Circuit-slide-27

Circuit-slide-27 - Chapter IV Linear antennas Theory of...

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Chapter IV Linear antennas Theory of Linear Dipole Antenna Arrays of Linear Dipole Antennas
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Antennas Antennas are transducers that transfer electromagnetic energy between a transmission line and free space.
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From a circuit point of view, a transmitting antenna behaves like an equivalent impedance that dissipates the power transmitted The transmitter is equivalent to a generator .
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A receiving antenna behaves like a generator with an internal impedance corresponding to the antenna equivalent impedance. The receiver represents the load impedance that dissipates the time average power generated by the receiving antenna.
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Antennas are in general reciprocal devices, which can be used both as transmitting and as receiving elements. This is how the antennas on cellular phones and walkie talkies operate. The basic principle of operation of an antenna is easily understood starting from a two wire transmission line , terminated by an open circuit. Imagine to bend the end of the transmission line, forming a dipole
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antenna . Because of the change in geometry, there is now an abrupt change in the characteristic impedance at the transition point, where the current is still continuous. The dipole leaks electromagnetic energy into the surrounding space, therefore it reflects less power than the original open circuit the standing wave pattern on the transmission line is modified In the space surrounding the dipole we have an electric field. At
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zero frequency (d.c. bias), fixed electrostatic field lines connect the metal elements of the antenna, with circular symmetry . At higher frequency , the current oscillates in the wires and the field
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emanating from the dipole changes periodically. The field lines propagate away from the dipole and form closed loops.
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The electromagnetic field emitted by an antenna obeys Maxwell’s equations Under the assumption of uniform isotropic medium we have the wave equation: Note that in the regions with electrical charges ρ
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In general, these wave equations are difficult to solve, because of the presence of the terms with current and charge. It is easier to use the magnetic vector potential and the electric scalar potential . The definition of the magnetic vector potential is Note that since the divergence of the curl of a vector is equal to zero we always satisfy the zero divergence condition We have also
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We define the scalar potential φ first noticing that and then choosing (with sign convention as in electrostatics) Note that the magnetic
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Circuit-slide-27 - Chapter IV Linear antennas Theory of...

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