Monthly Archives: May 2014


Reflector antennas have been of importance for decades in several areas of electrical engineering, ranging from telecom­munications and radars to deep-space exploration and radio astronomy. This is due to the high gain of reflector antennas, typically above 30 dBi. If we extend the concept of a reflector antenna to a reflecting mirror and view the human eye as the feed antenna operating in receiving mode, reflector antennas have been known for centuries. Optical astronomers have long been using reflecting mirrors in telescopes to enhance the visibility of stars, planets, and other celestial bodies.

The basic principle of operation of a parabolic reflector is that all rays emanating radially from a point source located at the focal point are reflected as a concentrated bundle of parallel rays, which can propagate for very long distances without loss due to speading. Inversely, incident rays parallel to the axis of symmetry of the paraboloid are all reflected to­ward its focal point, which concentrates the received signal at a single point. In that case, if the human eye or camera is placed a little bit behind the reflector focal point, an image with enhanced luminosity and definition is formed (Fig. 1).

However, reflector antennas can be designed to be wide­band devices, not limited to operation at frequencies covered by the spectrum of visible light. Radio telescopes, for example, search for celestial radio sources over a wide range of frequen­cies (e. g., 300 MHz to 40 GHz). In this case, the radio sources and corresponding frequencies are marked on charts ac-


Figure 1. Basic principle of operation of a parabolic reflecting mir­ror. The paraboloid surface is formed by rotating the parabolic curve about its axis of symmetry (s axis).





–f-s ^–Ь-S






Figure 2. The evolution of reflector antenna systems: (a) single axi – symmetric reflector, (b) dual axisymmetric reflector, (c) single offset reflector, and (d) dual offset reflector. The main reflectors are para­bolic.

cording to their physical locations in the sky, forming maps similar to the ones elaborated by optical astronomers. Feed antennas are employed to receive the signals from the celes­tial radio sources at different bandwidths.

One of the first reflector antennas operating at radio fre­quencies was built by Hertz in 1888 and consisted of a sheet of zinc of about 2 m by 1.2 m, molded as a parabolic cylinder and illuminated by a dipole feed (1). Since then, reflector an­tenna technology has gradually evolved toward the state of the art known today for the purpose of improving electrical performance and/or simplifying mechanical structure (Fig. 2). The most basic form is the single axisymmetric parabolic re­flector shown in Fig. 2(a), which is still in widespread use primarily at low frequencies and for low-cost applications. Large reflectors frequently use an axisymmetric dual reflector system with a parabolic main reflector, as shown in Fig. 2(b). The subreflectors are hyperbolic (Cassegrain system) or ellip­tical (Gregorian system). These systems offer a shorter trans­mission line (or waveguide) run to the feed antenna and are often used as earth terminal antennas in satellite communi­cation networks.

Axisymmetric single and dual reflectors suffer from aper­ture blockage due to the presence of feed/subreflector and supporting mechanical structures in front of the main reflec­tor aperture. This problem is solved by using an offset system with a main reflector that is a section of a parent reflector, normally a paraboloid of revolution, as shown in Fig. 2(c) and (d). Design and construction of offset reflectors are more elab­orate than for their symmetrical counterparts.

Remarkable technological advancements were achieved during World War II, as reflectors were widely employed in radar and communication systems (2). However, it was only with the proliferation of digital computers in the late 1960s that the most accurate analysis and synthesis algorithms were developed, especially the ones related to the configura­tions of Fig. 2(c, d) and (3-7). Closed-form analysis algorithms are generally only applied to symmetrical reflectors (4,8).

In addition, substantial improvements on the electrical performance of both axisymmetric and offset dual reflector configurations were obtained with shaping algorithms, an ef­fort only possible with efficient numerical processing com­bined with a solid knowledge of differential geometry and electromagnetics (5,9,10). The axisymmetric dual shaped re­flector was introduced in the 1970s (9) and is popular for large earth station antennas. The offset dual shaped reflector has reportedly achieved aperture efficiencies of about 85% (11) and has been enjoying an increase in popularity. As a conse­quence, the analysis and design of reflector antennas is nowa­days a specialized and unique area in applied electromagnet­ics, occupying many distinguished workers in industry and academia.

For the past two decades, reflector antennas have been ap­plied primarily to satellite communications and networks, deep-space exploration, and electronics defense. The reflector antenna carried by the Voyager spacecraft, for example, is a dual-reflector antenna shaped for high gain (Fig. 3). Besides specific designs applied to unique purposes, such as space­crafts and radiotelescopes, reflector antennas are also being produced on a very large scale for commercial applications, a multimillion-dollar market directly related to the globaliza­tion of communication currently underway. In particular, VSAT systems (very small-aperture terminals) are proliferat­ing and connecting together branches of large corporations, such as chains of stores, banks, and car manufacturers. The VSAT market is expected to grow at a rate of 20% per year (12).

Other examples of substantial economic importance are the satellite-based cellular communication systems, such as the Motorola IRIDIUM, in which well-defined multibeam cov­erage is required, and direct-to-home (DTH) satellite TV sys­tems, such as Hughes DirecTV and others, which employ small offset parabolic antennas to receive satellite signals. Thus, reflector antennas are present in our lives as major gateways for the exchange of information at home and, less conspicuously, in defense systems. Reflector antennas can therefore be considered one of the most successful electrical devices of all time, in view of their importance in many mod­ern engineering systems and applications, such as cellular communications, satellite TV, and electronic defense, as well as in the exploration of our galaxy and beyond.