Antenna Primer Wang Ng

References Balanis; Antenna Theory Collin; Antennas and Radiowave Propagation

What is an antenna? The American Heritage Dictionary: A metallic apparatus for sending and receiving electromagnetic waves. Webster’s Dictionary: A usually metallic device (as a rod or wire) for radiating or receiving radio waves. Balanis; Antenna Theory: An antenna is a transitional structure between free-space and a guiding structure. Assumption: Lossless antennas

Types of Antennas Wire antennas Aperture antennas Array antennas Reflector antennas Lens antennas Patch antennas

Radiation Mechanism

Basic Antenna Parameters Radiation pattern: The relative distribution of radiated power as a function of direction in space – an (hypothetical) isotropic antenna radiates equally in all directions. Gain G : The ratio of the radiated power in the maximum direction to the radiated power of an isotropic antenna. The gain of an antenna represents the ability to focus its beam in a particular direction – an isotropic antenna has a gain of 0 dB.

Basic Antenna Parameters Radiation Resistance R A : The equivalent resistance which would dissipate the same amount of power as the antenna radiates. Input impedance Z A : The ratio of the voltage to the current at the antenna terminals (Thevenin equivalent circuit). Polarization: The direction of the E-field.

Basic Antenna Parameters Beamwidth: The angle between the two directions in which the radiated power is half of the maximum value of the beam. Bandwidth: Half-power frequency band. Antenna efficiency: e = P rad / P in Effective aperture: It describes the effectiveness of an antenna in the receiving mode. It is defined as the ratio of the power delivered to the load to the incident power density.

Friis Transmission Equation For polarization matched antennas aligned for maximum directional radiation and reception, the ratio of the transmitted and received power is: Space Loss Factor

Radiation Pattern of an Infinitesimal Dipole

Radiation Pattern of a Thin Dipole

Radiation Pattern of a Circular Loop