Due to the time varying electric and magnetic fields, electromagnetic waves are created and these travel between the conductors. As these waves approach open space, free space waves are formed by connecting the open ends of the electric lines. Since the sinusoidal source continuously creates the electric disturbance, electromagnetic waves are created continuously and these travel through the transmission line, through the antenna and are radiated into the free space. Inside the transmission line and the antenna, the electromagnetic waves are sustained due to the charges, but as soon as they enter the free space, they form closed loops and are radiated. Explain the following terms w.r.t an antenna. a) Radiation Pattern lobes b) Power theorem and Poynting vector c) Radiation intensity. Radiation Pattern lobes: 1. Main Lobe: This is the radiation lobe containing the direction of maximum radiation. 2. Minor Lobe: All the lobes other then the main lobe are called the minor lobes. Theselobes represent the radiation in undesired directions. The level of minor lobes is usually expressed as a ratio of the power density in the lobe in question to that of themajor lobe. This ratio is called as the side lobe level (expressed in decibels). • Back Lobe: This is the minor lobe diametrically opposite the main lobe. • Side Lobes: These are the minor lobes adjacent to the main lobe and are separated by various nulls. Side lobes are generally the largest among the minor lobes. In most wireless systems, minor lobes are undesired. Hence a good antenna design should minimize the minor lobes. 2.
Bnmvnmvmvmvhmjjhjhjhjhmj Power theorem and Poynting vector: The directions of E , H and r form a right handed set such that the Poynting vector is in the r direction and it indicates the direction of propagation of the electromagnetic wave. Hence the time average isualiz vector given by where E and H represent the peak values of the electric and magnetic fields respectively. The average power radiated by an antenna can be written as where ds is the vector differential surface = W is the magnitude of the time average isualiz vector (Watts /m2 ) Radiation intensity: in a given direction is defined as power radiated from an antenna per unith soilid angle. U = power radiated/ unith soilid angle = Prad/area/r2 Radiadiatin intensity is a far field parameter of an antenna. where U is the radiation intensity in Watts per unit solid angle. the total power radiated is obtained by integrating the radiation intensity over the entire solid angle of 4 Prad = U d = 2 0 0 USINdd For an isotropic source ,U will be independent of the angles &
2 Prad = Uo SINdd =4Uo 0 0 Uo = Pr ad 4 Beam area The beam area A the solid angle through which all the power radiated by the antenna Would stream if P ( , ) is maintained at its maximum value over A and was zero else where. Beam efficiency The ratio of main beam area to the total beam area is called beam efficiency Beam efficiency = M = M A Directive gain: 1. Of an antenna, the ratio of 4π times the radiation intensity in a given direction (i.e., power radiated per unit solid angle), to the total power. The directive gain is usually expressed in dB, and expresses the performance of the antenna relative to an isotropic antenna. Note that 4π steradians is the solid angle subtended by a complete sphere, so the total power radiated by the antenna is the power radiated into 4π sr. 2. Of an antenna, for a given direction, the ratio of the radiance produced in the given direction to the average value of the radiance in all directions. If the direction is not specified, the direction of maximum radiance is assumed. The directive gain is usually expressed in dB. Directivity: The directivity of an antenna has been defined as “the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions”. In other words, the directivity of a nonisotropic source is equal to the ratio of its radiation intensity in a given direction, over that of an isotropic source. where D is the directivity of the antenna U is the radiation intensity of the antenna Ui is the radiation intensity of an isotropic source P is the total power radiated Sometimes, the direction of the directivity is not specified. In this case, the direction of the maximum radiation intensity is implied and the maximum directivity is given by
D is the maximum directivity U max is the maximum radiation intensity Directivity is a dimensionless quantity, since it is the ratio of two radiation intensities. Hence, it is generally expressed in dB. The directivity of an antenna can be easily estimated from the radiation pattern of the antenna. An antenna that has a narrow main lobe would have better directivity, then the one which has a broad main lobe, hence it is more directive. Antenna Gain: Antenna gain is a parameter which is closely related to the directivity of the antenna. We know that the directivity is how much an antenna concentrates energy in one direction in preference to radiation in other directions. Hence, if the antenna is 100% efficient, then the directivity would be equal to the antenna gain and the antenna would be an isotropic radiator. Since all antennas will radiate more in some direction that in others, therefore the gain is the amount of power that can be achieved in one direction at the expense of the power lost in the others. The gain is always related to the main lobe and is specified in the direction of maximum radiation unless indicated. It is given as A lossless /2 dipole antenna with input impedance of 73 is to be connected to a transmission line whose characteristic impedance is 50. Assuming pattern of the antenna is given by U = Bosin3, find the overall gain of this antenna. Umax = Bo Prad = U = Bo(32/4) ecd = 1 G = etD = 0.965 1.697 = 1.64 = 2.14dB Effective aperture: In receiving mode, the maximum power received in a receiving antenna is PAm . Consider this power to be that intercepted from the incoming wave by a maximum effective area Aem. If the power density of the incoming wave is S, then Prec =SAem . Aem is called maximum effective aperture of the antenna. Effective height • a) Effective height of an antenna is the height of the antenna responsible for power radiation or reception. It may be defined as the ratio of the induced voltage to incident field i.e He =V/E. Another way of defining effective height is to consider the transmitting case and equate the effective height to the physical height multiplied by the (normalized) average current. i.e