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This set of Antennas Question Paper focuses on “Radiation from Rectangular Aperture”.

1. Which of the following antenna belongs to rectangular aperture?
a) Horn antenna
b) Helical antenna
c) Parabolic antenna
d) Conical antenna

Explanation: The aperture of antenna at the end determines its shape. Horn antenna has a rectangular aperture. Helical, Parabolic, conical antenna have circular apertures. Helical antenna belongs to frequency independent antenna.

2. The radiation pattern of rectangular is similar to line source integrated in two directions
a) True
b) False

Explanation: The radiation pattern for the rectangular distributions is similar to the line source distributions. In this the patterns is calculated by integration in two directions as rectangle have two different lengths in different direction.

3. The total pattern function for rectangular aperture f(x, y) if f(x) and f(y) are separable is given by ____
a) f(x, y)=f(x) f(y)
b) f(x, y)=f(x)+f(y)
c) f(x, y)=f(x)/f(y)
d) f(x, y)=f(x)-f(y)

Explanation: The radiation pattern for the rectangular aperture is likely relatable to the line source distributions. If the functions f(x) and f(y) are separable, then total pattern will be the product of the two functions. f(x, y)=f(x)f(y).

4. The first-level of the side lobe occurs at ______ dB for a uniform rectangular aperture antenna.
a) -13.26
b) -6.63
c) 3
d) 8.5

Explanation: The aperture of antenna at the end determines its shape. If the field is uniform in amplitude and phase along the rectangular aperture then it is called a uniform rectangular aperture antenna. The first side-lobe occurs at -13.26dB.

5. A rectangular aperture a ×b is placed in xy-plane, The HPBW in H-plane is given by _____
a) 0.886λ/a
b) 0.443λ/a
c) 0.5λ/b
d) λ/b

Explanation: By equating the field in H-plane to half power point
$$\frac{sin⁡(0.5kasin\theta)}{0.5kasin\theta} = \frac{1}{\sqrt{2}} => \theta =arcsin⁡(\frac{0.443\lambda}{a})$$
Now HPBW = $$2 arcsin⁡(\frac{0.443\lambda}{a}) \approx 0.886\lambda/a.$$

6. The relation between directivity and the effective aperture of the uniform aperture antenna is given by _____
a) $$D = \frac{4\pi}{\lambda^2}A_{eff}$$
b) $$A_{eff} = \frac{4\pi}{\lambda^2}D$$
c) $$A_{eff} = \frac{4\pi\lambda^2}{D}$$
d) $$D = \frac{4\pi\lambda^2}{A_{eff}}$$

Explanation:For a uniform aperture antenna, the physical and effective apertures are equal.
The relation between directivity and the effective aperture of the antenna is given by
$$D = \frac{4\pi}{\lambda^2}A_{eff}$$.

7. Which of the following is used to reduce side lobe levels in aperture antenna?
a) Tapering
b) Increasing the power
c) Using repeaters
d) Reducing power

Explanation: The uniform aperture produces the high SLL under a constant phase amplitude excitation. To reduce this SLL effect, tapering is done. Tapering is done maximum at center and reduces to zero at the edges for anequivalent source distribution.

8. The principle plane pattern of the E-plane of rectangular aperture of a×b is given by F(θ) = ______
a) $$\frac{sin⁡(0.5kbsin\theta)}{0.5kbsin\theta}$$
b) $$\frac{cos⁡(0.5kbcos\theta)}{0.5kbsin\theta}$$
c) $$\frac{sec⁡(0.5kbsin\theta)}{0.5kbsin\theta}$$
d) $$\frac{1-sin⁡(0.5kbsin\theta)}{0.5kbsin\theta}$$

Explanation: The principle plane patterns for the uniform rectangular aperture antenna is given by
F(θ) = $$\frac{sin⁡(0.5kbsin\theta)}{0.5kbsin\theta}$$ and main lobe occurs when θ =0.

9. Tapering is done in order to reduce the side lobe level.
a) True
b) False

Explanation: The uniform aperture produces the high SLL under a constant phase amplitude excitation. To reduce this SLL effect, tapering is done. Tapering is done maximum at center and reduces to zero at the edges for an equivalent source distribution.

10. If the aperture antenna is tapered only in H-plane then which of the following is true compared to uniform non-tapered aperture antenna?
a) Principle patterns in E-plane and H-plane are same in both cases
b) Principle patterns in E-plane and H-plane are different in both cases
c) Principle patterns in E-plane is same and H-plane is different
d) Principle patterns in E-plane is different and H-plane is same 