e) Reflector antennas A reflector is used to concentrate the EM energy in a focal point where the receiver/feed is located. Optical astronomers have long known that a parabolic cylinder mirror transforms rays from a line source on its focal line into a bundle of parallel rays. Reflectors are usually parabolic. A parabolic cylinder reflector was first used for radio waves by Hertz in 1888. Sometimes, corner reflectors are used. Reflector antennas have very high gain and directivity. Typical applications: radio telescopes, satellite communications. They are not easy to fabricate and, in their conventional technology, they are rather heavy. They are not mechanically robust. Reflector Reflector Feed Subreflector parabolic refiector with front feed Reflector Feed Feed Parabolic reflector with cassegrain feed 9:30 PM Corner reflector f) Lens antennas Lenses play a similar role to that of reflectors in reflector antennas. They collimate divergent energy into more or less plane EM wave. Lenses are often preferred to reflectors at higher frequencies (f> 100 GHz). They are classified 9:30 PM✓ according to their shape and the material they are made of. 2) Antenna arrays Antenna arrays consist of multiple (usually identical) radiating elements. Arranging the radiating elements in arrays allows achieving unique radiation characteristics, which cannot be obtained through a single element. The careful choice and control of the phase shift and the amplitude of the signal fed to each element allows the change of the radiation pattern electronically electropic scanning. Such arrays are called phased arrays. ට General review of antenna geometries: - 1) Single-element radiators a) Wire radiators (single-element) straight-wire elements (dipoles/monopoles) wire antenna elements loops 31 о helices 0+000° Wire antennas are simple to make but their dimensions are commensurable with the wavelength. This limits the frequency range of their applicability (at most 1-2 GHz). At low frequencies, these antennas become increasingly large. b) Aperture antennas (single element) Aperture antennas were developed before and during the WW2 together with the emerging waveguide technology. Waveguide transmission lines were primarily developed to transfer high power microwave EM signals (centimeter wavelengths), generated by powerful microwave sources such as magnetrons and klystrons. These types of antennas are preferable in the frequency gro 20 GHz. c) Printed antennas The patch antennas consist of a metallic patch etched on a dielectric substrate, which has a grounded metallic plane at the opposite side. They are developed in the beginning of 1970s. There is great variety of geometries and ways of excitation. Modern integrated antennas often use multi-layer designs with a feed coupled to the radiator electro-magnetically (no galvanic contact). W Ground Plane Ground Plane rectangular patch circular patch printed dipole d) Leaky-wave antennas These are antennas derived from millimeter-wave (mm-wave) guides, such as dielectric guides, microstrip lines, coplanar and slot lines. They are developed for applications at frequencies >30 GHz, infrared frequencies included. Periodical discontinuities are introduced at the end of the guide that lead to substantial radiation leakage (radiation from the dielectric surge traveling-wave antennas.