Test: Antenna - Electronics and Communication Engineering (ECE) MCQ

Concept

• A mobile satellite array antenna consists of 14 dual patch antennas
• These are located in a truncated cone elevated at an angle of 40^∘ to the ground plane
• Each element has a gain of approximately 8.5dBi
• This gain is increased by activating a subarray of M adjacent elements that face the satellite
• A radial divider is used which activates M adjacent elements in the array and deactivates the remaining (14-M) elements

Calculation

We have M =3 ,elements activated

⇒(14 - 3)=11 antenna elements are deactivated

Hence the correct option is A

A wideband antenna has approximately or precisely the same operating characteristics as a very wide Passband.

It is distinguished from broadband antennas, where the passband is significant, but the antenna gain and/or radiation pattern need not stay the same over the passband.

Marconi Antenna:

The Marconi antenna is used primarily with frequencies below 2 MHz.

The difference between the Marconi antenna and the Hertz antenna is that the Marconi type requires a conducting path to the ground, and the Hertz type does not. The Marconi antenna is usually a quarter-wave grounded antenna or any odd multiple of a quarter wavelength

And the radiation resistance is:

The radiation intensity which is given by:

The maximum directivity and maximum effective aperture are:

Yagi-Uda Antenna:

• It is a directional antenna and it consists of multiple parallel half-wave dipole elements.
• It is a directional antenna and it consists of multiple parallel half-wave dipole elements.
• The direction of maximum radiation is along the axis of the dipole array which is called an end-fire radiation pattern.
• The reflector and director are both parasitic types.

Characteristics:

• Usually, Yagi-Uda antenna arrays have low input impedance.
• It has a high gain.
• It has relatively low bandwidth.
• The length of the director is less than λ/2
• The length of the reflector is greater than λ/2.
• Its radiation pattern is almost uni-directional.

Concept:

• For transmitting a signal, we need an antenna or an aerial.
• This antenna should have a size comparable to the wavelength of the signal (at least 1/4 in dimension) so that the antenna properly senses the time variation of the signal.
• For example, if we have an electromagnetic wave of frequency (f) 20 kHz, the wavelength (λ) is 15 km

Calculation:

Given that,

Radio waves of frequency 900 MHz are transmitted, i.e.,

f = 900 × 10⁶ Hz

Hence the minimum length of the antenna is given as:

Here c is the velocity of light, λ is wavelength and f are frequency of the wave

∴ 

L = 8.3 cm

Concept:

The antenna efficiency of an antenna is given as:

R_r : Radiation resistance

R_a : Effective Antenna resistance

Calculation:

Given:

Radiation resistance (Rr) = 60 Ω 

Antenna resistance (Ra) = 30 Ω 

Concept:

• For transmitting a signal, we need an antenna or an aerial.
• This antenna should have a size comparable to the wavelength of the signal (at least 1/4 in dimension) so that the antenna properly senses the time variation of the signal.
• For example, if we have an electromagnetic wave of frequency (f) 20 kHz, the wavelength (λ) is 15 km

Calculation:

Given that,

Electromagnetic waves of frequency 500 kHz is transmitted, i.e.,

f =  500 kHz

Hence the minimum length of antenna = 

Here c is the velocity of light, λ is wavelength and f are the frequency of the wave

∴ length of antenna = 
= 150 m

The radiation pattern of a Hertzian dipole is shown below:

Let us consider, l(OB) = x

∴ 

Point A is  times electric field at point B

It is a half-power bandwidth point.

∠ AOD = Half power bandwidth is

HPBW=90°

Non-resonant Antennas:

• Non-resonant Antennas are those for which resonant frequency does not occur.
• The wave moves in the forward direction and hence does not form a standing wave.
• The radiation pattern of the radiated wave matches the load impedance in the non-resonant antennas.
• These non-resonant antennas are non-periodic in nature.
• Example: Yagi –Uda Antenna, Rhombic Antenna, Ground Antenna. 

Resonant Antennas:

• When the voltage and current distribution form standing wave patterns, then such types of antennas are called standing wave antennas or resonant antennas.
• Resonant antennas exhibit a periodic nature.
• They have traveling waves propagating in forward and backward directions and are called bi-directional traveling-wave antennas.
• In resonant antennas, radiated waves traveling in opposite directions establish incident and reflected waves.
• Example: Loop Antenna, Half Wave Dipole Antenna

1. A cylindrical coil of many tightly wound turns of insulated wire with a general diameter of the coil smaller than its length is called a solenoid.

2. A magnetic field is produced around and within the solenoid. The magnetic field within the solenoid is uniform and parallel to the axis of the solenoid.

From the above explanation, we can see that,

A solenoid is basically a tightly wound helical coil of wire which is made up of a smaller diameter compared to its length.

When a magnetic field is generated inside a solenoid it is very much proportional to both the applied current and the number of turns per unit length hence the magnetic field does not dependent on the diameter of the solenoid and the field inside remains constant.