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From: Alan G3NOQ
Date: 14 Jun 2002
Time: 07:15:05
Remote Name: 192.168.0.3
David.... I am glad you are half convinced so let me try to improve the point. Instead of a parallel wire line, you could think of a parallel plate line e.g. stripline and assume very small plate spacing compared to the wavelength, which is the basis for transmission line analysis. As you said, the field distribution inside the line approximates a plane wave if the line has a matched load. If it is not matched, there is a forward and a backward wave causing a standing wave..... At the end of the line there is an open circuit, so the current there is zero - it must be because there is nowhere for the current to flow. The forward and backward waves are then equal and from standard theory the voltage and the electric field are doubled, and the current and the magnetic field are zero..... In other words the plane-wave condition does not exist at the open end of the line, and it does not radiate (actually a line does radiate an amount that depends on the spacing, not on the impedance, but that is outside the scope of transmission line theory)..... Ramo's theory as correctly adapted by Ralph in his equation 5 says that when the radial transmission line has an open circuit on the outer periphery, the input impedance is always imaginary. This is consistent with the idea that no power is radiated, which is correct in the context of transmission line theory. If the plate spacing is large enough for radiation to occur, we are then outside the scope of the Ramo theory..... Best wishes, Alan