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Impedance
From the point of view of a programmer, the above description of the operation of an isolated bidirectional delay line is complete. In order to connect one bidirectional delay line to others, however, we must introduce the impedance , a positive number associated with a particular waveguide. The impedance allows us to define the relationship between the voltage waves and the current waves which were mentioned in the last section, which is:

(4.4a) 
where referring to Figure 4.1, which implies, from (4.1), that we have

(4.5) 
Thus current waves entering a bidirectional delay line are delayed by the same amount as their voltage wave counterparts, but with sign inversion. In view of (4.4), we need only propagate a particular type of wave (i.e., either voltage or current) in a particular waveguide. In a waveguide network, however, we are free to use different types of waves in different waveguides, converting between the different types with (4.4) where necessary.
The admittance of the waveguide is defined by
and we define the physical current at either end of the waveguide, like the voltage, to be the sum of the wave components. Thus we have

(4.6) 
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Stefan Bilbao
20020122