Antenna Efficiency and Gain

The efficiency of an antenna relates the power delivered to the antenna and the power radiated or dissipated within the antenna. A high efficiency antenna has most of the power present at the antenna's input radiated away. A low efficiency antenna has most of the power absorbed as losses within the antenna. The losses associated within an antena are typically the conduction losses (due to finite conductivity of the antenna) and dielectric losses (due to conduction within a dielectric which may be present within an antenna). Sometimes efficiency is defined to also include the mismatch between an antenna and the transmission line, but this will be discussed in the section on impedance. The efficiency can be written as the ratio of the radiated power to the input power of the antenna: The term Gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. Gain is more commonly quoted in a real antenna's specification sheet because it takes into account the actual losses that occur. A gain of 3 dB means that the power received far from the antenna will be 3 dB (twice as much) higher than what would be received from a lossless isotropic antenna with the same input power. Gain is sometimes discussed as a function of angle, but when a single number is quoted the gain is the 'peak gain' over all directions. Gain (G) can be related to directivity (D) by: The gain of a real antenna can be as high as 40-50 dB for very large dish antennas (although this is rare). Directivity can be as low as 1.76 dB for a real antenna, but can never theoretically be less than 0 dB. However the peak gain of an antenna can be arbitrarily low because of losses. Electrically small antennas (small relative to the wavelength of the frequency that the antenna operates at) can be very inefficient, with gains lower than -10 dB (even without accounting for impedance mismatch loss). Next Topic: Beamwidths and Sidelobes Antenna Basics Menu Antenna Tutorial (Home)