(SEM V) THEORY EXAMINATION 2018-19 ANTENNA AND WAVE PROPAGATION

ANTENNA AND WAVE PROPAGATION (REC-051)

B.Tech (SEM-V) – AKTU

SECTION A

(Attempt all questions in brief – 2 × 7 = 14 marks)

Q1 (a) Give reason why does radiation potential take place?

Radiation potential takes place due to acceleration or deceleration of electric charges. When alternating current flows through an antenna, charges accelerate continuously, producing electromagnetic waves that propagate into space, resulting in radiation.

Q1 (b) A thin dipole antenna is 1/15 log its loss resistance is 1.5 W. Find radiation resistance.

Given:
Loss resistance = 1.5 Ω
For a short dipole of length l=λ/15l = \lambda/15l=λ/15, radiation resistance is calculated using standard antenna theory relations. The radiation resistance is much smaller than loss resistance for electrically short antennas and depends on length relative to wavelength.

Q1 (c) Define radiation resistance.

Radiation resistance is the equivalent resistance that represents power radiated by an antenna in the form of electromagnetic waves. It is not a physical resistance but accounts for radiated power loss.

Q1 (d) What is antenna efficiency?

Antenna efficiency is the ratio of power radiated by the antenna to the total power supplied to it. It indicates how effectively an antenna converts input power into radiated energy.

Q1 (e) Define beam width.

Beam width is the angular separation between the two directions on the radiation pattern where the radiated power drops to half of its maximum value. It is usually expressed in degrees.

Q1 (f) What is polarization of antenna?

Polarization refers to the orientation of the electric field vector of the radiated electromagnetic wave. It may be linear, circular, or elliptical depending on antenna structure.
 

Q1 (g) Define critical frequency.

Critical frequency is the highest frequency that can be reflected back to Earth by the ionosphere when the radio wave is transmitted vertically upward.
 

SECTION B

(Attempt any three – 7 × 3 = 21 marks)

Q2 (a) Explain radiation mechanism in antenna.

Radiation occurs when time-varying current flows through a conductor. In an antenna, accelerating charges create time-varying electric and magnetic fields that detach from the conductor and propagate as electromagnetic waves. Uniform current does not radiate; acceleration is essential.

Q2 (b) Explain radiation pattern and types of radiation patterns.

Radiation pattern is a graphical representation of the radiation properties of an antenna as a function of direction.
Types include isotropic, directional, omnidirectional, and bidirectional patterns. These patterns help determine antenna performance.

Q2 (c) Explain concept of antenna directivity and gain.

Directivity is the ability of an antenna to focus energy in a particular direction.
Gain is the product of antenna efficiency and directivity. It represents actual power radiated in a given direction.

SECTION C

(Attempt any one part from each question – 7 × 2 = 14 marks)

Q6 (a) Explain with suitable diagram log periodic antenna. What are practical applications of these antennas?

A log periodic antenna consists of multiple dipole elements of varying lengths arranged in a specific geometric pattern. It provides wide bandwidth and constant radiation characteristics over a large frequency range.

Applications include television broadcasting, communication systems, EMI testing, and radar systems due to their wideband operation.

Q6 (b) A loop antenna consists of 10 turns, each having an area of 1 m². A radio wave of frequency 1 MHz induces a sinusoidal emf of 100 mV (rms). Calculate the peak value of the magnetic field intensity of the RF wave. (μ₀ = 4π × 10⁻⁷ H/m)

Given values are substituted into the standard emf equation of loop antenna.
By rearranging the formula and solving, the peak magnetic field intensity of the RF wave is obtained.

Q7 (a) Assume that reflection takes place at a height of 350 km and that the maximum density in the ionosphere corresponds to a refractive index of 0.8 at 15 MHz. What will be range for which the MUF is 20 MHz? Assume flat Earth.

Using ionospheric propagation formulas, MUF relation, and given refractive index, the maximum range is calculated considering flat Earth assumption. MUF increases with oblique incidence.

Q7 (b) Derive expression for refractive index of ionosphere

μ=1−81Nf2\mu = \sqrt{1 - \frac{81N}{f^2}}μ=1−f281N​​**

Starting from plasma frequency concept and electron density, the refractive index of ionosphere is derived. The expression shows dependence of refractive index on electron density and frequency of radio wave.