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Engineering LibreTexts

10: Antennas

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  • An antenna is a transducer; that is, a device which converts signals in one form into another form. In the case of an antenna, these two forms are (1) conductor-bound voltage and current signals and (2) electromagnetic waves. Traditional passive antennas are capable of this conversion in either direction.

    • 10.1: How Antennas Radiate
      In this section, we consider the transmit case, in which a conductor-bound signal is converted into a radiating electromagnetic wave. Radiation from an antenna is due to the time-varying current that is excited by the bound electrical signal applied to the antenna terminals.
    • 10.2: Power Radiated by an Electrically-Short Dipole
      In this section, we determine the total power radiated by an electrically-short dipole (ESD) antenna in response to a sinusoidally-varying current applied to the antenna terminals. This result is both useful on its own and necessary as an intermediate result in determining the impedance of the ESD.
    • 10.3: Power Dissipated by an Electrically-Short Dipole
      The power delivered to an antenna by a source connected to the terminals is nominally radiated. However, it is possible that some fraction of the power delivered by the source will be dissipated within the antenna. In this section, we consider the dissipation due to the finite conductivity of materials comprising the antenna. Specifically, we determine the total power dissipated by an electrically-short dipole (ESD) antenna in response to a sinusoidally-varying current applied to the antenna ter
    • 10.4: Reactance of the Electrically-Short Dipole
      Any antenna may be characterized in terms of this impedance. The real-valued component of this impedance accounts for power which is radiated away from the antenna and dissipated within the antenna. The imaginary component of this impedance – i.e., the reactance – typically represents energy storage within the antenna, in the same way that the reactance of a capacitor or inductor represents storage of electrical or magnetic energy, respectively.
    • 10.5: Equivalent Circuit Model for Transmission; Radiation Efficiency
      A radio transmitter consists of a source which generates the electrical signal intended for transmission, and an antenna which converts this signal into a propagating electromagnetic wave. Since the transmitter is an electrical system, it is useful to be able to model the antenna as an equivalent circuit. From a circuit analysis point of view, it should be possible to describe the antenna as a passive one-port circuit that presents an impedance to the source.
    • 10.6: Impedance of the Electrically-Short Dipole
    • 10.7: Directivity and Gain
    • 10.8: Radiation Pattern
      The radiation pattern of a transmitting antenna describes the magnitude and polarization of the field radiated by the antenna as a function of angle relative to the antenna. A pattern may also be defined for a receiving antenna, however, we defer discussion of the receive case to a later section.
    • 10.9: Equivalent Circuit Model for Reception
    • 10.10: Reciprocity
    • 10.11: Potential Induced in a Dipole
    • 10.12: Equivalent Circuit Model for Reception, Redux
    • 10.13: Effective Aperture
    • 10.14: Friis Transmission Equation

    Thumbnail: Polar plots of the horizontal cross sections of a (virtual) Yagi-Uda-antenna. Outline connects points with 3 dB field power compared to an ISO emitter. (CC BY-SA 4.0 International; Timothy Truckle via Wikipedia)