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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

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 $$\widehat{a_\theta} \,and\, \widehat{a_\emptyset}$$

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

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 $$\frac{3 sin⁡θ}{r^2}a_r W/m^2$$?

Explanation: Radiation intensity is defined as the power radiated per unit solid angle.

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

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

Explanation: Effective length of the receiving antenna $$L_{eff}=\frac{V_{oc}}{E_i}=\frac{1}{0.2}=5m.$$

Sanfoundry Global Education & Learning Series – Antennas. 