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Roy Lewallen, W7EL wrote:
"If you`ll read what I`ve written, you`ll hopefully see that my only point of contention is with your claim that waves reflect from a "virtual short". They do not." Seems to me they do. If you are lucky enough to have a copy of Terman`s 1955 opus, we can reason together. On page 91 is found Fig. 4-3 Vector (phasor) diagrams showing manner in which incident and reflected waves combined to produce a voltage distribution on the transmission line. At an open circuit, the voltage phasors are in-phase. E2, the reflected phasor, rotates clockwise as it travels back toward the source. E1, the incident phasor, rotates counter-clockwise as we look back toward the source. Looking 1/4-wavelength back from the open-circuit, E2 and E1, each having rotated 90-degrees, but in opposite directions, are now 180-degrees out-of-phase. On page 92, Fig. 4-4 shows the current, which summed to zero at the open circuit, has risen to its maximum value at 1/4-wavelength back from the open-circuit while the voltage dropped to its minimum, nearly zero, maybe close enough to declare a "virtual short-circuit", 1/4-wavelength back from the open-circuit. What`s a short-circuit? Little voltage and much current. What`s the difference between a physical short and the virtual short? Nothing except the shunting conductor. Is there current flowing at the open-circuit end of the 1/4-wave line segment? No, the open-circuit won`t support current. If a high-impedance generator of the same frequency were connected to the virtual short point on the line, would it also be shorted? Yes. Where? At the virtual short, not the open-circuit at the end of the line. Best regards, Richard Harrison, KB5WZI |
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