Anand K. Verma - Introduction To Modern Planar Transmission Lines

The parameter p, defined below, could be determined from the characteristic impedance at the input and output ends of the line:

(2.3.18)

If the impedances at both ends of a line are fixed, changing the line length, ℓ, can change the parameter p. The parameter p also determines the propagation characteristics of a nonuniform transmission line. The series impedance and shunt admittance p.u.l . of the exponential line can be written as follows:

(2.3.19)

In case of an exponential line, the voltage and current wave equations (2.3.9)and (2.3.10)reduce to

(2.3.20)

(2.3.21)

Let us assume the following exponential form of the solution for the above wave equations with separate propagation constants for the voltage and current waves:

(2.3.22)

The above differential equations provide the following characteristic equations:

(2.3.23)

On solving the above equations, the following expressions are obtained for the complex propagation constants:

(2.3.24)

In the case of a uniform transmission line (p = 0), the propagation constants for the voltage and current waves are identical. The parameter p determines the attenuation constant, i.e. α of a nonuniform line. It is positive for the condition Z 0(x = ℓ) > Z 0(x = 0). Thus, there is an attenuation factor even for a lossless nonuniform line. The factor under the radical sign provides the propagation constant, i.e. the phase‐shift constant β. At the cut‐off frequency, ω = ω c, β is zero. The cut‐off frequency is given by

(2.3.25)

where phase velocity of the voltage and current waves on the line at x = 0 is

(2.3.26)

The complex propagation constants can be rewritten as follows:

(2.3.27)

The propagation constants β 1and β 2are imaginary quantities for the signal below the cut‐off frequency ω < ω c. Under such conditions, no wave propagates on the nonuniform line. The initial signal only gets attenuated. It is called the evanescent mode . The wave propagation takes place only for ω > ω c. Therefore, a nonuniform transmission line behaves like a high‐pass filter (HPF). However, real parts of the complex propagation constants γ 1and γ 2are nonzero. For p > 0, the voltage wave gets attenuated while the current wave is increased in the positive direction of wave propagation. In the backward direction, the reflected voltage and current waves have opposite behavior. The attenuation in the signal is not due to any ohmic loss of a line. It is due to the continuous reflection of the wave as it progresses on the line. The opposite behavior of the voltage and current waves maintains the constant flow of power (P) at any location on a line:

(2.3.28)

where . Unlike a lossless uniform transmission line, the phase velocity of the voltage and current waves on a lossless nonuniform transmission line is dispersive as given below:

(2.3.29)

The phase velocity shows singularity at the cut‐off frequency. After the cut‐off frequency, i.e. for ω > > ω c, it decreases, with an increase in frequency, to a value given by expression (2.3.26).

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