ANTENNAS HISTORY OF ANTENNAS

Marconi’s first experiments with transmitting electromag­netic waves in 1901, antennas have found several important applications over the entire radio frequency range and, nu­merous designs of antennas now exist. Antennas are an inte­gral part of our everyday lives, used for a multitude of pur­poses. An antenna is used to either transmit or receive electromagnetic waves and it serves as a transducer con­verting guided waves into free space waves in the transmit­ting mode or vice versa in the receiving mode. All antennas operate on the same basic principles of electromagnetic theory formulated by James Clark Maxwell. Maxwell put forth his unified theory of electricity and magnetism in 1873 (1) in his famous book, A Treatise on Electricity and Magnetism, incorpo­rating all previously known results on electricity and magne­tism and expressing these mathematically through what we refer to as Maxwell’s equations which hold over the entire electromagnetic spectrum. His theory was met with much skepticism and it wasn’t until 1886 that Heinrich Hertz (2), considered the father of radio, was able to validate this theory with his experiments. The first radio system, at a wavelength of 4 m, consisted of a A/2 dipole (transmitting antenna) and a resonant loop (receiving antenna) as shown in Fig. 1 (3). By turning on the induction coil, sparks were induced across the

A

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d

M

Figure 1. Heinrich Hertz’s radio system.

gap A which were detected across the gap B of the receiving antenna.

Almost a decade later, Guglielmo Marconi, in 1901, was able to receive signals across the Atlantic in St. Johns, New­foundland, sent from a station he had built in Poldhu, Corn­wall, England. Marconi’s transmitting antenna was a fan an­tenna with 50 vertical wires supported by two 6 m guyed wooden poles. The receiving antenna was a 200 m wire pulled up with a kite (3). For many years since Marconi’s experi­ment, antennas operated at low frequencies, up to the ultra high frequency (UHF) region and were primarily wire type antennas. The need for radar during World War II launched antenna design into a new era and opened up the entire radio frequency spectrum for their use. Since the 1950s many new antenna types such as reflector, aperture, and horn antennas came into use, most of them operating in the microwave re­gion. Their applications range from communications to as­tronomy to various deep space applications. These antennas have been discussed in several books and some of these have been included in Ref. 4-26. A good explanation of how an antenna radiates is given in Refs. 20 and 23. To understand how the antenna radiates, consider a pulse of electric charge moving along a straight conductor. A static electric charge or a charge moving with a uniform velocity does not radiate. However, when charges are accelerated along the conductor and decelerated upon reflection from its end, radiated fields are produced along the wire and at each end (20,21). The In­stitute of Electrical and Electronic Engineers (IEEE) stan­dard definitions of terms for antennas (24) and Balanis (25) provide a good source of definitions and explanations of the fundamental parameters associated with antennas.