Antennas are reciprocal devices. It means that they can be used both as transmitting and receiving elements; in two-way communication, the same antenna can be used for both transmission and reception (receiver protection may be used in some cases). For example, this is how the antennas on cellular phones operate. The reciprocity principle affirms that the transmitting and receiving patterns of an antenna are identical at a specified wavelength; there is no functional difference between receive and transmit modes of an antenna except that the power flow is directed inwards to the receive antenna and outwards from the transmit antenna. An antenna has the same efficiency, directivity, and polarization characteristics in receive and transmit modes. This property is called reciprocity, and it occurs due to the symmetry of the electromagnetic equations when the direction of time is reversed [JEF200401]. The basic characteristics of an antenna are its gain and its half-power beamwidth (HPBW), both of which are described in detail later. Some common physical antenna types include the following:

Linear dipole fed by a two-wire line 7 Linear monopole fed by a single wire over a ground plane 7 Coaxial ground-plane antenna 7 Loop antenna 7 Uda-Yagi (or Yagi-Uda) dipole array 7

Multiple loop antenna wound around a ferrite core 7 Log-periodic array 7 Parabolic (dish) antenna 7 Toroidal antenna (multiple feed), including simulsat types 7

An antenna leaks electromagnetic energy into the surrounding space. For example, in a dipole antenna, because of the change in geometry in the transmission line, there is an abrupt change in the characteristic impedance at the transition point where the current is still continuous; hence, the dipole leaks electromagnetic energy into the surrounding space (and so, it reflects less power than the original open circuit.) The space surrounding the dipole has an electric field. At higher frequency, the current oscillates in the wires, and the field emanating from the dipole changes periodically; the field lines propagate away from the dipole and form closed loops [AMA200101]. See Figure 3.2. Note that an ideal (hypothetical) isotropic antenna radiates equally in all directions.