This chapter and the following one deal with the two principal types of quasi-optical antenna: reflectors and lenses. The principles of ray optics, as opposed to guided-wave or constrained-wave theory, are instrumental in their design. The term “optics”, however, is considered in its most general sense, and diffraction (e.g. physical optics, geometric optics and the geometric theory of diffraction) and aberrations are also included. In fact, the commonest microwave antenna, the collimating reflector, is a diffraction-limited device which is incapable of being analysed using ray optics alone. Other material in these chapters goes significantly beyond the normal realm of optical principles: e.g. spherical wave theory, tolerance theory, aperture blocking, frequency-selective surfaces, contoured beams and low-noise antennas. Yet these subjects are held together by the common thread of their direct applications to quasi-optical antennas.

In a quasi-optical antenna system, it makes little difference, so far as the illumination characteristics of the feed are concerned, whether the collimating device is reflective (i.e. single or multiple mirrors) or refractive (i.e. a lens). In either case it is the function of the feed to provide a radiation pattern which is largely confined to the cone defined by the solid angle subtended at the focus by the optical aperture.

In this chapter, the fundamental characteristics of the multiple beam antenna (MBA) are presented, with some of the associated physical limitations governing the synthesis of the desired radiation patterns. Since these antennas have a wide variety of performance capability, a 'figure of merit' is presented as an aid in assessing the performance of an MBA. These systemic features, a description of classical versions of the MBA, the associated beam-forming networks, and some examples are presented as an introduction to the multiple beam antenna.

In recent years, the variety of antenna measurement techniques has expanded considerably and these are dealt with in the following Sections. The topics covered commence with a study of the antenna as an unknown radiating structure in space, which is to be investigated by using a source and a detector. A discussion on impedance measurements is followed by current distribution measurements where a probe is moved close to the antenna structure. This leads on to an extensive explanation of near-field techniques where the field of the test antenna is sampled close to the antenna and transformed to obtain the far-field. Several techniques carried out in the intermediate range, between near-field scanning and the conventional far-field distance, are described, including compact ranges and de-focusing methods. The traditional far-field methods for measuring directivity, gain, phase centre, boresight and scattering are then considered. Lastly, a description is given of both outdoor and indoor test ranges and the methods used to evaluate their performance.