ptical polarization should be regarded as the most general form of polarization, circular and linear being special cases of elliptical polarization with axial ratios of unity and zero, respectively. An elliptically polarized wave can be completely defined by three parameters: axial ratio, orientation angle, and sense of rotation. These are defined in the list that follows, using Figure 7.14 as a reference. In this figure yM is given by equation 7.56: (7.56) Axial ratio: The axial ratio is defined as the ratio of the minor axis to the major axis. This ratio is often expressed in decibels. Orientation angle: The orientation angle, B, is defined as the angle between the major axis and the reference axis. In terrestrial communications this reference axis is normally taken as parallel to the surface of the Earth. Sense of rotation: The convention for defining the sense of rotation of the electric vector is the same as for circular polarization; the commonly used terms are clockwise (or right- hand) and counterclockwise (or left-hand). Reception of Electromagnetic Waves Ellipticity angle: This is an alternative way of defining the axial ratio and is equal to the angle, a, defined in Figure 7.14 for the circumscribing rectangle CDEF. Reception of Electromagnetic Waves Electromagnetic waves may be received using a simple rod antenna, a loop antenna, or an array of rod attennas (a "Yagi" aerial). The concept of a hypothetical istropic antenna is described because it is useful in understanding the performance of aerials. The Simple Rod Antenna It can be assumed that a receiving rod antenna consists of an infinite number of current generators of length dx driven by fields E(x). Then, if the total impedance at the terminals is Z and the current generators have a normalized value of D(x) relative to the value at the terminals (x = 0), the elemental current generators will each contribute an amount, E(x)D(x)dx/z, to the current, I. The current distribution, D(x), on a rod antenna is assumed to be analogous to the sinusoidal st