In general, an antenna is best defined in a "free space", with nothing around. We live, however, on the Earth, and for many antennas we use it as a relatively conductive plane. For low frequency operation it is not a good conductor, so we improve its conductivity by metal wires, etc. The conductive ground becomes an important part of an antenna, the radiation pattern is formed by it. In some cases it is used as a mirror, in others, as a conductive line, or a part of a waveguide...