CHAPTER 1 ANTENNA BASICS An antenna is a transition device , or transducer , between a guided wave and a free space , or vice versa . The antenna is a device which interfaces the circuit and space. According to the website dictionary , an antenna is defined as “a metallic device for radiating or receiving radio-waves . According to IEEE standard definitions of terms for antennas , as antenna is a means for radiating or receiving radio waves. Radiation mechanism and radiation patterns In order to know how an antenna radiates, let us first consider how radiation occurs. A conducting wire radiates mainly because of time-varying current or an acceleration (or deceleration) of charge. If there is no motion of charges in a wire, no radiation takes place, since no flow of current occurs. Radiation will not occur even if charges are moving with uniform velocity along a straight wire. However, charges moving with uniform velocity along a curved or bent wire will produce radiation. If the charge is oscillating with time, then radiation occurs even along a straight wire. The radiation from an antenna can be explained with the help of Figure which shows a voltage source connected to a two conductor transmission line. When a sinusoidal voltage isapplied across the transmission line, an electric field is created which is sinusoidal in nature and this results in the creation of electric lines of force which are tangential to the electric field. The magnitude of the electric field is indicated by the bunching of the electric lines of force. The free electrons on the conductors are forcibly displaced by the electric lines of force and the movement of these charges causes the flow of current which in turn leads to the creation of a magnetic field.
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