__ANTENNA TERMS
IEEE STD-145__

__Antenna Factor__**That quality by which the voltage developed across this impedance must be multiplied to obtain the incident field strength in which the antenna is immersed......usually refers to 50 ohms output**__Aperture iIIumination__**The field over the aperture as described by amplitude, phase, and polarization distribution.**__Axial Ratio__**The ratio of the major axis to the minor axis of the polarization ellipse defines the magnitude of the axial ratio. The tilt angle describes the orientation of the ellipse in space.**__Circular Polarization__**It may be either right hand circular polarization (RHCP) or left hand circular polarization (LHCP). The sense of polarization id determined by observation of the direction of the rotation of the electric field vector from a point behind the source.**__Co-Polarized__**The polarization which the antenna is intended to radiate or receive. Also "like polarized".**__Cross Polarization__**In a specific plane containing the reference polarization ellipse, the polarization that is orthogonal to a specified polarization. NOTE: The reference polarization is the co-polarization.**__Directive Gain__**In a given direction, 4 pie times the ratio of the radiation intensity in that direction to the total power radiated by the antenna.**__Directivity__**The value of the directive gain in the direction of its maximum value, as represented by the distribution in space of the power radiated by the antenna. The directivity is defined as 4 pie times the ratio of the maximum radiation intensity ( power radiated per unit solid angle) to the total power radiated by the antenna. The directivity of an antenna is independent of its radiation efficiency and its impedance match to the connected transmission line.**__Effective Area of an Antenna__**In a given direction, the ratio of power available at the terminals of a receiving antenna to the power per unit area of a plane wave incident on the antenna from that direction, polarized coincident with the polarization that the antenna would radiate.**__Far Field Region__**That region of the field of an antenna where the angular field distribution is essentially independent of the distance from a specified point in the antenna region. NOTE: In free space, if the antenna has a maximum dimension, D, which is large compared to the wavelength, the far field region is commonly taken to exist at distances greater that 2D sq / Lambda from the antenna, Lambda being the wavelength. The far field pattern of certain antennas, such as multi-beam reflector antennas, are sensitive to variations in phase over their apertures. For these antennas the above equation may be inadequate.**__Front-to-Back Ratio__**The ratio of the maximum directivity of an antenna to its directivity in a specified rearward direction.**__Gain dBi__**The gain expressed in decibels relative to an isotropic radiator.**__Gain dBic__**The gain expressed in decibels relative to an isotropic radiator that is circularly polarized.**__Gain dBil__**The gain expressed in decibels relative to an isotropic radiator that is linearly polarized.**__Half -Power Beam width__**In a plane containing the direction of the maximum of a beam, the angle between the two directions in which the radiation intensity is one half the maximum value of the beam. NOTE: The half power beam width is equal to the 3dB beam width or the 0.707 voltage beam width.**__Input Impedance__**Input impedance is the parameter which relates the antenna to its transmission line. It is of primary importance in determining the transfer of power from the transmission line to the antenna and visa versa. The impedance match between the antenna and the transmission line is usually expressed in terms of voltage standing wave ratio (VSWR) or the reflective coefficient of the antenna when connected to a transmission line of a given impedance. The reflection coefficient expressed in decibels is return loss.**__Isotropic Radiator__**A hypothetical antenna having equal radiation intensity in all directions. NOTE: An isotropic radiator represents a convenient reference for expressing the directive properties of actual antennas. It may have any polarization.**__Near-Field Region__**The part space between the antenna and the far field region.**__Null__**That region of a radiation pattern, either computed or measured, where the amplitude goes through a minimum value.**__Parallel____Polarization__**The condition where the electric vector is parallel to the local conducting surface.**__Phase Center__**The location of a point associated with an antenna such that, if it is taken as the center of a sphere whose radius extends into the far-field, the phase of a given component over the surface of that radiation sphere is essentially constant, at least over the portion of the sphere where the radiation is significant.**__Polarization__**The polarization of an antenna is defined as the polarization of the electromagnetic wave as described by the shape and orientation of an ellipse, which is the locus of the extremity of the field vector, and the sense in which the ellipse is traversed with time. The elliptical locus is called the polarization ellipse and the wave is said to be elliptically polarized. Circular polarization**__Power Gain__**In a given direction, 4 pie times the ratio of the radiation intensity in that direction to the net power accepted by the antenna from the connected transmitter.**__Radiation Electromagnetic__**The emission of energy in the form of electromagnetic waves.**__Radiation Intensity__**In a given direction, the power radiated from an antenna per unit solid angle.**__Radiation Lobe__**A portion of the radiation pattern bounded by regions of relatively weak radiation intensity.**__Radiation Pattern__**A graphical representation of the radiation properties of the antenna as a function of space coordinates.**__Radiation Efficiency__**The ratio of the total power radiated by the antenna to the net power accepted by the antenna from the connected transmitter. It may also be expressed as the ratio of the maximum gain to the maximum direction.**__Realized Gain__**The power gain of an antenna in its environment, reduced by the losses due to the mismatch of the antenna input impedance to a specified impedance.**__Realized Radiation Efficiency__**The efficiency of an antenna in its environment reduced by all its losses suffered by it, including: ohmic losses, mismatch losses, feeding transmission losses, and radome losses.**__Relative Power Gain__**The ratio of the power gain in a given direction to the power gain of a reference antenna in its reference direction. NOTE: Common reference antennas are isotropic radiators, half-wave diploes, electric dipoles, magnetic dipoles, monopoles, and calibrated horn antennas.**__Return Loss__**The reflection co-efficient of a mismatch expressed in decibels. NOTE: Modern swept VSWR techniques actually sense the reflected component which is normalized to the forward component to yield a return loss, A 2:1 VSWR is equivalent to 9.5dB return loss.**__Side Lobe____Level, Relative__**The maximum relative directivity of a side lobe with respect to the maximum directivity of an antenna, usually expressed in decibels.**__Squint__**The deviation of the electrical peak of the beam from the mechanical axis.**__VSWR__**The voltage standing wave ratio of a component such as an antenna. It is referred to the characteristic impedance of the transmission line used. NOTE: The most common impedance is 50 ohms, but 75 and 300 ohms are frequently used in coaxial or twin lines for VHF,UHF applications.**