The phenomenon is known as "mutual coupling". The field from the antenna
induces a current in the conductor. That current produces a field which
does two things: it vectorally adds to the field from the antenna, which
alters the overall field; and it in turn induces a current into the
antenna conductors, which alter the field from the antenna.

A quantitative study of this phenomenon is complex and requires solution
of Maxwell's equations as well as fundamental equations for radiation.
The latter can't be solved at all in closed form, and the former can be
far from trivial if the geometry isn't extremely simple. Many
approximations have been developed over the years, but they can be
applied only to a few simple geometrical configurations. You won't have
any trouble finding an abundance of information in any antenna textbook
or in papers of various publications like the IRE/IEEE Proceedings.

But with a home computer, you can now get very accurate solutions to
even very complex geometries in seconds. They'll tell you not only the
overall pattern, but the currents on each part of each conductor. As the
author and seller of EZNEC, I recommend that you download the free demo
version of that program from http://eznec.com. There are, however, a
number of other readily available programs which all give essentially
the same information with equal accuracy.

Roy Lewallen, W7EL

Rohit wrote:
i m studying the distortion of the antenna pattern located near any
conductor. i want to know how do i interpret the patterns obtained. i
know the ideal pattern of the antenna but when i place them near any
conductor, can the pattern distortion be studied by knowing the surface
currents in the object due to the antenna? how the E field and H field
behaviour changes as it travels from free space to any finite conductor
plate?
i m very new to antennas so i need answer in detail to understand.
thanks
rohit