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Waveguide coupling methods
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Except possibly for the case where an oscillator exists inside a waveguide, it is necessary to have some form of input or output coupling in a waveguide system There are three basic types of coupling used in a microwave waveguide: capacitive (or probe), inductive (or loop), and aperture (or slot) Capacitive coupling is shown in Fig 19-17 This type of coupling uses a vertical radiator inserted into one end of the waveguide Typically, the probe is a quarter-
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Waveguide coupling methods 391 wavelength in a fixed-frequency system The probe is analogous to the vertical antennas used at lower frequencies A characteristic of this type of radiator is that the E field is parallel to the waveguide top and bottom surfaces This arrangement satisfies the first boundary condition for the dominant TE10 mode The radiator is placed at a point that is a quarter-wavelength from the rear wall (Fig 19-17B) By traversing the quarter-wave distance (90 phase shift), being reflected from the rear wall (180 phase shift), and then retraversing the
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Rotating joint 19-15 Basic rotating joint TE10 Mode
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392 Microwave waveguides and antennas
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C 19-17 Probe (capacitive) coupling
D Large diameter probe Low power Large diameter small diameter High power Broadband probes
quarter-wavelength distance (another 90 phase shift), the wave undergoes a total phase shift of one complete cycle, or 360 Thus, the reflected wave arrives back at the radiator in phase to reinforce the outgoing wave Hence, none of the excitation energy is lost Some waveguides have an adjustable end cap (Fig 19-17C) in order to accommodate multiple frequencies The end cap position is varied to accommodate the different wavelength signals Figure 19-17D shows high- and low-power broadband probes that are typically not a quarter-wavelength except at one particular frequency Broadbanding is accomplished by attention to the diameter-to-length ratio The degree of coupling can be varied in any of several ways: the length of the probe can be varied; the position of the probe in the E field can be changed; or shielding can be used to partially shade the radiator element Inductive, or loop coupling, is shown in Fig 19-18 A small loop of wire (or other conductor) is placed such that the number of magnetic flux lines is maximized This form of coupling is popular on microwave receiver antennas, in order to make a waveguide-to-coaxial cable transition In some cases, the loop is formed by the pigtail lead of a detector diode that, when combined with a local oscillator, downconverts the microwave signal to an IF frequency in the 30- to 300-MHz region Aperture, or slot coupling, is shown in Fig 19-19 This type of coupling is used to couple together two sections of waveguide, as on an antenna feed system Slots
Waveguide coupling methods 393 can be designed to couple either electric, magnetic, or electromagnetic fields In Fig 19-19, slot A is placed at a point where the E field peaks, so it allows electrical field coupling Similarly, slot B is at a point where the H field peaks, so it allows magnetic field coupling Finally, we see slot C, which allows electromagnetic field coupling Slots can also be characterized according to whether they are radiating or nonradiating A nonradiating slot is cut at a point that does not interrupt the flow of currents in the waveguide walls The radiating slot, on the other hand, does interrupt currents flowing in the walls A radiating slot is the basis for several forms of antenna, which are discussed at the end of this chapter
Coaxial cable Loop
Loop
H-lines 19-18 Loop (inductive) coupling
Waveguide H-field Possible location for loop
19-19 Slot coupling
394 Microwave waveguides and antennas
Microwave antennas
Antennas are used in communications and radar systems at frequencies from the very lowest to the very highest In both theory and practice, antennas are used until frequencies reach infrared and visible light, at which point optics becomes more important Microwaves are a transition region between ordinary radio waves and optical waves, so (as might be expected) microwave technology makes use of techniques from both worlds For example, both dipoles and parabolic reflectors are used in microwave systems The purpose of an antenna is to act as a transducer between either electrical oscillations or propagated guided waves (ie, in transmission lines or waveguides) and a propagating electromagnetic wave in free space A principal function of the antenna is to act as an impedance matcher between the waveguide, or transmission line, impedance and the impedance of free space Antennas can be used equally well for both receiving and transmitting signals because they obey the law of reciprocity That is, the same antenna can be used to both receive and transmit with equal success Although there might be practical or mechanical reasons to prefer specific antennas for one or the other mode, electrically they are the same In the transmit mode, the antenna must radiate electromagnetic energy For this job, the important property is gain G In the receive mode, the job of the antenna is to gather energy from impinging electromagnetic waves in free space The important property for receiver antennas is the effective aperture Ae, which is a function of the antenna s physical area Because of reciprocity, a large gain usually infers a large effective aperture and vice versa Effective aperture is defined as the area of the impinging radio wavefront that contains the same power as is delivered to a matched resistive load across the feedpoint terminals
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