In many literature, beam pattern is a narrow band concept so it is related to the steering vector. Since you are using time domain beamforming, I would do the following:
1. Simulate incoming signals for each direction you would like to calculate the beam pattern. It is the same as you simulate your source.
2. Process each signal using the SAME beamformer you use for your desired source, i.e., beamform each signal to (30, 45) degrees.
3. Now you get a beamformed signal for each direction and you can now compute the power or energy for those signals. You can then plot power or energy as a function of angle and it should match your beam pattern.
HTH
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