Courtesy of Pasternack : Antenna Performance Criteria Part 2
Antennas are essential components of RF and microwave devices and are used in a wide variety of applications including radio and television broadcasting, radar, cellular transmission, and satellite communications to name a few. Antennas are designed to transmit and receive radio waves determined by the design of the application intended to receive the transmissions and can be in all horizontal directions equally as in omnidirectional antennas, or in a designated direction as in directional or high gain antennas. An antenna in the receiving mode, in the form of a wire, horn, aperture, array, dielectric rod, for example, is used to collect electromagnetic waves and to extract power from them. Important properties related to the design of an antenna include gain and radiated efficiency, as discussed in an earlier article, aperture, directivity, bandwidth, polarization, radiation pattern, effective length, and resonance and are discussed here:
Power received by the antenna is associated with a collective area known as the effective aperture measured as the area of a circle to the incoming signal as the power density (watts per square meter) x aperture (square meters) = available power from antenna in watts. Antenna gain is proportional to aperture and gain is increased by focusing waves in a single direction while reducing other directions. Thus, the larger the aperture, the higher gain and narrower the beam-width. In most cases, larger antennas tend to have a higher maximum effective area.
Antenna directivity is the measure of concentrated energy radiated in a particular direction expressed as the ratio of radiation intensity in a given direction to the average radiation intensity. In other words, it is the ability of an antenna to focus energy in a specific direction when transmitting or receiving.
The bandwidth of an antenna refers to the range of frequencies over which the antenna can operate and is conceived of in terms of percentage of the center frequency of the band. Bandwidth is constant relative to frequency and antennas of different types have different bandwidth limitations.
Polarization is the orientation of the electric field of an electromagnetic wave, usually described as an ellipse. Electromagnetic waves emitted from an antenna can be polarized vertically and horizontally. The initial polarization of a radio wave is determined by the antenna. For example, if the wave is polarized in the vertical direction, then the E vector is vertical and it requires a vertical antenna. Circular polarization is a combination of both horizontal and vertical waves and, in the electric field vector, appear to be rotating with circular motion around the direction of propagation, making one full turn for each RF cycle.
- Radiation Pattern
Because antennas do not radiate power equally in all directions, antenna radiation patterns or polar diagrams are important tools to quickly evaluate the overall picture of antenna response. The radiation pattern of a transmitting antenna is a plot that describes the strength of the power field radiated by the antenna in various degrees. Radiation plots are often shown in the plane of the axis of the antenna (E plane) or the plane perpendicular to the axis (H-plane) and are usually shown in relative dB (decibels).
- Effective Length
The effective length describes the efficiency of an antennas in transmitting and receiving electromagnetic waves. It is used to determine the voltage induced on the open-circuit terminals of the antenna when a wave hits it. In a receiving antenna, the effective length is the length and orientation of a uniform current required to produce the same electric field as the transmitting antenna. It is a useful tool in determining the effect of polarization mismatch between the propagated waves of the transmitting antenna and the receiving antenna.