the line through RS. When the switch is closed, a voltage equal to V/2 is applied to the load, the charge stored in the line begins to discharge through the load with a current of V/2Z0, and a voltage step travels up the line toward the source. The source end of the line is approximately an open circuit due to the high RS, so the step is reflected uninverted and travels back down the line toward the load. The result is that a pulse of voltage is applied to the load with a duration equal to 2D/c, where D is the length of the line, and c is the propagation velocity of the pulse in the line. The propagation velocity in typical transmission lines is generally more than 50% of the speed of light. For example, in most types of coaxial cable the propagation velocity is approximately 2/3 the speed of light, or 20 cm/ns. High-power PFNs generally use specialized transmission lines consisting of pipes filled with oil or deionized water as a dielectric to handle the high power stress. A disadvantage of simple PFN pulse generators is that because the transmission line must be matched to the load resistance RL to prevent reflections, the voltage stored on the line is divided equally between the load resistance and the characteristic impedance of the line, so the voltage pulse applied to the load is only one-half the power-supply voltage. Blumlein transmission line A transmission line circuit which circumvented the above problem, producing an output pulse equal to the power-supply voltage V, was invented in 1937 by British engineer Alan Blumlein and is widely used today in PFNs. In the Blumlein generator (animation, right), the load is connected in series between two equal-length transmission lines, which are charged by a DC power supply at one end (note that the right line is charged through the impedance of the load). To trigger the pulse, a switch short-circuits the line at the power-supply end, causing a negative voltage step to travel toward the load. Since the characteristic impedance Z0 of the line is made equal to half the load impedance RL, the voltage step is half-reflected and half-transmitted, resulting in two symmetrical opposite-polarity voltage steps, which propagate away from the load, creating between them a voltage drop of V/2 − (−V/2)= V across the load. The voltage steps reflect from the ends and return,