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As this post from electronics stack exchange suggested, EM wave emitted by the antenna tends to become spherical waves(plane wave with a local analysis) in the far field region regardless of the shape of the source antenna. I extended this conclusion and assume that when designing antenna receiving the EM wave, the radiation pattern (not the one given by transmitter) of the EM wave can be neglected since in the operating region(far field) they are all the same.

But in another post from the same site also suggested that the radiation pattern of an antenna depicts the intensity of EM wave for points in space. By this information, I concluded, for designing an antenna adapting to such EM wave radiated, radiation pattern needs to be considered for a better gain.

So isn't the radiation pattern of the EM wave still technically considered?

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I think typically when they say "spherical wavefront" they don't mean that the wave forms a whole sphere. They mean that the segment of the wave you can observe has a spherical curvature.

There is a big difference between saying a wavefront has spherical curvature and an antenna is isotropic.

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Yes, as you correctly concluded, you cannot make any inferences on the phase over angle (which is what shapes the wave locally) from the radiation pattern, which gives you intensity over angle.

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