Ferrites at Microwave Frequencies
Includes a full mathematical treatment of the interaction of an electromagnetic wave with a gyromagnetic ferrite material.
Inspec keywords: rectangular waveguides; perturbation theory; ferrite isolators; microstrip lines; ferrite waveguides; circular waveguides; microwave circulators
Other keywords: stripline devices; microstrip devices; rectangular waveguides; perturbation theory; magnetised ferrite; Y-junction circulators; circular waveguides
Subjects: Waveguides and microwave transmission lines; Waveguide and microwave transmission line components; Microwave magnetic devices
- Book DOI: 10.1049/PBEW023E
- Chapter DOI: 10.1049/PBEW023E
- ISBN: 9780863410642
- e-ISBN: 9781849193818
- Page count: 280
- Format: PDF
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Front Matter
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1 Introduction
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This chapter discusses the introduction of microwave ferrite circulators, magnetism, spinel ferrites, magnetic garnets and, permanent magnetic ferrites.
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2 Plane waves in an infinite ferrite medium
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In all ferromagnetic and ferrimagnetic materials, there is interaction between microwave fields and the spinning electron, but it is only in ferrites and garnets, which are electrically insulating, that useful interaction between the magnetic properties of the material and electromagnetic waves can be obtained. A classical description of the spinning electron will be used to give a pictorial explanation of the way magnetic materials act on microwave fields.
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3 Longitudinally magnetised ferrite in circular waveguide
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In a microwave system, the electromagnetic field is guided along a transmission line, such as microstrip, or enclosed in hollow metal waveguide pipe. In this chapter we consider microwave transmission inside circular metal waveguide.
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4 Transversely magnetised ferrite in circular waveguide
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In this chapter, we consider the propagating conditions for a circularly symmetric system of ferrite in circular waveguide which is magnetised in some direction perpendicular to the direction of propagation along the waveguide. Four different systems of transverse magnetisation are shown. The radially magnetised ferrite tube is almost impossible to realise in practice and will not be considered further. The circumferentially magnetised ferrite tube is used in the design of a latching phase changer. Using the square-loop magnetic property of microwave ferrite materials, the ferrite may be magnetised to saturation circumferentially by passing a pulse of electric current through a wire threading the tube. The ferrite will then remain magnetised to remanence until it is demagnetised. The analysis of the circumferentially magnetised ferrite tube is given. Both the simple transverse magnetisation and the four-pole field have been used to provide birefringence to the microwave field in the waveguide. The effect of the transverse magnetisation is described, the effect of the four-pole magnetisation is described and the governing equations for a theoretical analysis are given.
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5 Circular waveguide devices
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This chapter discusses the following circular waveguide devices: longitudinally magnetised rotator; Faraday rotation circulator; isolator and circular polariser; circumferentially magnetised ferrite tube (latching phase changer); variable attenuator; quarter-wave plate; and half-wave plate.
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6 Transversely magnetised ferrite in rectangular waveguide
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The effect of a transversely magnetised ferrite medium on a plane wave is discussed. However, for the dominant TE10-mode in rectangular waveguide, the field approximates to a plane wave only at the centre of the waveguide. To get a maximum interaction between the ferrite material and a microwave field the magnetic component of the microwave field needs to be circularly polarised in a plane perpendicular to the direction of magnetisation in the ferrite.
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7 Rectangular waveguide devices
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The general theory of transversely magnetised ferrite in rectangular waveguide is given in Chapter 6. In particular, Fig. 6.1 shows the planes where the magnetic field in the rectangular waveguide is circularly polarised in a plane parallel to the broad face of the waveguide. However, transversely magnetised ferrite devices are not the only ones to give satisfactory performance in rectangular waveguide. It is also possible to provide variable phase change by using a longitudinally magnetised ferrite rod of appropriate diameter at the centre of rectangular waveguide. It appears that some kind of circular waveguide mode is generated in the ferrite which ought to experience Faraday rotation. With the constraint of the rectangular waveguide walls, rotation cannot occur and a large phase change takes place instead. The longitudinally magnetised phase changer is described in Section 7.7.
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8 Y-junction circulator
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This chapter discusses: scattering matrix theory, junction cavity, E-plane junction, isocirculator, matching networks, and striplines. These are all considered in relation to the Y-junction circulator.
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9 Stripline and microstrip devices
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Stripline is often used as a generic term for any transmission line consisting of a thin planar conductor supported on a dielectric sheet parallel to an earthed metallic plane. Symmetrical stripline has the conductor in the region between two earthed metallic conductors, and is sometimes called triplate line. It has the advantage that the microwave field is enclosed and is symmetrical and can be analysed from electrostatic principles assuming a pure TEM-mode. It can be constructed with a printed conductor sandwiched between two dielectric plates and their ground planes. A good summary of stripline design theory is given by Gupta, Garg and Chadha,' who quote all the formulas needed to determine the characteristic impedance and loss of the line and the corresponding inverse design formulas. The most accurate empirical formulas have been given by Wheeler, who also publishes some curves which may be used for design. Some curves of characteristic impedance for some particular standard sizes of stripline substrate are given in the 'Microwave Engineers' Handbook'.
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10 Millimetre wave devices
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In the millimetre wave frequency range, scaling becomes a problem so that copies of microwave devices become difficult to manufacture with adequate accuracy. Then special techniques suitable only for millimetre wave devices become necessary. This chapter concentrates on those techniques which are peculiar to ferrite devices operating at millimetre wave frequencies. The rest of this section discusses the different transmission structures that have been developed particularly for use as millimetre wave lines. In the next section, we consider the limitations due to the fact that the ferrite material is much less efficient as a gyromagnetic material at these higher frequencies. For free-space propagation, there are a number of atmospheric gas absorption frequencies which limit the range of useful propagation, but there are also windows at 35, 94, 140 and 230 GHz, which is why there are a large number of publications concentrating particularly on these frequencies.
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11 High-power and nonlinear effects
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In the rest of this book, the precession of the magnetisation in any small region of ferrite is assumed to be uniform, and any variation of the microwave fields is negligible. However, there are other modes of motion of the magnetisation which vary with very short wavelengths within a small region of ferrite. They are called spin waves, and can be excited in the ferrite when the microwave magnetic field intensity exceeds a certain critical field value. They contribute to the attenuation in the ferrite and to nonlinear effects at high peak power. Above a threshold critical field, spinwaves are excited which increase exponentially and contribute to excess microwave loss owing to the ferrite. The mathematical mechanism for their generation and propagation is beyond the scope of this book.
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12 Perturbation theory and measurements
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The microwave properties of ferrite and other dielectric materials are often measured by observing the change in resonant frequency and Q-factor of a microwave resonant cavity when a small sample of ferrite or other material is inserted. If the ferrite sample is small, it will have only a small effect on the undisturbed fields in the cavity. Calculations can be simplified, if it is assumed that the fields external to the ferrite body are unchanged from those of the empty cavity and it is only the fields in the ferrite body which are different. The theory is called perturbation theory because it is assumed that the ferrite makes only a small perturbation to the fields existing in the unperturbed activity. Perturbation theory, as it is applied to a resonant cavity, is given in this chapter. Then it is applied to the measurement of ferrite material properties. The particular cavity systems suitable for the measurement of permittivity and tensor permeability are described and theoretical calculations are given.
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Back Matter
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