What is antenna? Give its types and their radiation pattern.Ripunjay Tiwari
Ans. The wireless communication is an efficient way of transmitting signals through space without guidance. We do not need any ‘medium’ for the transport of electromagnetic energy from the transmitter to the outside world and in reverse from the outside world to the receiver. Antennas couple electromagnetic energy to and from space to and from a wire or coaxial cable. An antenna will radiate power on all directions but, typically, does not perform equally well in all directions. A common way to characterize the performance of an antenna is the radiation pattern, which is a graphical representation of the radiation properties of an antenna as a function of space coordinates.
A theoretical reference antenna is the isotropic radiator, a point in space radiating equal power in all direction, i.e., all point with point with equal power are located on a sphere with the antenna as its center. The radiation pattern is symmetric in all direction, as shown in fig 1.4
However, such an antenna does not exist in reality. Real antennas all exhibit directive effects, i.e, the intensity of radiation is not the same in all directions from the antenna. The simplest real antenna is a thin center-fed dipole (dipole antenna), also called Hertzian dipole as shown in fig. 1.5 (right hand side). The dipole consists of two collinear conductors of equal length, separated by a small feeding gap. The length, of the dipole is not arbitrary, but, for example, half the wavelength of the signal to transmit result in a very efficient radiation of the energy. If mounted on the roof if a car, the length of l/4 is efficient. This is also known as marconi antenna.
A ½ dipole has a uniform or omnidirectional radiation pattern in one place and a figure eight pattern in the other two planes as shown in fig. 1.6. This types of antenna can only overcome environmental challenges by booting the power level of the signal. Challenges could be mountains, valleys, buildings etc.
If an antenna is positioned e.g., in a valley or between building, an omni-directional radiation pattern is not very useful. In this case directional antennas with certain fixed preferential transmission and receptions can be used. Fig. 1.7. Shown the radiation pattern of a directional antenna with the main lobe in the direction of the x-axis. A special example of directional antennas is constituted by satellite dishes.
Several directed antennas can be combined on a single pole to construct a sectorized antenna. A cell can be sectorized onto, for example, three or six sectors, thus enabling frequency reuse fig 1.8 shows the radiation pattern of these sectorized antennas.
Two or more antenna can also be combined to improve reception by counteracting the negative effects of multipath propagation. These antennas, also called multi-element antenna arrays allow different diversity schemes. One such scheme is switched diversity or selection diversity, where the receiver always used the antenna element with the largest output. Diversity combining constitutes a combination of the power of all signals to produce gain. The phase is first corrected to avoid cancellation, fig. 1.9 shows different schemes. On the left, two ¼ antennas are combined with a distance of ½ between them on top of ground plane. On the right, three standard ½ dipoles are combined with a distance of ½ between them. Spacing could also be in multiples of 1/2.
A more advanced solution is provided by smart antennas which combine multiple antenna elements with signal processing to optimize the radiation/ reception pattern is response to the signal environment.