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
Magnetic field
Polarisation is defined by the electric field, and in a plane wave the magnetic field is perpendicular to it — so the plane of polarisation is the plane perpendicular to H. That is option 2.
Start from the passage's own definition : "Polarization is the direction of the electric field and is therefore the same as the antenna's physical configuration." So a vertical wire carries a vertical current, produces a vertical E field, and is described as vertically polarised.
Now the geometry of a plane wave. The three vectors form a right-handed orthogonal set:
\(\overline{E}\perp\overline{H}\perp\overline{k}\) with \(\overline{E}\times\overline{H}=\overline{S}\)
the Poynting vector giving the direction of propagation. Since E and H are mutually perpendicular, the plane containing E — the plane of polarisation — is necessarily perpendicular to H.
| Antenna | E field | H field | Polarisation |
|---|---|---|---|
| Vertical dipole | Vertical | Horizontal | Vertical |
| Horizontal dipole | Horizontal | Vertical | Horizontal |
Why option 1 is wrong although it looks natural : the plane of polarisation contains the electric field, it is not perpendicular to it. Option 3 is impossible — a plane cannot be perpendicular to both E and H at once, since those two are already perpendicular to each other; such a plane would collapse to the direction of propagation. Option 4 describes a field parallel to both, which for orthogonal vectors does not exist.
The question's wording is awkward, which is why the answer is flagged: "polarized in the plane of the field perpendicular to X" has to be unpicked as "the plane of the E field, which is perpendicular to X", and only H makes that true. Read the other way — as asking which field the polarisation plane is defined by — it would point at the electric field, so a key could conceivably differ.
Why polarisation matching matters, as the passage stresses : the received power varies as \(\cos^{2}\psi\) with the angle between transmitting and receiving polarisations, so a 90° mismatch gives, in principle, zero output. That is why broadcast FM and television use one convention consistently across a service area, and why mobile systems, whose handsets are held at arbitrary angles, resort to diversity or circular polarisation.
Hence, the plane of polarisation is perpendicular to the magnetic field.
Similar Questions
For a half wave dipole the directivity ‘D’ in dB is of the order of :
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
Important Questions from Basics of Antenna
The antenna which provides circularly polarized waves is
A dipole carries RMS current of about 300 A across the radiation resistance 2 Ω. What would be the power radiated by an antenna?
A Yagi antenna is a directional antenna consisting of parasitic elements-
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:
