Where to find dbm value in CST?
Any help would be appreciated.
Yes, because only that makes sense for an antenna pattern.
dBm is a measure for power. For the antenna, you decide what power you feed into the antenna. Then this power, less the internal losses in the antenna, will be radiated. The total radiated power is input power * antenna efficiency.
The far field plot shows how radiated power is distributed in different directions, compared to an isotropic (omnidirectional) antenna. That direction-dependent gain over isotropic is the dBi value. Antenna pattern can be shown for gain (relative to input power) or directivity (relative to total radiated power). Gain = Directivity * Efficiency
If you want to know the power received by another antenna at a distance, have a look at the radar equation
http://www.radartutorial.eu/01.basic...uation.en.html
and Friis equation
https://en.wikipedia.org/wiki/Friis_...ssion_equation