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CHAPTER 2 - Conveying Power at Radio Frequency 42 The velocity factor of any cable type, coaxial or otherwise, may be calculated quite simply by means of (2.3-3). √ Where, = Relative permittivity of insulation between conductors = Velocity of wave propagation = Velocity of light in a vacuum Equations (2.3-1) and (2.3-2) show that a transmission line's characteristic impedance (Z 0 ) increases as the conductor spacing increases. If the conductors are moved away from each other, the distributed capacitance will decrease (greater spacing between capacitor 15 "plates"), and the distributed inductance will increase (less cancellation of the two opposing magnetic fields). Less parallel capacitance and more series inductance results in a smaller current drawn by the line for any given amount of applied voltage therefore the characteristic impedance of the line also increases. Conversely, bringing the two conductors closer together increases the parallel capacitance and decreases the series inductance. Both changes result in a larger current drawn for a given applied voltage, equating to a lesser impedance. A more detailed analysis of transmission lines geometry and physical construction and their effects on characteristic impedance is presented in section 2.11. Ignoring any dissipative effects such as dielectric "leakage" and conductor resistance, the characteristic impedance of a transmission line is equal to the square root of the ratio of the line's inductance per unit length divided by the line's capacitance per unit length: √ Where, = Characteristic impedance of the line L = Inductance per unit length of the line C = Capacitance per unit length of the line 15 For a parallel plate capacitor , where is the distance between the plates. (2.3-4) (2.3-3) Conquer Radio Frequency 42 www.cadence.com/go/awr

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