1. A linear antenna having length less than λ/8 is called as _______
a) Short monopole
b) Short dipole
c) Half-wave dipole
d) Quarter-wave monopole
Explanation: Short monopoles have length less than λ/8 and the current distribution is triangular. Short dipole has length less than λ/2. Half-wave dipoles have length equal to λ/2. Quarter-wave monopoles have length equal to λ/4.
2. Find the power radiated by an antenna whose radiation resistance is 100Ω and operating with 3A of current at 2GHz frequency?
a) 900W
b) 1800W
c) 450W
d) 700W
Explanation: Power radiated Pr=I2 Rr=100×32=900Watts.
3. Front-to-Back ratio is defined as ratio of power radiated in desired direction to the power radiated in back lobe.
a) True
b) False
Explanation: Front-to-Back ratio (FBR) =\(\frac{Power\, radiated\, desired\, direction}{Power\, radiated\, in\, backward\, direction}.\) If more power is diverted backside, then the gain of the antenna decreases.
4. Relation between beam solid angle Ω, horizontal half-power beam width ∅A, vertical half-power beam width ∅E is __________
a) Ω≈∅A.∅E
b) Ω≈∅A+∅E
c) Ω≈∅A/∅E
d) Ω≈∅A-∅E
Explanation: Half-power beam width is the angular range of antenna pattern in which half power is radiated. The relation between Ω, ∅A, ∅E is given by Ω≈∅ A.∅E.
5. Which of the following field varies inversely with r2?
a) Far field
b) Near field
c) Radiation field
d) Electrostatic field
Explanation: Induction field is also known as ‘Near field’, varies inversely with r2. Electrostatic field varies inversely with r3. Far field is also known as Radiation field, varies inversely with r.
6. Find the effective area of a half-wave dipole operating at frequency 100MHz and directive gain 1.8?
a) 1.28m2
b) 2.18m2
c) 0.128m2
d) 12.8m2
Explanation: The effective area \(A_e=\frac{\lambda^2}{4π}D \)
\(\lambda = \frac{c}{f}=3×\frac{10^8}{100×10^6}=3m\)
\(A_e=\frac{3^2}{4π}×1.8=1.28m^2\)
7. Which of the following option is false?
a) Omni-directional antenna is a special case of directional antenna
b) Directional antenna radiates power effectively in particular directions compared to other directions
c) Isotropic antenna radiates power in all directions
d) End-fire array antenna has its main beam normal to the axis containing antenna
Explanation: End-fire array has its main beam parallel to the axis of antenna (θ=0° or 180°). For broadside antenna it is normal to the axis of antenna. Omni-directional antenna radiates power in only one direction and is non-radiating in other directions. So it is a special case of directional antenna.
8. The angular distance between two successive nulls of main lobe is called as ______
a) FNBW
b) HPBW
c) Beam width
d) FBR
Explanation: The angular distance between two successive nulls of main lobe is called First Null Beam width. Half-power beam width is the angular distance when 50% of power is radiated. FBR is the Front-to-Back ratio defined as ratio of power radiated at 0° to power radiated at 180°.
9. If beam width of the antenna increases, then directivity ________
a) Decreases
b) Increases
c) Remains unchanged
d) Depends on type of antenna
Explanation: As beam width of antenna increases its area coverage broadens, thereby directivity decreases. Beam area and directivity are inversely proportional. \(D=\frac{4\pi}{Beam \,Area}.\)
10. The receiving antenna is designed to have ____ side-lobe-ratio and ____ SNR.
a) Low, high
b) High, high
c) Low, low
d) High, low
Explanation: Side lobe ratio is ratio of power density in side lobes to main lobe. A receiving antenna is said to be efficient if side lobes are minimized and receives most of the transmitted signal. So it should have low SLR and high SNR.