s Elliptical polarisation should be regarded as the most general form wave is propagating with the speed of light. This statement lacks of polarisation, circular and linear being special cases of elliptical some rigour, but is accurate enough for most practical purposes and polarisation with axial ratios of unity and zero, respectively. An has stood the test of time. elliptically polarised wave may be completely defined by three The current distribution shown as a dotted line on Figure 6.15, is parameters, axial ratio, orientation angle and sense of rotation. for a half wavelength dipole antenna. In this case, if the field is These are defined as follows, using Figure 6.13 as a reference. In this figure Y is given by Equation 6.56. tan Y = Ey Ex (6.56) Current distribution 1. 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. 2. Orientation angle: The orientation angle B is defined as the angle between the major axis and the reference axis. In terre- strial communications this reference axis is normally taken as parallel to the surface of the Earth. 3. Sense of rotation: The convention for defining the sense of Figure 6.15 Current distribution on a half wavelength rotation of the electric vector is the same as for circular polari- dipole sation and the commonly used terms are clockwise (or right hand) and counter clockwise (or left hand). considered constant along the antenna, the current is given by 4. Ellipticity angle: This is an alternative way of defining the axial Equation 6.57 where R is the load impedance and the intrinsic ratio and is equal to the angle a defined in Figure 6.14 for the radiation resistance of the half wavelength dipole is 73 ohms. circumscribing rectangle CDEF. Reception of the electromagnetic wave (6.57) 6.3.1 The simple rod antenna From Equation 6.57 it follows that the open circuited voltage e across the terminals is given by Equation 6.58. It can be assumed that a receiving rod antenna consists of an in