This set of Antennas Multiple Choice Questions & Answers (MCQs) focuses on “Isotropic Radiators”.
1. An ideal source in which the power is radiated equally in all directions is called as ________ radiator.
a) Isotropic
b) Omni-directional
c) Directional
d) Transducer
View Answer
Explanation: Isotropic radiators radiate power in all directions uniformly. Omni-directional antenna radiates only in one direction. Directional antenna radiates maximum power in particular direction. Transducer converts one form of energy to other.
2. In Isotropic radiation, which of the following vector component is absent in pointing vector?
a) \(\widehat{a_r}\)
b) \(\widehat{a_\theta}\)
c) \(\widehat{a_\emptyset}\)
d) Both \(\widehat{a_\theta} \,and\, \widehat{a_\emptyset} \)
View Answer
Explanation: Isotropic radiators will radiate power equally in all directions. Due to this symmetrical distribution, components of θ, Φ get cancelled. So it will have only radial component.
3. What is the amount of Electric field present at a distance of 10km for an isotropic radiator with radiating power 3kW?
a) 30mV/m
b) 60mV/m
c) 15mV/m
d) 10mV/m
View Answer
Explanation: For isotropic radiator, power per unit area = \(\frac{P_t}{4πr^2}\) — (1)
From pointing theorem, \(P=\frac{\mid E_{max}^2\mid}{2\eta}\), where η=120π. —– (2)
Equating (1) & (2) and Erms=Emax/√2, we get
\(E_{rms}=\frac{\sqrt{30P_t}}{r}=\frac{\sqrt{30×3000}}{10000}=30mV/m.\)
4. What is the radiation intensity for isotropic antenna having radiation power density \(\frac{3 sinθ}{r^2}a_r W/m^2\)?
a) 3sinθ ar W/Steradian
b) 3cosθar W/Steradian
c) 6πsinθ ar W/Steradian
d) 6πcosθ ar W/steradian
View Answer
Explanation: Radiation intensity is defined as the power radiated per unit solid angle.
U= Wr.r2= 3sinθ ar W/Steradian.
5. For an isotropic source, Radiation intensity will be _____ on θ and ______ on Φ.
a) Dependent, independent
b) Independent, independent
c) Independent, dependent
d) Dependent, dependent
View Answer
Explanation: Due to symmetric distribution in isotropic source, there will be no θ, ф components. Only radial component will be present. \( P_{total \,rad}=∯ Usinθdθd∅=4πU (For \,θ: 0 \,to \,π, \phi: 0 \,to\, 2π).\)
∴ Radiation intensity \(U=\frac{P_{total \,rad}}{4π}\)
6. Find the effective length of a receiving antenna with open circuit voltage 1V and incident electric field 200mV/m?
a) 0.2m
b) 50m
c) 5m
d) 5cm
View Answer
Explanation: Effective length of the receiving antenna \(L_{eff}=\frac{V_{oc}}{E_i}=\frac{1}{0.2}=5m.\)
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