Download a pdf copy of this application note: The Antenna Factor Concept
What is an Antenna's Antenna Factor?
An antenna's antenna factor (AF) is a comparison between the field level presented to the
antenna and the actual voltage produced by the antenna at it's connector. This can be seen in
. Note, the antenna factor varies with frequency, that is for the same presented field
strength, but at a different frequency, the voltage produced at the connector is not constant.
Creating the AF Data
By way of further explanation, a not particularly accurate yet simple way to generate AF data for
an antenna would be to illuminate it with a known field strength at a specific frequency, and use
a receiver to measure the volts produced at the antenna connector. The reason it is not
particularly accurate in this example is because we will use the RF train used in an immunity
system to create a known field strength (say 10V/m) over the antennas frequency range
(covered in equally spaced spot frequencies). This is shown in Figure 2. The field probe is then
replaced by the antenna in question, the spot frequencies stepped through and the voltage at
the antenna connector measured and recorded.
Using the AF Data
Far more accurate AF data is supplied in plot form by the antenna supplier. A typical plot is
shown in Figure 3. This data is input to the EMC emissions software so it can add the antenna
factor to the entered cable loss etc. All the data entered into the software is in dBs.
The field level emitted by the EUT can then be determined by simple algebraic addition of the
voltage reading on the receiver and the entered data.
That is the voltage reading in dBs is added to the AF at that frequency and the cable loss at that
frequency to obtain the actual field strength measured by the antenna.
Download this article of VSWR Explained
Here is a useful "cheat sheet
" converting VSWR to the percentage of reflected power.
This online VSWR Calculator
is also very useful