Antennas Some Properties and Principles of Antennas An antenna may be viewed as a transducer used to match the transmission line to the surrounding medium or vice versa. (Sadiku 588) Transmission lines are designed to guide electromagnetic energy with a minimum ofradiation. All antennas involve the same basic principle that radiation is produced by accelerated (or decelerated) charge. (Kraus 5 th ed 247) Note: Time-varying current means that electrons are accelerated and decelerated. 248 In a transmission line, if the spacing between wires is a small fraction of wavelength, waves are guided along the sine with little loss by radiation. As the separation approaches the order of a wavelength or more, the wave tends to be radiated so that the opened-out line acts like an antenna which launches a free-space wave. Ulaby Fig 9.1 Note: This is consistent with our model of transmission li nes in which the energy is passed on to inductive and capacitive elements. When the energy gets to the antenna, the inductive and capacitive elements become the permittivity and permeability properties ofspace, and Maxwell’s equations require that the varying electric field between the conductors creates a magnetic field which creates an electric field, etc. in a self- perpetuating fashion. The currents in the conductors were created by an electric field. In order to create a current in a receiving conductor (antenna), there must be an electric field induced in that conductor. That electric field can be created either by the received electric field
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An antenna may be viewed as a transducer used to match the transmission line to the
surrounding medium or vice versa. (Sadiku 588)
Transmission lines are designed to guide electromagnetic energy with a minimum of radiation. All antennas involve the same basic principle that radiation is produced byaccelerated (or decelerated) charge. (Kraus 5th ed 247) Note: Time-varying current meansthat electrons are accelerated and decelerated.
248 In a transmission line, if the spacing between wires is a small fraction of wavelength,waves are guided along the sine with little loss by radiation. As the separation approachesthe order of a wavelength or more, the wave tends to be radiated so that the opened-outline acts like an antenna which launches a free-space wave.
Ulaby Fig 9.1
Note: This is consistent with our model of transmission lines in which the energy is passed on to inductive and capacitive elements. When the energy gets to the antenna, theinductive and capacitive elements become the permittivity and permeability properties of space, and Maxwell’s equations require that the varying electric field between theconductors creates a magnetic field which creates an electric field, etc. in a self- perpetuating fashion.
The currents in the conductors were created by an electric field. In order to create a
current in a receiving conductor (antenna), there must be an electric field induced in that
conductor. That electric field can be created either by the received electric field
orientation of the magnetic field is perpendicular to the electric field, one can think of the
electric field as having been generated by the magnetic field.)
Polarization of an electromagnetic wave refers to the orientation of the electric field
component of the wave. For a linearly polarized wave, the orientation stays the same as
the wave moves through space. If we choose our axis system such that the electric field is
vertical, we say that the wave is vertically polarized. If our transmitting antenna is
vertically oriented, the electromagnetic wave radiated is vertically polarized since, as we
saw before, the electric field is in the direction of the current in the antenna.
The convention is to refer to polarization with reference to the surface of the earth.
Precise orientation is less problematic than one might think, since waves bounce of theground and other objects so do not maintain their original orientation anyway. In space,
horizontal and vertical lose their meaning, so alignment of linearly polarized sending and
receiving antennas is more difficult to achieve. These difficulties are somewhat
circumvented by circular polarization of waves. (RFID 3.34) With circular polarization,
the tip of the electric field vector traces out a circle when viewed in the direction of