# Basics of Antenna MCQ || Antenna Questions and Answers

Ques.1. Which of the following is NOT true with respect to antennas?

1. Lower frequencies are radiated near the end of transmission line
2. Horn antennas are used to feed parabolic dish antennas
3. Yagi-Uda antennas have high bandwidth and low gain
4. Higher frequencies are radiated near the feedpoint

Answer.3. Yagi-Uda antennas have high bandwidth and low gain

Explanation:-

Yagi-Uda Antenna:

• It is a directional antenna and it consists of multiple parallel half-wave dipole elements.
• It is a directional antenna and it consists of multiple parallel half-wave dipole elements.
• The direction of maximum radiation is along the axis of the dipole array which is called an end-fire radiation pattern.
• The reflector and director are both parasitic types.

Characteristics:

• Usually, Yagi-Uda antenna arrays have low input impedance.
• It has a high gain.
• It has a relatively low bandwidth.
• The length of the director is less than λ/2
• The length of the reflector is greater than λ/2.
• Its radiation pattern is almost uni-directional.

Ques.2. A Cassegrain reflector antenna has a sub-reflector whose geometrical shape is:

1. Paraboloid
2. Hyperboloid
3. Ellipsoid
4. Spherical

Explanation:-

A Cassegrain reflector antenna has a sub-reflector whose geometrical shape is Hyperboloid.

Cassegrain reflector antenna

The Cassegrain feed, which is shown in Figure, has a dual reflector.

• The main reflector is a parabola
• The secondary reflector is a hyperbola, and the feed is placed along the axis of the parabola at or near the vertex.

The rays that emanate from the feed illuminate the secondary reflector, which is located at the focal point of the paraboloid.

The rays are then reflected by the primary reflector and are converted to parallel rays.

The Cassegrain feed arrangement is much easier for servicing and adjustment since both the transmitting and receiving equipment can be located behind the primary reflector.

Ques.3. To make antenna more directional, either its size must be increased or

1. The number of its feed horns must be increased
2. The frequency of its transmission must be increased
3. Its effective isotropic radiated power (eirp) must be increased
4. Its footprint must be increased

Answer.2. The frequency of its transmission must be increased

Explanation:-

An antenna gives the wireless system three fundamental properties, i.e. Gain, Direction, and Polarization.

• The direction is the shape of the transmission pattern.
• As the gain of a directional antenna increases, the angle of the radiation usually decreases.
• A directional antenna is an antenna that radiates or receives greater power in a specific direction allowing increased performance.
• To make the antenna more directional, either its size must be increased or the frequency of its transmission must be increased.

Ques.4. Two isotropic antennas are separated by a distance of two wavelengths. If both the antennas are fed with currents of equal phase and magnitude, the number of lobes in the radiation pattern in the horizontal plane are

1. 2
2. 4
3. 6
4. 8

Explanation:-

Array factor is defined as $= 2\cos \left( {\frac{\psi }{2}} \right)$

Where ψ = α + βd cos θ

α = phase difference between two point sources.

d = distance between two point sources

θ = horizontal angle

Calculation:

Since Antenna are in the same phase α = 0

d = 2λ

$\begin{array}{l} \psi = \alpha + \beta d\cos \theta = 0 + \frac{{2\pi }}{\lambda } \times 2\lambda \cos \theta \\ \\ \:\:\therefore \beta = \frac{{2\pi }}{\lambda } \end{array}$

Array factor A.F. = 2 cos (2π cos θ)

No. of lobes in the Radiation pattern in the horizontal plane

= no. of times Array factor has maximum values when θ is varying from 0° to 360°

|A.F| = |2 cos (2π cos θ|

 θ 0° 60° 90° 120° 180° 240° 270° 300° A.F 2 2 2 2 2 2 2 2

Since |AF| has 8 times its maximum value.

So no. of lobes will be 8 in the radiation pattern.

Ques.5. An antenna has 40 Ω antenna resistance and 60 Ω radiation resistance. The efficiency of the antenna will be

1. 30%
2. 40%
3. 50%
4. 60%

Explanation:-

Given:

Radiation resistance (Rr) = 60 Ω

Antenna resistance (Ra) = 40 Ω

The antenna efficiency is given as;

η = Rr/(Rr + Ra)

Thus,

η% = 60/(60 + 40) × 100%

η% = 60%

Ques.6. The half-power beam width (HPBW) of an antenna in the two orthogonal planes are 120º and 40º respectively. The directivity of the antenna is approximately equals to

1. 10 dB
2. 6.5 dB
3. 12 dB
4. 8.5 dB

Explanation:-

Given: θE = 20° and θH = 20°

Since half power beam widths are small.

Therefore, the approx. directivity will be:

D = 4000/(θE × θH)

= 4000/(120 × 40) = 8.33

in dB,

D  = 10 log(8.33) ≈ 8.5 dB

Ques.7. What is the length of the antenna needed for a signal of 500 KHz frequency?

1. 6 km
2. 150 m
3. 600 m
4. 1 km

Explanation:-

Given that,

Electromagnetic waves of frequency 500 kHz are transmitted, i.e.,

f =  500 kHz

Hence the minimum length of the antenna will be

λ/4 = c4f

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

Length of antenna = (3 × 108)/(4 × 5 × 105) = 150 m

Ques.8. A short current element has a length l = 0.03 λ, where λ is the wavelength. The radiation resistance for uniform current distribution is

1. 0.072π2 Ω
2. 80π2 Ω
3. 72 Ω
4. 80 Ω

Explanation:-

Given dl = 0.03 λ

${R_{rad}} = 80{\pi ^2}{\left[ {\frac{{dl}}{\lambda }} \right]^2}{\rm{\Omega }}$

Where

λ = c/f

f = frequency

dl = length of Antenna

${R_{rad}} = 80{\pi ^2}{\left[ {\frac{{0.03\lambda }}{\lambda }} \right]^2}$

Ques.9. What should be the length of the transmitting Antenna for radiating radio waves of 900 MHz?

1. 16.7 cm
2. 1.67 cm
3. 8.3 cm
4. None of the above

Explanation:-

Given that,

Radio waves of frequency 900 MHz are transmitted, i.e.,

f = 900 × 106 Hz

Hence the minimum length of the antenna is given as:

L = λ/4 = c/4f

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

L = (3 × 108)/(4 × 900 × 106) = 0.83 m

Ques.10. Assertion (A): A Cassegrain antenna uses a main paraboloidal reflector and a relatively small hyperboloidal sub-reflector with a small horn-feed at the vertex of the main paraboloidal reflector.

Reason (R): The optical technique developed by William Cassegrain was used in telescope design to obtain large magnification with a physically short telescope. This configuration is found to be effective in the design of microwave antennas also.

1. Both A and R are individually true and R is the correct explanation of A
2. Both A and R are individually true but R is not the correct explanation of A
3. A is true but R is false
4. A is false but R is true

Answer.2. Both A and R are individually true but R is not the correct explanation of A

Explanation:-

Casse grain is a type of feed given to the reflector antenna. This technique was developed by William Cassegrain. In this type, the feed is located at the vertex of the paraboloid. A convex-shaped reflector, which acts as a hyperboloid is placed opposite to the feed of the antenna. It is also known as a secondary hyperboloid reflector or sub-reflector. It is placed such that one of the foci coincides with the focus of the paraboloid. Thus, the wave gets reflected twice.

Working of a Cassegrain Antenna

When the antenna acts as a transmitting antenna, the energy from the feed radiates through a horn antenna onto the hyperboloid concave reflector, which again reflects back onto the parabolic reflector. The signal gets reflected into the space from there. Hence, wastage of power is controlled and the directivity gets improved.

When the same antenna is used for reception, the electromagnetic waves strike the reflector, get reflected on to the concave hyperboloid and from there, it reaches the feed. A waveguide horn antenna presents there to receive this signal and sends it to the receiver circuitry for amplification

So both statements A and R are true but R is not the correct explanation of A.

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