air boundary condition comsol

Hi, I am currently using the COMSOL to calculate the far-field distribution of a radiation source. I first started with a very simple case --- a point source in air, to simply test. I am using the RF module-->In Plane waves--->Harmonic Propagation.

I am using the Cartesian coordinate in the two dimensional space. The point source is located at center (x=0, y=0) of a square area (air). Then I set the boundary condition of the square area as scattering boundary condition and calculated the far-field pattern on one side versus the angle. The angle is defined as atan2(x,y)[1/deg], the same as in its Help document.

However, the far-field radiation pattern is like a parabolic curve with minimum at normal direction. Isn't it supposed to be a straight line (constant) for just a single point source in the far field?

I am confused how the COMSOL is actually relating the far-field to the angle inside the software.

Any suggestion or comment on this topic would be much appreciated.

Thanks