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Reg, G4FGQ wrote:
"It is impossible for an E-field to exist without an H-field." Agreed. By definition an electromagnetic wave includes an electric component and a magnetic component. That does not mean the components are inseparable. The purpose of a Faraday screen is to eliminate capacitive coupling while permitting magnetic coupling. I`ve worked in several medium wave broadcast plants. In these, each tower was coupled through a 1:1 air-core transformer to its transmission line. The transformer consisted of two identical coils, one on either side of a Faraday screen. The coils shared a cmmon axis. Electrically, the transformer was transparent at the transmitting frequency. It coupled the transmitting frequency as if the transformer did not exist to impede. Its purpose was to eliminate capacitive coupling, The Faraday screen provided a place where electric field lines are shunted to ground. The problem with capacitive coupling between a transmitter and a tower is that the higher the frequency, the less the reactance or opposition. The coupling is better through a capacitance to the harmonics of a frequency than it is for the fundamental. The Faraday screen removes this unwanted bias for imroved harmonic propagation. A side effect of the Faraday screen is that it removes lightning strokes before they reach the transmission line from the tower. The Faraday screen looks like a metal rake. Its back where the teeth or tines join is firmly grounded. The teeth are open-circuited. Current cannot circulate between and through the teeth, so no counter electromotive force can be generated to oppose magnetic coupling between primary and secondary coils. The rake is transparent for magnetic coupling but it is a stopper for electric coupling. By complete shielding, that is metalllically enclosihng one or both coils of an impedance coupling pair, magneric coupling between them can be practically eliminated. A coupling capacitor between the coils allows only the electric field to be effective. There`s no magnetic field involved. I`m no advocate of the E-H antenna, but the electric and magnetic components of a wave are easily separated. Best regards, Richard Harrison, KB5WZI |
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