load and the open‐circuited load, are discussed below. The voltage and current distributions on a transmission line for both the cases are also obtained.
Short‐Circuited Receiving End
For the short‐circuited load ZL = 0, the line voltage at the load end is zero
(2.1.75)
The input impedance at any distance (ℓ − x) from the source end is
(2.1.76)
At the load end, the voltage is zero. However, the line current is not infinite like the lumped element circuit with a short‐circuited termination at output. A short‐circuited transmission line draws only a finite current from the source. The ℓ < λ/4 short‐circuited line section behaves as an inductive element. The electrical nature of the line section can be controlled by changing its electrical length [B.9–B.15].
Open‐Circuited Receiving End
The load impedance is ZL → ∞ for an open‐circuited transmission line and the load current
(2.1.77)
(2.1.78)
The ℓ < λ/4 open‐circuited line section behaves as a capacitive element. The electrical nature of the line section can be controlled by changing its electrical length.
Matched and Mismatched Termination
The input impedance at any location on a line is Zin (x) = Z0 if it is matched terminated in its characteristic impedance, i.e. ZL = Z0. Normally, the characteristics impedance of a microwave line is a real quantity. The line terminated in Z0 does not create any reflected wave on a transmission line. However, for the mismatched termination ZL ≠ Z0, there is a reflected wave on a transmission line, traveling from the load end to the source end.
Exponential Form of Solution
The wave nature of the line voltage and line current becomes more obvious from the exponential form of solutions of the wave equations. The solution of wave equation (2.1.37), for the phasor line voltage and line current, can also be written in the exponential form:
The distance x is measured from the source end. The time‐dependent harmonic form of the voltage wave is
(2.1.80)
For an outgoing wave on a lossy line, the wave amplitude decays and its phase lags; whether the distance is measured from source end or load end. It is accounted for by the proper sign of distance x. The amplitude of a wave is exponentially decaying due to the line losses. It is expressed by the attenuation constant α (Np/m).
The expression of the traveling current wave on a line could be written as follows:
(2.81)
If the source at x = 0 is connected to a line of infinite extent, there is no reflection from the load end, as the wave will never reach to the load end to get reflected. Therefore, for the forward traveling voltage and current waves on an infinite line, V− = 0 and the above solutions of the wave equations are written as follows:
(2.1.82)
The input impedance of an infinite line at any location x is Zin (x) = Z0. Thus, a finite line terminated in its characteristic impedance, i.e. ZL = Z0 behaves as an infinite extent transmission line without any reflection. The characteristic impedance shows a specific feature of a line that is determined by the geometry and physical medium of the line. Once a finite extent line is terminated in any other load impedance, i.e. ZL ≠ Z0, the voltage and current waves are reflected from the load. The wave reflection is expressed by the reflection coefficient, Γ(x). The reflection coefficient is a complex quantity and its phase changes over the length of a line. However, its magnitude remains constant over the length of a lossless line. The interference of the forward‐moving and backward‐moving reflected waves produces the standing wave with maxima and minima of the voltage and current on a line. Figure (2.8c) shows the voltage standing wave. The position and magnitude of the voltage maxima Vmax and voltage minima Vmin are measurable quantities. Their positions are measured from the load end. The reflection coefficient, and also the voltage standing wave ratio, VSWR, i.e. S, is defined using the Vmax and Vmin. The VSWR could be measured with a VSWR meter. Therefore, the reflection on a transmission line is expressed by both the reflection coefficient and the VSWR. The reflection at the input terminal of a line is also expressed as the return‐loss, RL = − 20 log10|Γin|. The wave behavior in terms of the reflection parameter is an important quantity in the design of circuits and components in the microwave and RF engineering.
At the load end, the total line voltage is a sum of the forward and reflected waves, and it is equal to the voltage drop VR across the load impedance ZL. It is shown in Fig (2.8a). To simplify expressions for the line voltage and current, the origin is shifted from the source end to the load end. In that case, expressions for both the line voltage and line current, given by equation (2.1.79), have to be modified for the new distance variable x < 0. Now the source and load are located at x = − ℓ and at x = 0, respectively. For the origin at the load end, the reflected wave V−e(ωt
