antenna-theory.com

[cui]

A simple question to you guys, but I'm confused about this.

My question is : Dose the antenna 3D pattern variable at a fixed frequency in far field?
As we know the E-field, H-field and current is variable at a fixed point, so I suppose that the 3D pattern in far field is variable too.
But we see that the antenna pattern is invariable, dipole's far field pattern -a fixed apple-shaped pattern for instance.
How can I explain this?

[admin]

If you observed the fields very far from an antenna, you would be in the far field:
http://www.antenna-theory.com/basics/fieldRegions.php

You would then have plane waves, where E- and H- fields are in phase but pointing orthogonally to each other and the direction of radiation.

So if you observed the E-field at a location in the far field, it would oscillate in time at the frequency of the wave. The radiation pattern is a peak magnitude for all directions. It is frequency dependent.