Phase center of antenna

Hi ,everyone!

There is a concept of antenna which puzzle me all the time.

That is: What's the phase center of an antenna?

Who can give me the explict words?

Which book do you introduce me to read?

It seems to be difficult to find the concepts in the book ?

Thanks!

page 7

http://www.qsl.net/n1bwt/chap1.pdf

The reference point which makes the phase variations of the antenna at the far field to be a constant independent of angle variations is called the antenna phase center.
When referenced to the phase center, the fields radiated by the antenna are spherical waves with ideal sherical wave fronts or equiphase surfaces. Hence, the phase center is a reference point from which radiation is said to emanate.

for more info refer to Balanis's Antenna book, chapter 13, pages 799 and 800.