Eleclromagnetic waves (radiant energy) encompass a number of familiar types of radiation, such as light, radio waves, X-rays, heat and Cosmic waves. Despite this variety, all these forms of radiation are similar in that they obey the same physical laws, differing only in wavelength and frequency. The electromagnetic spec- trum may be defined in terms of wavelength, or fre- quency, ranging from extremely long waves of low frequency inherent in the magnetic field of the earth; through long, short, and microscopic radio waves; in- frared waves; light waves; ultraviolet and X-rays; and into the infinite region of gamma and Cosmic waves of unknown origin. Of this vast range of electromagnetic radiation bathing the earth, those waves of immediate interest in the field of communication are of a dimen- sion comparable with the size of man himself-radio waves.
Every electric system that carries alternating or pul- sating current radiates a certain amount of this energy into space in the form of electromagnetic waves. The amount of radiated ener is small when the waves are large compared to the radiating system, as in the case of 60-Hz industrial current. As the frequency of alterna- tion is raised and the corresponding wavelength is shortened, a portion of the electromagnetic spectrum is reached wherein the radiated energy becomes of some practical use for long-distance radio communication. This region is termed the communication region of the electromagnetic spectrum and is composed largely of radio waves.

The earliest use of a radio antenna was recorded by Hertz in his communication experiments in 1884. As the knowledge of radio grew, so the knowledge and use of radio antennas grew likewise. Today's antennas re- present modern design applied to ideas nearly 100 years old. And while the antennas of today bear little resem- blance to those of yesteryear, so the antennas of to- morrow will seem strange and unusual to the communicator of today.
The antenna (aerial) is made up of a system of con- ductors designed to radiate or intercept electromagnetic waves. Antennas come in many shapes and sizes, but they all have one factor in common-they are made up of conducting material and require a feed system to extract or accept radio energy. Many antennas are com- plicated, but most of them are not. Some of the better hf and vhf antennas are described in this Handbook.

The Complete Antenna The antenna is a device for converting guided electric waves into electromagnetic waves in free space. A matching device of some sort is generally employed to ease this abrupt transition, and a transmission line is often used to efficiently guide the electric waves from the transmitter to the antenna (Figure I). It is under-

Figure 1. Representative Antenna System The antenna is a device for converting guided electric waves into electromagnetic waves in free space. A matching device is often used to ease this abrupt transi- tion, and a transmission line guides the electric waves from the transmitter to the antenna.

stood, moreover, that the antenna system follows the general laws of reciprocity and can extract electromag- netic waves from free space and convert them to electric waves capable of being detected by a radio receiver. The range of frequencies (bandwidth) over which a reasonable match or transformation between guided waves and free waves can be achieved depends to a degree on the amplitude and nature of the mismatch in the antenna system. If the transformation is gradual so that wave parameters do not undergo a sudden change, but vary gradually between the guided and the free condition, the transition is smooth and the frequency span of efficient operation may be quite large. Accord- ingly, the disturbance or unwanted reflection of the guided wave may be quite small. If, on the other hand, the transition between the

guided and the free-space waves is abrupt, a region of reflection exists in the system such that a portion of the wave is sent back down the transmission line. The re- flected wave may be compensated for, to a degree, by adjustments made to a matching device which creates equal and opposite reflections to annul the original re- flection generated by the abrupt transition in the an- tenna system. In any case, the frequency span, or bandwidth, of the antenna system is considerably re- duced over that achieved by a perfect transformation between guided and free waves. The bandwidth of an antenna system is relative, and one way of specifying it is to define the limit of wave reflection alloed on the transmission line feeding the antenna. For example, if it is specified that the reflected wave shall be limited in amplitude to one quarter the value of the incident (direct) wave on the line, the over- all system bandwidth may be defined by this limit, as measured under actual operating conditions. It is common practice to specify antenna system bandwidth in terms of the amplitude of the reflected wave with respect to the incident wave. This specifica- tion may be expressed as a voltage standing-wave ratio (abbreviated VSWR, or simply SWR) which is measur- able by an inexpensive instrument placed in series with the transmission line. The SWR figure bears a definite relationship to the amplitude of the reflected wave, and it is simpler to measure and plot the SWR of an an- tenna and then to define the operating limits by SWR readings than it is to interpret the SWR in terms of the amount of reflection. Generally speaking, SWR values up to 3 are acceptable in simple antenna systems, while a somewhat lower SWR value of 2 is often specified as a maximum limit for various forms of beam antennas. On the other hand, some antennas employ so-called tuned feeders which operate with SWR values as high as 100. Strictly speaking, the maximum value of SWR acceptable in a system is often limited by the economics of the problem and is subjective rather than objective, being a relative concept rather than an absolute limita- tion arbitrarily imposed.

A time-varying electromagnetic field, or wave, may be propagated through empty space at the velocity of light. The wave is considered to be made up of interre- lated electric (E) and magnetic (H) fields at right angles to each other and lying in a plane, as pictured in Figure 2. The wave energy is divided equally between the two fields. If the wave is pictured as originating at a point source in space, the wave spreads out in an ever-grow- ing sphere with the source as the center. The path of an energy ray from the source to any spot on the sphere is a straight line and, at a large distance from the source, the wavefront does not appear to be spherical, but is assumed to be a flat surface, as shown in the illustra- tion. The plane electromagnetic wave may be represented in terms of its fields, with the vertical arrows represent- , ,

Figure 2. The Plane Electromagnetic Wave When a wave has travelled far enough from the source the wavefront appears flat and it is called a plane wave. The plane contains the perpendicular eledric (E) and magnetic (H) lines representing the wave front which is always perpendicular to the direaion of wave travel. In (A) the wave is traveling out of the page toward the reader. A cross section of a traveling wave is shown in (B). Arrows which go into the plane of the page are shown by small "Xs" for the tail, and those which come out of the page are shown by dots for the points of the arrows. The particular configuration of an electromag- netic field is termed a "mode."

ing the direction and strength of the electric field and the horizontal arrows the direction and strength of the magnetic field. The wave shown is said to be vertically polarized because the electric field is vertical. If the electric field were horizontal, the wave would be hori- zontally polarized. Other waves may be circularly po- larized, corresponding to left-handed and right-handed helices.
The abstract concept of an electromagnetic wave travelling through space is difficult to comprehend without the assistance of mathematical proof. Viewed from the theory of electron flow in a conductor, there is no suggestion of energy radiation into space. A set of relationships termed Maxwell's equations form the ba- sic tools for the analysis of most electromagnetic wave problems.

Maxwell's Equations James C. Maxwell (1831-1879), ahtaking concept of brilliant student of the natural sciences, derived a

HOME
NEXT PAGE