Hi imtiaz,
It is expected for the results of Radiation efficiency and Gain to have this behavior.
The Radiation Efficiency and Gain are calculated using the Accepted Power. Accepted Power is calculated using the s11 parameter.
Please have a look at the following links of AEDT Help,
Ansys HFSS 2023 R2 - Accepted Power
Ansys Electronics 2023 R2 - Radiation Efficiency
Ansys HFSS 2023 R2 - Peak Gain
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