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.
An antenna is polarized in the plane of the field perpendicular to
Electric field
Electric field — option (A).
The convention that settles it. The polarisation of an antenna is defined, universally and by IEEE standard, in terms of the electric field of the wave it radiates — specifically the orientation of the E vector as seen looking along the direction of propagation. The magnetic field is never used for the definition, even though it carries exactly the same information, being perpendicular both to E and to the direction of travel.
| Polarisation | Orientation of E | Antenna that produces it |
|---|---|---|
| Vertical | Perpendicular to the earth’s surface | A vertical dipole or monopole — broadcast and mobile masts |
| Horizontal | Parallel to the earth’s surface | A horizontal dipole — television Yagi arrays |
| Circular | Rotates once per cycle, constant magnitude | Helical antenna, or crossed dipoles fed \(90^{\circ}\) apart |
| Elliptical | Rotates, magnitude varying | The general case |
Why the convention is on E and not on H. A wire antenna responds to the electric field along its own length: the field drives charges up and down the conductor, so a vertical wire couples to a vertical E field. Since the antenna’s physical orientation and the E direction line up directly, defining polarisation on E makes the geometry immediate — and the two fields are locked at right angles anyway, so nothing is lost by the choice.
Why it matters in practice — polarisation mismatch loss. A receiving antenna picks up only the component of the incident field aligned with it :
\(\text{PLF}=\cos^{2}\psi\)
where \(\psi\) is the angle between the transmit and receive polarisations. At \(\psi=90^{\circ}\) the loss is theoretically infinite — a vertical antenna receives essentially nothing from a horizontally polarised transmitter, which is why the two are used on adjacent channels to reduce interference, and why a handset held at an angle loses signal.
Circular polarisation and its use. Satellite links use it because the relative orientation of a spacecraft and a ground station is not fixed, and because Faraday rotation in the ionosphere twists a linearly polarised wave unpredictably. A circularly polarised wave suffers a constant 3 dB loss into a linear antenna regardless of angle, which is far preferable to an unpredictable deep null.
Hence, the answer is Electric field.
Similar Questions
The angular distribution of the transmitted power around the antenna is generally known as :
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.
For a half wave dipole the directivity ‘D’ in dB is of the order of :
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:
