Question 1

Q.1. (a) Define and explain the following terms of antenna: (i) Beam-width (H…

Accepted Answer

Beam-width (half-power and first-null), directivity, effective aperture, and antenna efficiency are the four fundamental parameters characterizing an antenna's radiation pattern shape, directional gain, power-capture capability, and internal loss performance respectively; an antenna serves as the essential transducer between guided-wave (transmission line/waveguide) energy and free-space electromagnetic radiation, with Maxwell's introduction of displacement current resolving Ampere's law's inconsistency for open (capacitor-like) circuits and enabling the self-sustaining propagation of EM waves that antennas launch into space.

Question 2

Q.1. (a) With the help of Maxwell's equation, explain how radiation and recep…

Accepted Answer

Radiation and reception of EM waves occur through Maxwell's coupled curl equations, in which a time-varying antenna current produces a changing magnetic field that induces a changing electric field (and vice versa via displacement current), allowing the disturbance to propagate outward as a self-sustaining wave detached from the source; an isotropic radiator (radiating uniformly in all directions) is a purely theoretical reference standard, since Maxwell's equations show that any real antenna current distribution necessarily produces a non-uniform radiation pattern, making a true isotropic radiator physically unrealizable. For the given radiation intensity U(theta,phi)=10*sin(theta)*sin(phi) W/sr (noting the stated function integrates to zero net power as literally written and is very likely OCR-corrupted from the standard textbook form U=B0*sin(theta)*sin^2(phi)), using this corrected, physically consistent form gives radiated power Prad=49.35 W and directivity D0=2.55 (4.06 dB).

Question 3

Q.2. What is phased array antenna? What is the main difference between BSA an…

Accepted Answer

A phased array antenna is a multi-element array whose individual element excitation phases are electronically controlled to steer the composite radiation beam direction without physical (mechanical) movement of the antenna structure; the main difference between a Broadside Array (BSA, maximum radiation perpendicular to the array axis) and an End-Fire Array (EFA, maximum radiation along the array axis) is the relative phase difference between adjacent elements, with the normalized field pattern for an 8-element array (spacing lambda/2) showing a narrow main beam broadside to the array for BSA and along the array axis for EFA.

Question 4

Q.2. Define the principle of pattern multiplication of an antenna array. A un…

Accepted Answer

The principle of pattern multiplication states that the total radiation pattern of an array of identical elements equals the product of the individual element's own radiation pattern and the array factor (the pattern of an equivalent array of isotropic point sources with the same geometry, spacing, and excitation); for a 12-element uniform linear array with lambda/4 spacing and phase difference psi=-90 degrees (the ordinary end-fire condition, since beta*d=90 degrees), the calculated directivity is D0=6 (approximately 7.78 dB), with corresponding effective aperture Aem=0.4775*lambda^2.

Question 5

Q.3. (a) Design a three-element Yagi-Uda antenna to operate at a frequency of…

Accepted Answer

A three-element Yagi-Uda antenna at 570 MHz (wavelength approximately 52.6 cm) is designed with a driven element (half-wave dipole, approximately 0.473 lambda long), a reflector slightly longer than the driven element (approximately 0.5 lambda) spaced approximately 0.15-0.25 lambda behind it, and a director slightly shorter than the driven element (approximately 0.45 lambda) spaced approximately 0.1-0.2 lambda in front of it; lens antennas (dielectric, E-plane/H-plane metal-plate, and zoned/stepped lenses) convert a diverging spherical wavefront from a feed source into a collimated plane wavefront by introducing a path-length-dependent phase delay across the lens aperture, with the dielectric lens's shaping governed by the fundamental lens equation relating path lengths through the lens material and free space.

Question 6

Q.3. (a) Calculate the design data of a rhombic antenna to operate at 50 MHz …

Accepted Answer

A rhombic antenna designed for 50 MHz operation (wavelength 6 m) with an elevation angle of 30 degrees is sized using the standard design relations linking tilt angle, leg length, and elevation angle, with typical leg lengths for this elevation angle falling in the range of several wavelengths (several tens of metres); the rectangular microstrip patch designed for 10 GHz center frequency, epsilon_r=10.2, and substrate height 0.127 cm has calculated physical dimensions of approximately W=0.634 cm and L=0.426 cm (after accounting for fringing-field length extension).

Question 7

Q.4. (a) Explain the formation of ionosphere. What are the various layers of …

Accepted Answer

The ionosphere is formed by solar ultraviolet and X-ray radiation ionizing the upper atmosphere's gas molecules, creating distinct D, E, F1, and F2 layers at progressively increasing altitude, each with different electron density and hence different effects on radio wave propagation (absorption in the D-layer, reflection of MF/lower-HF waves by the E-layer, and reflection of HF waves enabling long-distance skywave communication by the F-layers); multiple-hop transmission occurs when a wave successively reflects between the ionosphere and the earth's surface over several hops to reach a distant receiver beyond the range of a single hop, with the earth's magnetic field causing the ionosphere to become anisotropic (birefringent), splitting an incident wave into ordinary and extraordinary components that propagate with different characteristics.

Question 8

Q.4. (a) What do you understand by duct propagation? Under what conditions ar…

Accepted Answer

Duct propagation is an anomalous tropospheric propagation mode occurring when a temperature inversion or strong humidity gradient creates a thin atmospheric layer (duct) with a steeply decreasing refractive index with height, trapping VHF/UHF/microwave radio waves within the duct and guiding them far beyond the normal radio horizon, though limited to specific frequency bands and highly dependent on unpredictable, transient weather conditions; tropospheric scattering exploits weak forward-scattering of microwave signals from turbulent irregularities in the lower troposphere to achieve reliable beyond-horizon communication in the 300 MHz-10 GHz range, requiring high transmitter power and large, high-gain antennas at both ends of the link.

Question 9

Q.5. (a) Define the critical frequency and critical angle. How is critical fr…

Accepted Answer

Critical frequency is the highest frequency that a vertically-incident radio wave can have and still be reflected by a given ionospheric layer, directly related to the layer's maximum electron density (fc=9*sqrt(Nmax), with Nmax in electrons per cubic metre); critical angle is the largest angle of incidence (from vertical) at which the critical frequency wave is still reflected, and both quantities increase with increasing electron density; grounded antennas use the earth itself as part of the radiating/return-current system (via image theory), while ungrounded antennas are self-contained, complete radiating structures not relying on ground connection. For the given communication problem (60MHz, 2kW, directive antenna field 5x half-wave reference, ht=60m, hr=6m, required field 100 microV/m), the calculated maximum range is approximately 105.3 km.

Question 10

Q.5. (a) What is meant by fading? Also define skip distance and give the reas…

Accepted Answer

Fading is the random, time-varying fluctuation in received signal strength arising from multipath interference, ionospheric absorption variation, and polarization changes, while skip distance is the minimum ground distance from a transmitter within which sky-wave signals cannot be received (since waves transmitted within this radius escape into space rather than being reflected back to earth), varying due to changes in ionospheric height, electron density, and operating frequency, particularly with time of day and season. For the given numericals, the critical frequency for Nmax=1.3x10^6 electrons/cm^3 is calculated as approximately 10.26 MHz, and the Maximum Usable Frequency (MUF) for the 2500 km path with 200 km ionospheric height and 5 MHz critical frequency is calculated as approximately 31.65 MHz.

Antenna And Wave Propagation