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Analyzing Stub Matching with Reflection Coefficients
Roy Lewallen, W7EL wrote:
"Those traveling waves, and hence their sum, cannot cause a reflection of other waves, or alter those waves in any way." Let`s reason together on the situation in a quarter-wavelength short-circuited transmission line stub. I maintain it has a hard short on its far end and a high impedance on its near end. A high impedance means just what it says. You can put a high voltage on it and the resulting current is small. Reflection from a short-circuit results in a 180-degree voltage phase reversal at the short. A round-trip on a 1/4-wave stub produces an additional 180-degree phase reversal. This means thats volts returning to the open-circuit end of the stub are about of the same phase and magnitude as when they started out. Nearly identical voltages appear at the same pair of terminals from opposite directions. Significant current flows in either direction? I think it does not. Where voltage causes insignificant current flow, we have a high impedance. That is why King, Mimno, and Wing on page 30 of Transmission Lines, Antennas and Wave Guides say: "Since the input impedance of a short-circuited quarter-wavelength section of transmission line is a very high resistance, short-circuited stubs may be used to support the line." Best regards, Richard Harrison, KB5WZI |
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