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Module 11 - Microwave Principles: Waveguide Theory and Application
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Module 11 − Microwave Principles
...Pages 1-21 through 1-30Pages i, 1−1, 1−11, 1−21, 1−31, 1−41, 1−51, 1−61, 2−1, 2−11, 2−21, 2−31, 2−41, 2−51, 2−61, 3−1, 3−11, AI−1, Index−1, Assignment 1, Assignment 2 Waveguide Theory and Application - Signal Coupling When less efficient coupling is desired, you can rotate or move the loop until it encircles a smaller number of H lines. When the diameter of the loop is increased, its power-handling capability also increases. The bandwidth can be increased by increasing the size of the wire used to make the loop. When a loop is introduced into a waveguide in which an H field is present, a current is induced in the loop. When this condition exists, energy is removed from the waveguide.
Figure 1-40A - Loop coupling in a rectangular waveguide.
Figure 1-40B - Loop coupling in a rectangular waveguide.
Figure 1-40C - Loop coupling in a rectangular waveguide.
Figure 1-41 - Slot coupling in a waveguide. Slots or apertures are sometimes used when very loose (inefficient) coupling is desired, as shown in figure 1-41. In this method energy enters through a small slot in the waveguide and the E field expands into the waveguide. The E lines expand first across the slot and then across the interior of the waveguide. Minimum reflections occur when energy is injected or removed if the size of the slot is properly proportioned to the frequency of the energy. After learning how energy is coupled into and out of a waveguide with slots, you might think that leaving the end open is the most simple way of injecting or removing energy in a waveguide. This is not the case, however, because when energy leaves a waveguide, fields form around the end of the waveguide. These fields cause an impedance mismatch which, in turn, causes the development of standing waves and a drastic loss in efficiency. Various methods of impedance matching and terminating waveguides will be covered in the next section.
Q-24. What term is used to identify each of the many field configurations that can exist in waveguides? Q-25. What field configuration is easiest to produce in a given waveguide? Q-26. How is the cutoff wavelength of a circular waveguide figured? Q-27. The field arrangements in waveguides are divided into what two categories to describe the various modes of operation? Q-28. The electric field is perpendicular to the "a" dimension of a waveguide in what mode? Q-29. The number of half-wave patterns in the "b" dimension of rectangular waveguides is indicated by which of the two descriptive subscripts? Q-30. Which subscript, in circular waveguide classification, indicates the number of full-wave patterns around the circumference? Q-31. What determines the frequency, bandwidth, and power-handling capability of a waveguide probe? Q-32. Loose or inefficient coupling of energy into or out of a waveguide can be accomplished by the use of what method? Waveguide Impedance Matching Waveguide transmission systems are not always perfectly impedance matched to their load devices. The standing waves that result from a mismatch cause a power loss, a reduction in power-handling capability, and an increase in frequency sensitivity. Impedance-changing devices are therefore placed in the waveguide to match the waveguide to the load. These devices are placed near the source of the standing waves. Figure 1-42 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. The iris is located in the transverse plane.
Figure 1-42 - Waveguide irises.
Figure 1-43A - Conducting posts and screws. PENETRATING.
Figure 1-43B - Conducting posts and screws. EXTENDING Through. An inductive iris and its equivalent circuit are illustrated in figure 1-42, view (A). The iris places a shunt inductive reactance across the waveguide that is directly proportional to the size of the opening. Notice that the edges of the inductive iris are perpendicular to the magnetic plane. The shunt capacitive reactance, illustrated in view (B), basically acts the same way. Again, the reactance is directly proportional to the size of the opening, but the edges of the iris are perpendicular to the electric plane. The iris, illustrated in view (C), has portions across both the magnetic and electric planes and forms an equivalent parallel-LC circuit across the waveguide. At the resonant frequency, the iris acts as a high shunt resistance. Above or below resonance, the iris acts as a capacitive or inductive reactance. POSTS and SCREWS made from conductive material can be used for impedance-changing devices in waveguides. Figure 1-43A and 1-43B, illustrate two basic methods of using posts and screws. a post or screw which only partially penetrates into the waveguide acts as a shunt capacitive reactance. When the post or screw extends completely through the waveguide, making contact with the top and bottom walls, it acts as an inductive reactance. Note that when screws are used the amount of reactance can be varied.
Q-33. What is the result of an impedance mismatch in a waveguide? Q-34. What is used to construct irises? Q-35. An iris placed along the "b" dimension wall produces what kind of reactance? Q-36. How will an iris that has portions along both the "a" and "b" dimension walls act at the resonant frequency? Waveguide Terminations
Figure 1-44A - Waveguide horns. E Plane SECTORAL HORN.
Figure 1-44B - Waveguide horns. H Plane SECTORAL HORN.
Figure 1-44C - Waveguide horns. PYRAMID HORN.
Figure 1-45A - Terminating waveguides.
Figure 1-45B - Terminating waveguides.
Figure 1-45C - Terminating waveguides.
Figure 1-45D - Terminating waveguides Electromagnetic energy is often passed through a waveguide to transfer the energy from a source into space. As previously mentioned, the impedance of a waveguide does not match the impedance of space, and without proper impedance matching, standing waves cause a large decrease in the efficiency of the waveguide. Any abrupt change in impedance causes standing waves, but when the change in impedance at the end of a waveguide is gradual, almost no standing waves are formed. Gradual changes in impedance can be obtained by terminating the waveguide with a funnel-shaped HORN, such as the three types illustrated in figures 1-44A, 1-44B, and 1-44C. The type of horn used depends upon the frequency and the desired radiation pattern. As you may have noticed, horns are really simple antennas. They have several advantages over other impedance-matching devices, such as their large bandwidth and simple construction. The use of horns as antennas will be discussed further in chapter 3. A waveguide may also be terminated in a resistive load that is matched to the characteristic impedance of the waveguide. The resistive load is most often called a Dummy Load, because its only purpose is to absorb all the energy in a waveguide without causing standing waves. There is no place on a waveguide to connect a fixed termination resistor; therefore, several special arrangements are used to terminate waveguides. One method is to fill the end of the waveguide with a graphite and sand mixture, as illustrated in figure 1-45A. When the fields enter the mixture, they induce a current flow in the mixture which dissipates the energy as heat. Another method figure 1-45B is to use a high-resistance rod placed at the center of the E field. The E field causes current to flow in the rod, and the high resistance of the rod dissipates the energy as a power loss, again in the form of heat. Still another method for terminating a waveguide is the use of a wedge of highly resistive material, as shown in of figure 1-45C. The plane of the wedge is placed perpendicular to the magnetic lines of force. When the H lines cut through the wedge, current flows in the wedge and causes a power loss. As with the other methods, this loss is in the form of heat. Since very little energy reaches the end of the waveguide, reflections are minimum. All of the terminations discussed so far are designed to radiate or absorb the energy without reflections. In many instances, however, all of the energy must be reflected from the end of the waveguide. The best way to accomplish this is to permanently weld a metal plate at the end of the waveguide, as shown in figure 1-45D.
Q-37. What device is used to produce a gradual change in impedance at the end of a waveguide? Q-38. When a waveguide is terminated in a resistive load, the load must be matched to what property of the waveguide? Q-39. What is the primary purpose of a dummy load? Q-40. The energy dissipated by a resistive load is most often in what form? Waveguide Plumbing Since waveguides are really only hollow metal pipes, the installation and the physical handling of waveguides have many similarities to ordinary plumbing. In light of this fact, the bending, twisting, joining, and installation of waveguides is commonly called waveguide plumbing. Naturally, waveguides are different in design from pipes that are designed to carry liquids or other substances. The design of a waveguide is determined by the frequency and power level of the electromagnetic energy it will carry. The following paragraphs explain the physical factors involved in the design of waveguides. Waveguide Bends
Figure 1-46 - Gradual E bend.
Figure 1-47 - Gradual H bend.
Figure 1-48 - Sharp bends.
Figure 1-49 - Waveguide twist.
Figure 1-50 - Flexible waveguide. The size, shape, and dielectric material of a waveguide must be constant throughout its length for energy to move from one end to the other without reflections. Any abrupt change in its size or shape can cause reflections and a loss in overall efficiency. When such a change is necessary, the bends, twists, and joints of the waveguides must meet certain conditions to prevent reflections. Waveguides may be bent in several ways that do not cause reflections. One way is the gradual bend shown in figure 1-46. 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. Another common bend is the gradual H bend (figure 1-47). It is called an H bend because the H fields are distorted when a waveguide is bent in this manner. Again, the radius of the bend must be greater than two wavelengths to prevent reflections. Neither the E bend in the "a" dimension nor the H bend in the "b" dimension changes the normal mode of operation. A sharp bend in either dimension may be used if it meets certain requirements. Notice the two 45-degree bends in figure 1-48; the bends are 1/4λ apart. The reflections that occur at the 45-degree bends cancel each other, leaving the fields as though no reflections have occurred. Sometimes the electromagnetic fields must be rotated so that they are in the proper phase to match the phase of the load. This may be accomplished by twisting the waveguide as shown in figure 1-49. The twist must be gradual and greater than 2λ. The flexible waveguide (figure 1-50) allows special bends which some equipment applications might require. It consists of a specially wound ribbon of conductive material, most commonly brass, with the inner surface plated with chromium. Power losses are greater in the flexible waveguide because the inner surfaces are not perfectly smooth. Therefore, it is only used in short sections where no other reasonable solution is available. Waveguide Joints Since an entire waveguide system cannot possibly be molded into one piece, the waveguide must be constructed in sections and the sections connected with joints. The three basic types of waveguide joints are the PERMANENT, the SEMIPERMANENT, and the ROTATING JOINTS. Since the permanent joint is a factory-welded joint that requires no maintenance, only the semipermanent and rotating joints will be discussed. Sections of waveguide must be taken apart for maintenance and repair. a semipermanent joint, called a CHOKE JOINT, is most commonly used for this purpose. The choke joint provides good electromagnetic continuity between sections of waveguide with very little power loss. A cross-sectional view of a choke joint is shown in figures 1-51A and 1-51B. The pressure gasket shown between the two metal surfaces forms an airtight seal. Notice in figure 1-51B that the slot is exactly 1/4λ from the "a" wall of the waveguide. The slot is also 1/4λ deep, as shown in figure 1-51A, and because it is shorted at point (1), a high impedance results at point (2). Point (3) is 1/4λ from point (2). The high impedance at point (2) results in a low impedance, or short, at point (3). This effect creates a good electrical connection between the two sections that permits energy to pass with very little reflection or loss.
Figure 1-51A - Choke joint.
Figure 1-51B - Choke joint.
Figure 1-52 - Rotating joint. Whenever a stationary rectangular waveguide is to be connected to a rotating antenna, a rotating joint must be used. a circular waveguide is normally used in a rotating joint. Rotating a rectangular waveguide would cause field pattern distortion. The rotating section of the joint, illustrated in figure 1-52, uses a choke joint to complete the electrical connection with the stationary section. The circular waveguide is designed so that it will operate in the TM0,1 mode. The rectangular sections are attached as shown in the illustration to prevent the circular waveguide from operating in the wrong mode. Distance "O" is 3/4λ so that a high impedance will be presented to any unwanted modes. This is the most common design used for rotating joints, but other types may be used in specific applications. Waveguide Maintenance The installation of a waveguide system presents problems that are not normally encountered when dealing with other types of transmission lines. These problems often fall within the technician's area of responsibility. a brief discussion of waveguide handling, installation, and maintenance will help prepare you for this maintenance responsibility. Detailed information concerning waveguide maintenance in a particular system may be found in the technical manuals for the system. Since a waveguide naturally has a low loss ratio, most losses in a waveguide system are caused by other factors. Improperly connected joints or damaged inner surfaces can decrease the efficiency of a system to the point that it will not work at all. Therefore, you must take great care when working with waveguides to prevent physical damage. Since waveguides are made from a soft, conductive material, such as copper or aluminum, they are very easy to dent or deform. Even the slightest damage to the inner surface of a waveguide will cause standing waves and, often, internal arcing. 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. The previously mentioned dents are only one type of physical damage that can decrease the efficiency of the system. Another problem occurs because waveguides are made from a conductive material such as copper while the structures of most ships are made from steel. When two dissimilar metals, such as copper and steel, are in direct contact, an electrical action called ELECTROLYSIs takes place that causes very rapid corrosion of the metals. Waveguides can be completely destroyed by electrolytic corrosion in a relatively short period of time if they are not isolated from direct contact with other metals. Any inspection of a waveguide system should include a detailed inspection of all support points to ensure that
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