Discuss antenna gain in detail.Ripunjay Tiwari
Ans. Antenna gain is a measure of the directionality of an antenna. Antenna gain is defined as the power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna). For example, if an antenna has a gain of 3 dB, or a factor of 2. The increased power radiated in a given direction is at the expense of other directions. In effect, increased power is radiated in one direction by reducing the power radiated in other directions. It is important to note that antenna gain does not refer to obtaining more output power that but rather to directionality.
A transmission line connects the radio transmitter and its radio frequency (RF) power amplifier (PA to transmit antenna. In the receiver the antenna is connected to the front-end low-noise amplifier (LNA). The amplifier received signal is fed to a down converter and demodulated as shown in fig. 1.11 in the mobile or handheld transmitter/receiver, the same antenna is used for transmit antenna to radiate electromagnetic waves.
An isotropic antenna is an ideal lossless antenna that radiates power equally well in all directions. In mobile communications, omnidirectional antenna are most frequently used. These transmit/receive antenna are an approximation of ideal isotropic antenna. As transmit antennas they radiate well in all directions, and as receiving antennas they receive signals from all direction equally well. These have an approximate unity gain G=1, or 0 dB. Assume that transmit RF amplifier provides p watts of an isotropic transmit antenna illustrated in fig. 1.11. The radiated power density p or the outward flow of electromagnetic energy, measured at a distance r from antennas is
Directional antennas concentrate the radiated power in a particular direction. Antenna directivity (D) of a directional antenna is defined as
Power density at distance r in direction
To use the antenna directivity definition, we require knowledge of the power actually transmitted by the antenna. The power differs from the power supplied at the transmitter and receiver terminals by the losses in the antenna itself.
A receiving antenna with an effective aperture A and at distance r from the omnidirectional transmitting antenna receives a power of pr (watts) given by-
Antenna gain G is related to the aperture of the antenna and the wavelength (in metres) of the radio signal.
And c = 3 * 108 m/sec (velocity of light)
f = frequency of transmitted carrier.
Ideal omnidirectional antennas have a unity gain G = 1, thus from equation (iv)