open-area test site, the ground reflection, receive antenna height scan and near-field effects at low frequencies (below about 100 MHz at 3 m) all break the simple 1/d law. That is why EMC standards specify site validation (NSA or sVSWR) instead of relying on the formula.
Q6. I have a received power in dBm. What is the field?
With AF in hand: E (dBuV/m) = P (dBm) + 107 + AF + cable loss. Example: -40 dBm at the receiver, AF = 18 dB/m, cable loss 2 dB gives 87 dBuV/m.
Q7. What about dBpT or A/m for magnetic fields?
In the far field, H = E / 377. In dB terms, dBuA/m = dBuV/m - 51.5. Near the source (loop antennas, below 30 MHz) this ratio does not hold and the field must be measured with a calibrated loop.
Checklist: always state the impedance (50 or 75 ohm), whether the value is peak, quasi-peak or average, and the measurement bandwidth. A level without its detector and RBW is not a complete number.
Questions or corrections? Reply below and we will fold good ones into this FAQ.
— rfforum 编辑部 / Editorial Team