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.
Which of the following antennas is best exerted from a waveguide ?
Horn
The passage answers this directly: "A horn is an ideal antenna for terminating a waveguide" — option 3.
Why a horn and not something else. A waveguide carrying a wave to its open end faces an abrupt discontinuity: the guide's wave impedance differs sharply from free space's 377 Ω, so most of the energy reflects back down the guide instead of radiating. A horn cures this by flaring the guide open gradually, so the impedance changes smoothly from that of the guide to that of free space over a distance of several wavelengths. The reflection is small and almost all the power leaves the aperture.
It is also the mechanically natural termination — a horn is simply the waveguide itself, opened out. No transition, no balun, no feed network: the guide becomes the antenna. That is why the passage calls it "ideal for terminating a waveguide", and why horns come in the shapes it lists — sectoral (flared in one plane), pyramidal (both planes) and conical (from circular guide).
| Antenna | Natural feed | Why not a waveguide |
|---|---|---|
| Horn | Waveguide | — it is the flared guide |
| Biconical | Balanced two-wire line | Needs a balanced feed at its apex |
| Folded dipole | 300 Ω twin lead | A wire antenna, fed at a gap |
| Discone | Coaxial cable | Disc to inner, cone to outer |
The gain follows from the aperture :
\(G=\dfrac{4\pi A_{e}}{\lambda^{2}}\)
so a larger flare gives more gain — but only up to a point. Flare too abruptly and the path from the throat to the aperture edge exceeds that to the centre, so the aperture field is no longer in phase and the gain falls again. The optimum flare balances aperture size against that phase error, and the resulting beamwidths are typically 10° to 30°.
Where horns are used : as standard-gain references for antenna measurement, since their gain can be computed accurately from their dimensions; as the feed for a parabolic dish, illuminating the reflector from its focus; and in radiometry and satellite ground stations. The first of these is worth noting — the horn is one of the few antennas whose absolute gain is predictable from geometry alone.
Hence, the antenna best excited from a waveguide is the horn.
Similar Questions
Helical antenna is used for satellite tracking because of
(a) the circular polarisation (b) the good front to back ratio (c) the Tropo scatter (d) the Faraday effect
Which of the above are correct ?
Which of the following antenna is not a wideband antenna ?
An antenna that is circularly polarized is :
A helical antenna is used for satellite tracking because of its
The discone antenna is
Which is a non-resonant antenna ?
Match the following :
List – I List – II a. Loop antenna i. sharp broadside null b. Rhombic antenna ii. long wire antenna c. Folded dipole iii. 300 ohms d. Horizontal antenna iv. horizontal polarization Codes :
Important Questions from Important Antenna Types - Teaching
Helical antenna is used for satellite tracking because of
(a) the circular polarisation (b) the good front to back ratio (c) the Tropo scatter (d) the Faraday effect
Which of the above are correct ?
Which of the following antenna is not a wideband antenna ?
An antenna that is circularly polarized is :
A helical antenna is used for satellite tracking because of its
The discone antenna is
