Lecture 4

 1.5 The terminated lossless transmission line What is a voltage reflection coefficient? Assume an incident wave ( ) generated from a source at z < 0. We have seen that the ratio of voltage to current for such a traveling wave is Z 0, the characteristic impedance. But when the line is terminated in an arbitrary load Z L  Z 0, the ratio of voltage to current at the load must be Z L. Thus, a reflected wave must be excited with the appropriate amplitude to satisfy this condition.

What is a voltage reflection coefficient? Total voltage and current on the line (superposition of incident and reflected waves): Voltage reflection coefficient Γ: The total voltage and current at the load are related by the load impedance, so at z = 0, we must have (V 0 + : incident; V 0 - : reflected) (Phase difference: π)

What is a voltage reflection coefficient? The total voltage and current waves on the line : Consider the time-average power flow along the line at the point z: which can be simplified: Constant average power flow at any point on the line; Total power delivered to the load = incident power – reflected power

To understand the reflection coefficient, eg, standing wave (Z l = 0,  0 = -1), we have Define Voltage standing wave ratio to measure the reflection intensity at  0  < 1 (z = -l) (1  SWR <  ， where SWR=1 implied a match load.) l = -z l

The reflection coefficient at z = -l: At a distance l = -z from the load, the input impedance seen looking toward the load is A more usable form of input impedance: ZLZL Z in Z0Z0 l z 0 (l)(l)  (0) Input impedance of oe portion of transmission line with an arbitrary load impedance. Transmission line impedance Equation. What is a voltage reflection coefficient?

Special termination conditions > Voltage and current along the line: > Input impedance of lossless transmission line ZLZL Z in Z0Z0 l z 0 > Characteristic impedance

(1). Short circuit transmission line (Z L = 0) voltage current impedance 0 l =-z Z in Voltage: Current: Input impedance:

voltage current impedance (2). Open circuit transmission line Z L =  Z in Voltage: Current: Input impedance:

(3). Quarter-wave transmission line

(4). Interface of two transmission lines Reflection coefficient: Transmission coefficient: Insertion loss: (return loss)

 1.6 Sourced and loaded transmission lines Total input voltage:

The incident input voltage: Here the input impedance is and the reflection coefficient seen looking into the generator is Eliminate Z in

Power consideration:

Homework 3. A lossless transmission line of electrical length l = 0.3, is terminated with a complex load impedance as shown below. Find the reflection coefficient at the load, the SWR on the line, the reflection coefficient at the input of the line, and the input impedance to the line. Z L = 30 –j20  ZLZL l = 0.3 Z in Z 0 = 75  4. A radio transmitter is connected to an antenna having an impedance 80 + j40  with a 50  coaxial cable. If the 50  transmitter can deliver 30 W when connected to a 50  load. How much power is delivered to the antenna?