Antennas are used for receiving and transmitting the electromagnetic signals. Their size depends upon the operating frequency / wavelength. Higher is the frequency, lower is the size of antenna. They work on Maxwell equations for field theory. They are of various types for different applications like TV transmission, AM transmission, FM transmission and satellite transmission. The waves travel in free space.
For a half wave dipole the directivity ‘D’ in dB is of the order of :
2.15 dB
Know the two numbers and which is which. For a half-wave dipole the directivity as a ratio is
\(D=1.64\)
and the question asks for it in decibels:
\(D_{dB}=10\log_{10}(1.64)=10\times0.2148\)
\(D_{dB}=2.15\ \text{dB}\)
which is option 3.
The trap is option 4. 1.64 is the correct directivity, but as a plain ratio, not in decibels. The question is testing whether the two forms are kept apart — and 1.64 dB would actually correspond to a ratio of about 1.46, not 1.64. Whenever both a number and its decibel value appear among the choices, check which the question asked for.
Option 1 is the other classic distractor. 1.76 dB is the directivity of the Hertzian (infinitesimal) dipole, whose ratio is 1.5:
\(10\log_{10}(1.5)=1.76\ \text{dB}\)
The half-wave dipole is slightly more directive than the elementary one because its current distribution is sinusoidal rather than uniform, which narrows the beam a little — from a half-power beamwidth of 90° down to 78°.
Where 1.64 comes from. The far field of a half-wave dipole is
\(E_\theta \propto \dfrac{\cos\left(\dfrac{\pi}{2}\cos\theta\right)}{\sin\theta}\)
Integrating \(|E_\theta|^{2}\) over the sphere gives a radiation resistance of 73 Ω and, on forming \(D=4\pi U_{max}/P_{rad}\), the value 1.64.
Why 2.15 dB is worth memorising above all. It is the conversion between the two gain references:
\(G_{dBi}=G_{dBd}+2.15\)
Manufacturers quote antenna gain sometimes against isotropic (dBi) and sometimes against a half-wave dipole (dBd), and 2.15 dB is the constant that reconciles them.
Summary of the standard values. Half-wave dipole: D = 1.64 = 2.15 dBi, Rrad = 73 Ω, length λ/2. Hertzian dipole: D = 1.5 = 1.76 dBi.
Hence, the directivity of a half-wave dipole is 2.15 dB.
Similar Questions
The angular distribution of the transmitted power around the antenna is generally known as :
An antenna is polarized in the plane of the field perpendicular to
The value of radiation resistance of a Hertzian dipole of length \(\dfrac{\lambda}{80}\) is given by
An antenna is generally a metallic object, often a wire or collection of wires, used to convert high frequency current into electromagnetic waves and vice-versa. Antenna serve either or both of the following two functions, generation or the collection of electromagnetic energy. In a transmitting system, a radio-frequency signal is developed, amplified, modulated and applied to the antenna. The R-F currents flowing through the antenna produce electromagnetic waves which radiate into the atmosphere. In a receiving system, electromagnetic waves "cutting" through the antenna induce alternating currents for use by the receiver.
Efficient operation also requires that the receiving antenna be of the same polarization as the transmitting antenna. Polarization is the direction of the electric field and is therefore, the same as the antennas physical configuration. Thus vertical antenna will transmit vertical polarized light, any antenna having a physical length that is one-half wavelength of the applied frequency is called a Hertz antenna. Hertz antenna are predominantly used with frequencies above 2 MHz. Usually, at frequencies below 2 MHz, a Marconi type of antenna is used. The Marconi antenna is usually a quarter-wave grounded antenna or any odd multiple of a quarter wavelength.
The Yagi-Uda antenna consists of a drivers element and two or more parasitic elements. Yagi-Uda has two parasitic elements, a reflector and a director. This Yagi-Uda antenna provides about 7 dB of power gain with respect to a half-wavelength dipole reference. More complex antennas may be circularly polarized both vertically and horizontally polarized waves are radiated with equal power in both. If the powers are unequal the antenna is said to be elliptically polarized.
A horn is an ideal antenna for terminating a waveguide and may be conical, rectangular or sectorial. Wide band antennas either when the transmissions are wide band or when working of narrow channels over a wide frequency range is the major application. Helical antenna, which consists of a loosely wound helix backed up by a metal ground plane. Loop antennas are often used for direction finding. Loops have many shapes and generally consists of a single turn of wire. Discone is a ground plane antenna evolved from the vertical dipole and having a very similar radiation pattern.
Important Questions from Basics of Antenna
Arrange the following antennas in ascending order of their radiation resistance.
A. Short dipole (L = \(\frac{\lambda}{10}\) )(I av = l o)
B. Short dipole (L= \(\frac{\lambda}{10}\) )( I av = l o/2)
C. Linear \(\frac{\lambda}{2}\) dipole (sinusoidal current distribution)
D. Small Loop (square loop) single turn of(L = \(\frac{\lambda}{10}\) )
Choose the correct answer from the options given below
If the frequency of the signal is 1 MHz, the minimum height of the transmitting antenna should be:
The Antenna employed in Television receivers
Which of the following is NOT true with respect to antennas?
An instrument to check whether RF power in the transmission line is transferred to the antenna is:
