Characterizing Reflector Antennas

One important property of any antenna is that its radiation characteristics when it is used as a transmitter are the same as when it is in the receiving-mode. This is a consequence of the well-known electromagnetic fields principle of reciprocity. Even though radio telescope antennas are generally used only for receiving signals, it is often simpler to characterize it by considering the antenna to be in the transmitting mode. Antenna terminology is also influenced by the reciprocity principle, for example we have been calling the dipole or horn placed at the focus of the reflector to receive the signal from distant sources as the ``feed'', i.e. as though it were coupled to a transmitter rather than a receiver.

All antennas can be described by the following characteristics (see also Chapter 3)

1. Radiation pattern The field strength that the antenna radiates as a function of direction. The simplest type of antenna normally radiates most of its energy in one direction called the `primary beam' or `main lobe'. The angular width of the main lobe is determined by the size and design of the antenna. It is usually parametrized by its full width at half maximum, also called its $3$dB beamwidth. Weaker secondary maxima in other directions are called side lobes. Although the pattern is a function of both elevation and azimuth angle, it is often only specified as a function of elevation angle in two special orthogonal planes, called the E-plane and the H-plane.
2. Directivity The radiated power in the direction of the main lobe relative to what would be radiated by an isotropic antenna with the same input power. A related quantity called the Gain also takes into account any electrical losses of the antenna. For reflector antennas, one can also define an aperture efficiency which is the ratio of the effecting collecting area of the telescope to its geometric area. For the relation between the gain and the effective collecting area see Chapter 3.
3. Polarization The sense of polarization that the antenna radiates or receives as a function of direction. This may be linear, circular, or elliptical. Note that when describing the polarization of a wave, it is sufficient to specify the polarization of the electric-field vector.
4. Impedance From the point of view of the microwave circuit behind the antenna, the antenna can be represented as a complex load impedance. The characteristics of this load depend on the radiation patterns of the antenna and hence the design of the antenna. The goal of a good design is to match the impedance of the antenna to the impedance of the transmission line connecting the antenna to the receiver. The impedance match can be characterized by any one of the following parameters:
   • the voltage reflection coefficient, ${{\rho}_v}$.
   • the return loss (in dB), ${{R}_{L}} = -20{\log\vert{{\rho}_v}\vert}$.
   • the voltage standing-wave ratio, $VSWR = {\frac{1 + \vert{{\rho}_v}\vert} {1 - \vert{{\rho}_v}\vert}}$.
5. Phase Center All horns and feeds have a phase center. This is the theoretical point along the axis of the feed which is the center of curvature of the phase fronts of the emerging spherical waves.