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2.1 Introduction 29 2 Conveying Power at Radio Frequency 2.1 Introduction As mentioned in chapter one, whereas at low frequency Electric and Magnetic fields may be considered quasi-static, at high frequency their rate of change is such that this assumption is no longer valid. The simplified, low-frequency models, which assume that most of the power is conveyed by currents, are therefore no longer suitable since, at Radio Frequency, most of the power is conveyed by Electric and magnetic fields intertwined in such a way as to form an Electromagnetic wave. Let us look at an example to illustrate what we mean by electromagnetic waves. The charged particle shown in Figure 2.1-1 is oscillating along a length of wire. Figure 2.1-1 Generation of an Electromagnetic Wave by oscillation of a charged particle As the charge moves and assumes different positions, the electric field which it generates also changes. If we were in free space and we looked at the electric field lines in the area surrounding the charge, we would see their direction change at the same speed as the charge oscillates. If we placed a test charge near the wire therefore, the force which this test charge would experience would vary in time as the oscillating charge moves. Also, the speed of the oscillating charge will be increasing as it leaves one end of the wire and decreasing as it approaches the opposite end. This translates into a time-varying current which generates a time-varying magnetic field as shown by equation (1.3-1) where the constant dc current is replaced by ( ). ( ) ( ) By Faraday's law of induction, this magnetic field generates an electric field which, through Ampere's law, generates a magnetic field and so on. This mechanism, which Maxwell's equations beautifully describe, is shown in Figure 2.1-2. Conquer Radio Frequency 29 www.cadence.com/go/awr

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