Common practice is to treat antenna systems as a two terminal device in free space.
Pickup most handbooks, and even text books, and antennas and often antenna systems are described in this way.
That model is quite inadequate for many or most antenna systems installed in proximity of natural ground. For example, a two terminal dipole and feed line system representation cannot have feed line common mode current, and it follows that thinking in terms of two terminal models denies a full understanding of the antenna system.
A three terminal model of an antenna system
(Schmidt nd) sets out a three terminal model of an antenna system in presence of ground using quite conventional linear circuit theory.
Above is Schmidt’s Y network based on values of three intermediate impedances, ZD, ZU, and ZC. These are found from measured values Za, Zb and ZC as explained by Schmidt: Continue reading Equivalent circuit of an antenna system