Difference between antenna factor and antenna gain

There is some misunderstanding regarding the terms antenna factor and the antenna gain. Both the items are related to field but in a different way.

Antenna Factor

The antenna factor is specified in 1/m, and is added to the reading of the spectrum analyser, which is in dBuV. Both units are logarithmic so adding one factor to the other is the same as multiplying them in a linear scale. The resulting unit is the dBuV/m, the unit used for Radiated Emission testing. The factor is always related to a certain distance, be aware when using it. Some manufactures/calibration houses supply multiple factors, check which on applies to your set up. An often made mistake is the antenna factor for 10 meters in used in a 3 meter side or the other way around.

example

• Spectrum Analyser reading is 45,5 dBuV
• The antenna factor is 4,5 dB [1/m]

The total reading is 50 dBuV/m

For more information see wikipedia:Antenna_factor

Antenna Gain

The antenna gain is often misunderstood because most antenna's do not have an active pre amplifier. The gain is generated by guiding the beam into one direction, so the gain related to an isotropic antenna the unit is then dBi. In other words the gain compared to an isotropic antenna. When knowing the gain of the antenna it is possible to calculate the field at a certain point. When in the far field the impedance is 377 ohm, and the added information of the antenna gain and the distance it is possible to calculate the V/m.

Example

• Power is 40 dBm.
• Gain is 10 dB.
• Distance is 3 meters.

The total power is 100 Watts or 50 dBm, this generates a voltage of ${\frac {100}{377}}\approx 0,265\ V$

The distance is 3 meters so this gives ${\frac {0,265}{3}}\approx 0,088\ V/m$

For more information see wikipedia:Antenna_gain

Decibel

Power expressed in milliWatt in dB values.

• 1 mW = 0 dBm

• 2 mW = 3 dBm

• 100 mW = 20 dBm

• 1 W = 30 dBm

• 100 W = 50 dBm