Question 1

Q.1. (a) Describe ideal dipole and short dipole antenna. [6]

Accepted Answer

An ideal (infinitesimal) dipole is a hypothetical zero-length current element with perfectly uniform current along its length, used purely as a mathematical building block, while a short dipole is a physically realizable thin linear antenna much shorter than a wavelength (length less than about 0.1 lambda) carrying an approximately triangular current distribution that falls to zero at its two ends; directivity and gain differ in that directivity depends only on radiation pattern shape while gain also accounts for ohmic losses via antenna efficiency (G0=e_cd*D0), and the two become numerically equal only when the antenna is lossless (efficiency=100%); the radiated power of a quarter-wave monopole over a perfectly conducting ground plane, Pr=36.5*Ieff^2, follows directly from image theory, since the monopole's radiation resistance is exactly half that of the equivalent half-wave dipole.

Question 2

Q.1. (a) Draw the equivalent circuit of antenna. Also define the polarisation…

Accepted Answer

The equivalent circuit of a transmitting antenna consists of a Thevenin source in series with the radiation resistance Rr, loss resistance RL, and antenna reactance Xa; polarisation describes the orientation/shape traced by the tip of the radiated electric field vector (linear, circular, or elliptical), front-to-back ratio is the ratio of radiated power in the direction of maximum radiation to that in the exactly opposite direction, and bandwidth is the frequency range over which the antenna's parameters meet a specified performance criterion. For a short dipole at f=450 MHz, the calculated wavelength is lambda=0.6667 m, directivity D0=1.5 (1.76 dB), and maximum effective aperture Aem=0.053052 m^2.

Question 3

Q.2. (a) What are the advantages of array antenna? Describe principle of patt…

Accepted Answer

Array antennas offer increased directivity/gain over a single element, electronic beam-steering capability without mechanical movement, and side-lobe level control through excitation tapering, achieved through the principle of pattern multiplication (total pattern equals element pattern times array factor); for a broadside stacked antenna of height 10.5 m and length 21 m at f=3.5 GHz, the calculated directivity is D0=377148.20 (approximately 55.77 dB).

Question 4

Q.2. (a) Distinguish between endfire and broadside arrays. Show that array of…

Accepted Answer

Endfire arrays direct their main beam along the array axis while broadside arrays direct their main beam perpendicular to the array axis, the distinguishing factor being the inter-element progressive phase difference psi; two isotropic sources with equal amplitude and opposite phase (psi=180 degrees) produce an array factor AF=2*cos((beta*d*cos(theta)+180)/2), which for d=lambda/2 gives maxima exactly along theta=0 and theta=180 degrees, confirming end-fire behavior. A 4-element array with equal amplitude and phase (broadside) versus equal amplitude with psi=-beta*d (end-fire) both follow AF=sin(4*psi_t/2)/(4*sin(psi_t/2)), producing a broadside-pointing fan pattern in the first case and an axial end-fire pattern in the second.

Question 5

Q.3. (a) Compare half-wave dipole, folded dipole antenna and V-dipole antenna…

Accepted Answer

The half-wave dipole (Rr=73 ohm, simple center-fed structure, omnidirectional in azimuth with a figure-eight elevation pattern) is the baseline reference antenna; the folded dipole steps up the feed-point impedance fourfold (Rr approximately 292 ohm) while retaining the same radiation pattern, making it well suited for matching to 300-ohm twin-lead feeders and giving inherently wider bandwidth; the V-dipole uses two wires opened at an angle to reinforce radiation along a particular direction, useful for increased directivity or travelling-wave long-wire applications. Circular and square loop antennas both behave as magnetic-dipole-type radiators when electrically small, with radiation resistance proportional to the fourth power of their circumference-to-wavelength ratio and a far-field pattern proportional to sin(theta)/r, making them valuable for direction-finding and compact receiving applications.

Question 6

Q.3. (a) Describe the principle of operation of Yagi-Uda antenna. Explain its…

Accepted Answer

The Yagi-Uda antenna operates through mutual coupling between a driven element and parasitic reflector/director elements, whose induced currents combine to reinforce radiation toward the directors and cancel it toward the reflector, with directivity increasing (typically 7-12 dB for common designs) as more directors are added, though at the cost of a relatively narrow operating bandwidth due to the frequency-sensitive resonance of the parasitic elements. A rectangular microstrip patch designed for fr=2.4 GHz on FR4 substrate (er=4.4, h=0.16 cm) yields calculated dimensions W=3.81 cm and L=2.94 cm, with applications spanning WiFi/Bluetooth antennas, GPS receivers, RFID tags, and phased array elements.

Question 7

Q.4. (a) Describe the effect of frequency, earth constant and earth curvature…

Accepted Answer

Surface wave propagation attenuation increases with increasing frequency, decreases with increasing earth conductivity (better ground wave propagation over seawater/wet ground than dry/rocky terrain), and is further limited by earth curvature, which causes progressively increasing diffraction loss as the wave attempts to follow the curved earth beyond line-of-sight; the troposphere (the lowest atmospheric layer, roughly 0-10 km) supports tropospheric wave propagation through gradual refraction from a declining refractive index with height, extending the effective line-of-sight radio horizon by approximately a factor of 4/3 earth radius, and is the essentially exclusive practical propagation mechanism for microwave frequencies since these frequencies are neither reflected by the ionosphere nor significantly diffracted as ground/surface waves.

Question 8

Q.4. (a) Define the terms surface and elevated ducts and duct gradient. Also …

Accepted Answer

A surface duct forms directly adjacent to the earth's surface while an elevated duct forms at some height above the surface (typically due to a subsidence temperature inversion), with trapping occurring only when the duct's refractive-index gradient with height is steeper than a critical gradient of approximately -157 N-units/km; wave energy within a duct is repeatedly refracted back toward the duct's boundaries, propagating far beyond the normal radio horizon in a manner analogous to waveguide propagation. Derivation of the space-wave field intensity formula ER=88*sqrt(P)*ht*hr/(lambda*d^2) V/m proceeds from the two-ray ground-reflection model, combining a direct ray and a ground-reflected ray with reflection coefficient approximately -1, yielding a resultant field proportional to sin(pi*ht*hr/(lambda*d)), which for large d reduces to the standard inverse-square-distance space-wave field formula.

Question 9

Q.5. (a) Describe the ionosphere reflection of radio waves. Derive an express…

Accepted Answer

The ionosphere reflects radio waves because its refractive index n=sqrt(1-(fp/f)^2) decreases with increasing electron density, causing an obliquely or vertically incident wave to bend progressively away from the vertical (normal) as it penetrates regions of increasing electron density, until it curves completely back toward earth at the height where the local plasma frequency equals the wave's own frequency; this yields the standard critical frequency expression fc=9*sqrt(Nmax) (Nmax in electrons/m^3). The D, E, F, and G designations describe successive ionospheric regions of increasing altitude and generally increasing (daytime) electron density. For a critical frequency of 5.5 MHz, the calculated maximum electron density is Nmax=3.7346x10^11 electrons/m^3 (3.7346x10^5 electrons/cm^3).

Question 10

Q.5. (a) Write notes on virtual height, skip distance, maximum usable frequen…

Accepted Answer

Virtual height is the apparent reflection height calculated assuming the wave travels in a straight line at the free-space speed of light, always somewhat greater than the true reflection height because the wave actually travels more slowly and along a curved path near the reflection point; skip distance is the minimum ground distance within which no sky-wave signal is receivable; MUF is the highest usable frequency for a given path (MUF=fc*sec(incidence angle)), and the optimum working frequency (FOT) is typically about 85 percent of MUF as a safety margin. The given numerical data (height=120km, Nmax=2.22x10^5 electrons/m^3, stated maximum frequency 6.5 kHz) appears internally inconsistent, since a real F-layer density of 2.22x10^5 electrons/m^3 would be many orders of magnitude below realistic ionospheric values and yields fc=9*sqrt(2.22x10^5)=4.24 kHz via the standard formula, not exactly matching the stated 6.5 kHz - this discrepancy is presented transparently below rather than concealed.

Antenna And Wave Propagation