Waveguide

“ WAVEGUIDE POSTS—A rod of conductive material used as impedance-changing devices in waveguides.
WAVEGUIDE SCREW—A screw that projects into a waveguide for the purpose of changing the impedance.
WAVELENGTH— (1) The distance in space occupied by 1 cycle of a radio wave at any given instant; (2) The distance a disturbance travels during one period of vibration.
WINDOW—See Slot. YAGI ANTENNA—A multielement parasitic array. Elements lie in the same plane as those of the end-fire array. ”

“ WAVEGUIDE MODE OF OPERATION— Particular field configuration in a waveguide that satisfies the boundary conditions. Usually divided into two broad types: the transverse electric (TE) and the transverse magnetic (TM) . ”

“ Circular waveguides are used in specific areas of radar and communications systems, such as rotating joints used at the mechanical point where the antennas rotate. Figure 3-37 illustrates the dominant mode of a circular waveguide. The cutoff wavelength of a circular guide is 1.71 times the diameter of the waveguide. Since the "a" dimension of a rectangular waveguide is approximately one half-wavelength at the cutoff frequency, the diameter of an equivalent circular waveguide must be 2/1.71, or approximately
Figure 3-37.—Dominant mode in a circular waveguide. ”

“ A waveguide horn, called a FEEDHORN, may be used to feed energy into a parabolic dish. The directivity of this feedhorn is added to that of the parabolic dish. The resulting pattern is a very narrow and concentrated beam. In most radars, the feedhorn is covered with a window of polystyrene fiberglass to prevent moisture and dirt from entering the open end of the waveguide. ”

“ Internal arcing causes further damage to the waveguide in an action that is often self-sustaining until the waveguide is damaged beyond use. Part of your job as a technician will be to inspect the waveguide system for physical damage. ”

“ Figure 3-44 illustrates three devices, called irises, that are used to introduce inductance or capacitance into a waveguide. An iris is nothing more than a metal plate that contains an opening through which the waves may pass. ”

“ Also, the inside surfaces of waveguides are often plated with silver or gold to reduce skin effect losses. These requirements increase the costs and decrease the practicality of waveguide systems at any other than microwave frequencies. ”

“ Efficiency Bandwidth coverage Power-handling capability Each of the above 3-49. A waveguide that is not perfectly impedance matched to its load is not efficient. Which of the following conditions in a waveguide causes this inefficiency?
Sine waves Dominant waves Standing waves Transverse waves A waveguide iris that covers part of both the electric and magnetic planes acts as what type of equivalent circuit at the resonant frequency? ”

“ When such a change is necessary, the bends, twists, and joints of the waveguides must meet certain conditions to prevent reflections.
Waveguides maybe bent in several ways that do not cause reflections. One way is the gradual bend shown in figure 3-48. This gradual bend is known as an E bend because it distorts the E fields. The E bend must have a radius greater than two wavelengths to prevent reflections. ”

“ The sea-surface acts somewhat like an optical reflector or mirror, and the electric waves glide along its surface. The rotundity of the earth within certain limits hardly makes any perceptible effect upon the ease of communication. The waves sent out by the transmitter of a long-distance wireless station are from 3000 to 20,000 feet in length, and there is, therefore, a considerable amount of bending or diffraction. It is a familiar fact, as already explained, that a wave-motion, whether on water or in air, spreads round an obstacle to a certain extent. ”

“ The whole theory of the trail of waves made by a canal-boat is only comprehensible if it is clearly seen that a water-surface wave has a certain velocity determined by its wave-length. If the wave-speed is small, the waves are short. As the speed increases the waves get longer. Or the matter may be put in another way. We may say that just as a pendulum has a certain rate of vibration depending on its length, so a water wave has a certain frequency, and therefore, a certain speed of propagation dependent upon the wave-length, or shortest distance from one wave-crest to the next. ”

“ At each oscillation some energy is frittered away into heat in the conductor, and if the conductor and condenser have a special form, energy may be rapidly removed from the system by the electric waves which are formed in the surrounding æther or dielectric. These waves consist in the propagation through the medium of lines of electric strain, just as an air wave consists in the propagation through the air of regions of air-compression, or a water wave consists in the propagation of an elevation on the surface. ”

“ As a wave progresses over the surface, wave-energy is continually being imparted to portions of the water in front, and it is transferred away from others in the rear. In the very act of setting a fresh particle of water in oscillation, the portions already vibrating must diminish their own motion. They may hand on the whole of their energy or only a part of it to their neighbours. This distinction is a very important one, and it determines whether a single act of disturbance shall create a solitary wave or wave-train in a medium. ”

“ The waves which run along a large and slack wire are large and slow, those along a small and tightly stretched wire are small and quick; and from the data we possess as to light, its velocity of transmission, its refraction when its waves pass from one medium into another of different density, and from the distance between the waves as shown by interference, it is easy to calculate the lengths and vibratory periods of the waves, and the elasticity of the medium through which such waves are transmitted. ”

“ The waves which spread round a central point when a stone is thrown into the water, give one the idea that the water which forms the wave really moves towards the observer. But it is not so, as may be readily proved by placing on the surface a floating body, which we shall find is but little, if at all, influenced by the undulations of the water. The appearance of rolling waves given on the stage by means of a painted cloth, to which an undulatory motion is given, is an instance of this apparent movement. ”

“ In free space æther waves of various wave-lengths all travel, as far as we know, at the same rate. This [256] equality in speed is, however, disturbed the moment the waves enter a transparent material substance such as glass. The velocity of propagation is then reduced in all cases, but it is generally more reduced for the shorter waves than for the longer ones; and as a consequence the rays of shorter-wave lengths are more bent or refracted than the rays of longer wave-length. ”

“ Finally, it may be noticed that the conditions of wave-propagation without change of type may be investigated in another manner. If we impress on the whole medium a velocity equal and opposite to that of the wave we obtain a “steady” or “stationary” state in which the circumstances at any particular point of space are constant. Thus in the case of the vibrations of an inextensible string we may, in the first instance, imagine the string to run through a fixed smooth tube having the form of the wave. ”

“ If, however, the strip is employed as a kumascope and connected at one end to the earth and at the other end to an aerial, when electric waves fall upon the aerial, the electrical oscillations thereby excited seem to have the property of stopping this dance of silver particles and the resistance of the gap is increased several times, but falls again when the wave ceases. ”

“ REFLECTED WAVE— (1) The wave that reflects back from a medium; (2) Waves traveling from the load back to the generator on a transmission line; (3) The wave moving back to the sending end of a transmission line after reflection has occurred.
REFLECTION WAVES—Waves that are neither transmitted nor absorbed, but are reflected from the surface of the medium they encounter.
REFLECTOR—The parasitic element of an array that causes maximum energy radiation in a direction toward the driven element. ”

“ But if the speed of the projectile reaches and exceeds the velocity of sound, then the head-wave, as we shall call it, augments noticeably in power, and is more and more extended, that is, the angle made by the contours of the wave with the direction of flight is more and more diminished, just as when the speed of a boat is increased a similar phenomenon is noticed in connexion with the bow-wave. ”

“ When the waves leave the generating area and continue as free waves, the wavelength and period continue to increase, while the height decreases. The rate of change gradually decreases. The speed (S) of a free wave in deep water is nearly independent of its height or steepness. ”

“ These three classes of waves, differing so immensely in magnitude and velocity, all agree in this,—that it is the wave that travels, and not the fluid or medium. The rapidity of the luminous wave is about nine hundred million times that of the sound-wave; hence we may suppose that the ether in which it moves is about as many times rarer or lighter than air, and the retina of the eye which it impresses as many times more delicate and sensitive than the drum of the ear. ”

“ We know how, as the circles widen, the height of the wave grows less and less, until, at no great distance from the centre of disturbance, the wave can no longer be discerned, so slight is the slope of its advancing and following faces. How tremendous, then, must have been the upheaval of the bed of ocean by which wave-circles were sent across the Pacific, retaining, after travelling 5000 miles from the centre of disturbance, the height of a two-storied house! ”

“ The waves generated in the ether by the swinging atoms of luminous bodies are of different lengths and amplitudes. The amplitude is the width of swing of the individual particles of the wave. In water-waves it is the height of the crest above the trough, while the length of the wave is the distance between two consecutive crests. The aggregate of waves emitted by the sun may be broadly divided into two classes: the one class competent, the other incompetent, to excite vision. But the light-producing waves differ markedly among themselves in size, form, and force. ”

“ Let be the rectangular coordinates of any point in the medium, which with the system of waves we may regard as indefinitely extended, and let be the components of electrical displacement at that point at the time being the average values of the components of electrical displacement at that time in a wave-plane passing through the point. Then are perfectly defined quantities, of which are connected with and by the ordinary equations of wave-motion, while each of the quantities has always zero for its average value in any wave-plane. ”

“ Among the waves which reach the eye many, nay, most, are so small that ordinary vision cannot perceive their action. Take, however, a telescope, and so gather them together as to intensify this action, and they are rendered perceptible, just as the unnoticed heaving of ocean becomes a manifest wave-motion when it reaches a regularly narrowing inlet. Thus, from stars so remote that their light has required thousands, or, even in some cases, perhaps, hundreds of thousands of years in reaching us, the light-waves flow steadily in upon us. ”

“ It will be remembered that in one of the first illustrations of wave motion light waves were compared with the waves along a cord, and it was stated that the vibrations which caused the phenomena of light are known to be vibrations of this character rather than of the character of sound waves. The sound waves consist of vibrations in the direction of the propagation of the sound itself. The motion of the particles in the light waves are at right angles to their direction of propagation. ”

“ The medium is supposed to be drifting downwards, across the field. The source may be taken as stationary at S. The horizontal arrows show the direction of waves in the medium; the dotted slant line shows their resultant direction. A wave centre drifts from D to 1 in the same time as the disturbance reaches A, travelling down the slant line D A. The angle between dotted and full lines is the angle between ray and wave-normal. ”

“ The most important and easily observed of the four sets of waves is the oblique bow wave. It is most easily seen when a boy’s boat skims over the surface of a pond, and readily observed whenever we see a duck paddling along on the water. Let any one look, for instance, at a duck swimming on a pond. He will see two trains of little waves or ripples, which are inclined at an angle to the direction of the duck’s line of motion. ”

“ To sum up, we may then say that whenever rapid electrical oscillations are created in open circuit, such as the two rods above described, the arrangement constitutes a device for creating an impulse or effect in the surrounding space called an electric wave in the æther or electro-magnetic medium; just as an organ-pipe or piano-string or other musical instrument constitutes a device for creating waves in the air by means of mechanical oscillations. ”

“ If we can send out a spherical wave front, and have it lengthen rapidly as it proceeds, we will have a wave front that is, at all points, different. Any entering wave would, sooner or later, meet a wave that was half a phase out, no matter what the motion was, nor what the frequency, as long as it lies within the comparatively narrow molecular wave band. ”

“ For almost a century it has been the common belief that light is merely a wave motion in the ether. Our eyes respond to ether waves of certain length only. Waves a little longer than those which affect our eyes are felt by us as heat waves. Waves still longer than those of heat are the so-called electric waves. These we use in wireless telegraphy. There are still shorter waves than those of light. These affect the sensitive plate in photography. They help to form the green material in the leaves of plants and the brilliant colours in flowers. ”