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CHAPTER 2 - Conveying Power at Radio Frequency 108 2.11.2 Microstrip Line Coaxial lines are extensively used at RF and Microwave frequencies since they are ideal structures to contain the electric and magnetic fields generated at high frequency. This is because they support the TEM 21 mode of propagation which means that they keep Electric and Magnetic fields perpendicular to one another and to the direction of propagation. The speed of propagation is of course affected by the dielectric medium used inside the line but, since this medium is homogeneous, such a speed may be easily calculated by means of equation (2.11-2). Figure 2.11-6 Electric and Magnetic Field inside a coaxial line, the direction of propagation is perpendicular to the page The problem with coaxial lines is that they are not very compact, cannot be easily integrated with printed circuits and are hard to prototype. For example, you could not easily construct a quarter-wave transformer (section 2.9) by means of a coaxial cable and you would need to use connectors to join the lines. An easier way to implement transmission lines with a specific characteristic impedance is to use microstrip lines. Such lines are very simple to construct and their basic structure is shown in Figure 2.11-7. Figure 2.11-7 Basic structure of a microstrip transmission line They consist of a large sheet of metal, usually copper, called ground plane, with a slab of dielectric material on top. On top of the dielectric, a strip of metal is used to create the equivalent of the inner conductor in a coaxial line which will carry the actual signal. The obvious problem with this type of line is that both magnetic and electric fields will be partially contained in the dielectric material and partially in the air surrounding the line. This is shown in Figure 2.11-8. 21 Transverse ElectroMagnetic Conquer Radio Frequency 108 www.cadence.com/go/awr

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