24-07-2012, 05:07 PM
Transmission Lines
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Load Reection and Transmission
In this section I address the reection and transmission of voltage, current, and power at terminations
of cables. Typically there are two types of terminations, source circuits from which the voltage/ current
waves in the cable originate, and load circuits to which the power carried by the cable is to be delivered.
In this section I address primarily the load termination. The behavior at the source termination is discussed
in a later section. Both of these termination circuits can be modeled as RLC circuits. RLC circuits
are characterized by complex impedances which are the ratio of the complex voltage to current amplitude
in the circuit. Consider the generic driven RLC circuit shown below.
SWR and Node Position
A slotted line probe can be used to determine the impedance of a load circuit. A slotted line probe
is a transmission line designed so that the the voltage amplitude can be measured as a function of z along
a transmission line. The SWR ratio and positions of the standing wave nodes can be measured by a
slotted line probe. The SWR ratio can be used to determine the magnitude of the voltage reection coe-
cient j V j through Equation 8. 54. The wavelength of the cable wave is twice the distance between standing
wave nodes. The phase of the voltage reection coecient 'R can then be found from noting the position
of the node nearest the load and using Equation 8. 52. Knowing the voltage reection coecient allows
one to determine the load impedance given the characteristic impedance of the the slotted line probe
cable. ( See Equation 8. 43) .