SUBJECT: ANTENNAS AND WAVE PROPAGATION
1. Define an antenna. Antenna is a transition device or a transducer between a guided wave and a free space wave or vice versa. Antenna is also said to be an impedance transforming device. 2. What is meant by radiation pattern? Radiation pattern is the relative distribution of radiated power as a function of distance in space .It is a graph which shows the variation in actual field strength of the EM wave at all points which are at equal distance from the antenna. The energy radiated in a particular direction by an antenna is measured in terms of FIELD STRENGTH.(E Volts/m) 3. Define Radiation intensity?
The power radiated from an antenna per unit solid angle is called the radiation intensity U (watts per steradian or per square degree). The radiation intensity is independent of distance. 4. Define Beam efficiency? The total beam area ( ΩA) consists of the main beam area ( ΩM ) plus the minor lobe area ( Ωm) . Thus ΩA = ΩM+ Ωm . The ratio of the main beam area to the total beam area is called beam efficiency. Beam efficiency = ΣM = ΩM / ΩA.
5.Define Directivity? The directivity of an antenna is equal to the ratio of the maximum power density P(θ,φ)max to its average value over a sphere as observed in the far field of an antenna. D = P(θ,φ)max / P(θ,φ)av. Pattern. D = 4π / ΩA. . area(ΩA ). Directivity from Directivity from beam
6.What are the different types of aperture.? i) Effective aperture. ii). Scattering aperture. iii) Loss aperture. iv) collecting aperture. v). Physical aperture. 7.Define different types of aperture.? Effective aperture(Ae). It is the area over which the power is extrated from the incident wave and delivered to the load is called effective aperture. Scattering aperture(As.) It is the ratio of the reradiated power to the power density of the incident wave. Loss aperture. (Ae). It is the area of the antenna which dissipates power as heat. Collecting aperture. (Ae). It is the addition of above three apertures. Physical aperture. (Ap). This aperture is a measure of the physical size of the antenna.
8. Define Aperture efficiency? The ratio of the effective aperture to the physical aperture is the aperture efficiency. i.e Aperture efficiency (dimensionless). = ηap = Ae / Ap
9. What is meant by effective height? The effective height h of an antenna is the parameter related to the aperture.It may be defined as the ratio of the induced voltage to the incident field.i.e H= V / E. 10. What are the field zone? The fields around an antenna ay be divided into two principal regions. i. ii. Near field zone (Fresnel zone) Far field zone (Fraunhofer zone)
11.What is meant by Polarization.?
The polarization of the radio wave can be defined by direction in which the electric vector E is aligned during the passage of atleast one full cycle.Also polarization can also be defined the physical orientation of the radiated electromagnetic waves in space. The polarization are three types. They are Elliptical polarization ,circular polarization and linear polarization. 12. What is meant by front to back ratio.? It is defined as the ratio of the power radiated in desired direction to the power radiated in the opposite direction. i.e FBR = Power radiated in desired direction / power radiated in the opposite direction. 13. Define antenna efficiency.? The efficiency of an antenna is defined as the ratio of power radiated to the total input power supplied to the antenna. Antenna efficiency = power Power radiated / Total input
14. What is radiation resistance ?
The antenna is a radiating device in which power is radiated into space in the form of electromagnetic wave. W’ = I2 R Rr = W’/ I2 Where Rr is a fictitious resistance called called as radiation resistance. 15 What is meant by antenna beam width? Antenna beamwidth is a measure of directivity of an antenna. Antenna beam width is an angular width in degrees, measured on the radiation pattern (major lobe)...
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