Basics of Antenna MCQ || Antenna Questions and Answers

Answer.1. Spherical wave

Explanation:-

The near field is a region in which there are strong inductive and capacitive effects from the currents and charges in the antenna that cause electromagnetic components that do not behave like far-field radiation. In the near field, the radiated waves take the form of Spherical waves.

Answer.1. Less radiative

Explanation:-

A microstrip line consists of a conductor built on a low-loss dielectric substrate with a grounded plane. It can be easily integrated into a microwave device.

1. Stripline is often described as a coaxial cable that was run over by a truck, with its outer ground planes, the inner conductor and isolating dielectric material between the three metal layers.

2. Stripline conductors are well isolated by the surrounding dielectric material due to which they do not radiate and are described as being non-dispersive.

Hence Microstripline is more radiative than stripline but it is easier for component integration due to stripline being totally covered structure.

Answer.2. Yagi antenna

Explanation:-

The Antenna employed in Television receivers is Yagi – Uda Antenna. It is a directional antenna and it consists of multiple parallel half-wave dipole elements.

Yagi – Uda Antenna has:

• A Driven element that is used to apply/receive the signal power.
• A reflector is used to reflect the signal radiation towards the driven element and directions.
• Directors that are used to increase the directivity of the Antenna.

Answer.1. Conserve the available limited battery power

Explanation:-

A communications satellite‘s transponder is the series of interconnected units that form a communications channel between the receiving and the transmitting antennas. Transponder performs mainly two functions those are amplifying the received input signal and translates the frequency of it.

Satellites generate power from solar cells. The power generated by the solar cell is limited. ∴ In order to save the available limited battery power, the transponder of a satellite is not operated at its maximum.

Answer.3. -π / 4

Explanation:-

Antenna array factor is given by

$\left| {{E_T}} \right| = \frac{{\sin \frac{{N\psi }}{2}}}{{\sin \frac{\psi }{2}}}$

and ψ = βd cosθ + α

α = phase shift between element

θ = maximum direction

N = Number of element

Calculation:

Given

d = λ/4

θ = 60°

|E_T| is maximum when

$\begin{array}{l} \frac{\psi }{2} = \pm m\pi \:\left( {m = 0,\:1,\:2..} \right)\\ \\ m = 0 \Rightarrow \frac{{\beta d\cos \theta + \alpha }}{2} = 0\\ \\ \frac{{2\pi }}{\lambda } \times \frac{\lambda }{4}\cos {60^ \circ } + \alpha = 0 \end{array}$

α = −(π/4)

Answer.2. uniformly in all directions

Explanation:-

Isotropic radiation is the radiation from a point source, radiating uniformly in all directions, with same intensity regardless of the direction of measurement.

An isotropic radiator is a theoretical point source of electromagnetic (or) sound waves that radiate the same intensity of radiation in all directions.

Answer.1. 0.244 V/m

Explanation:-

Directive gain G_d = 6 dB = 3.98

Rotating power P_rad = 16 kW

Distance r = 8 km

Calculation:

We know,

Avg power

$\begin{gathered} {P_{avg}} = \frac{{{P_{rod}} \cdot {G_d}}}{{area}} = \frac{{{P_{rad}} \cdot {G_d}}}{{4\pi {r^2}}} \hfill \\ \hfill \\ \frac{{{P_{rad}} \cdot {G_d}}}{{4\pi {r^2}}} = \frac{1}{2} = \frac{{E_0^2}}{{{\eta _0}}}\:\left\{ {\because \eta = {\eta _0}} \right\} \hfill \\ \hfill \\ \frac{{{P_{rad}} \cdot {G_d}}}{{4\pi {r^2}}} = \frac{1}{2} \cdot \frac{{E_0^2}}{{120\:\pi \:}}\left\{ {{\eta _0} = 120\pi = 377\:{\text{\Omega }}} \right\} \hfill \\ \hfill \\ {E_0} = \frac{{\sqrt {60 \cdot {P_{rad}} \cdot {G_d}} }}{r} = 0.244\:V/m \hfill \\ \end{gathered}$

Answer.2. 75 m

Explanation:-

Given that,

Radio waves of frequency 1 MHz are transmitted, i.e.,

f = 1 × 10⁶ Hz

Hence the minimum length of the antenna is given as:

L = λ/4 = c/4f   (since c = nλ)

Here c is the velocity of light, λ is wavelength and f are the frequency of the wave

L = (3 × 10⁸)/(4 × 1 × 10⁶) = 75 m

Answer.2. Inversely proportional

Explanation:-

For large antennas:

Directivity is given by:

D = 32400/(θ_E × θ_H)

For small antennas:

Directivity is given by:

D = 41253/(θ_E × θ_H)

Where:

D: Directivity

θ: Beam-width

Hence, The beam-width and the directivity of an antenna are Inversely proportional.

Answer.1. Directivity

Explanation:-

Directivity of antenna is defined as the ratio of radiation intensity in a given direction from antenna to the radiation intensity over all directions. The average radiation intensity is equal to the total power radiated by the antenna divided by 4π.