CHAPTER 2 - Conveying Power at Radio Frequency
68
Bear in mind that the battery at this point is still drawing charges from the bottom wire, hence the
pull. Charges flush through the short circuit, rapidly discharging the excess of charge on the top wire
and repopulating the bottom. These charges experience double acceleration, doubling the current!
This is the "current-doubling" effect that occurs in a short-circuited transmission line. The voltage at
the end point remains zero since there is no difference in the carrier concentration of top and
bottom wires (Figure 2.5-52). This is the exact opposite of what happens in an open-circuited line,
where the voltage doubles, but the current drops to zero.
Figure
2.5-52 After the end of the line is reached, current doubles and voltage progressively flattens to zero
By the time the bottom terminal of the battery is reached, the charge carrier density once
again becomes uniform (zero voltage everywhere), but the carriers are still drifting like mad and
hence the doubled current carries on flowing. You may see a simulated animation which
confirms this behaviour in video 2.2.
In this discussion the charge carriers, once set in motion, drift freely along the conductor
forever. This is only possible if the conductors have no resistance. Any practical conductor has some
resistance associated with it but we for simplicity we assume that this creates a negligible loss.
In section 3.4 we will see what happens the line is terminated with impedances which are
either higher (section 3.4.1) or lower (section 3.4.2) than the characteristic impedance of the line.
Vs
+
_
Zero Volts
Conquer Radio Frequency
68 www.cadence.com/go/awr
