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.
The discone antenna is
useful as a UHF receiving antenna
The discone is the classic wideband omnidirectional receiving antenna for VHF and UHF — option 4.
What it is. The name contracts disc and cone: a flat horizontal disc sits above the apex of a downward-flaring cone, the feeder's inner conductor going to the disc and its outer to the cone. The passage describes it as "a ground plane antenna evolved from the vertical dipole and having a very similar radiation pattern" — so it is vertically polarised and omnidirectional in the horizontal plane, exactly like a vertical dipole.
Why it is prized as a receiving antenna. Its defining property is bandwidth. Because the cone presents a gradually changing impedance rather than a resonant length, the discone maintains an acceptable VSWR over a range of roughly 10:1 in frequency, against a few per cent for a resonant dipole. One antenna therefore covers an entire scanning range with no tuning — which is why it is the standard antenna on a wideband scanner receiver, and why it appears at airfields and monitoring stations covering the whole VHF/UHF spectrum.
| Property | Discone |
|---|---|
| Bandwidth | ~10:1 |
| Polarisation | Vertical (linear) |
| Horizontal pattern | Omnidirectional |
| Gain | Low, about that of a dipole |
Why the other options fail.
Option 1 — direction finding needs a sharp null so that a bearing can be taken, which is why the passage assigns that role to the loop antenna. A discone is omnidirectional and has no null in azimuth at all, making it the worst possible DF antenna.
Option 2 — radar requires high gain and a narrow beam to resolve targets in angle, which means a parabolic reflector or an array. The discone's low gain and broad pattern are the opposite of what radar needs.
Option 3 — the discone is linearly polarised, vertically so. The word "circular" in the option refers to its physical shape, not to its polarisation; the passage's circularly polarised example is the helix.
The general trade-off it illustrates is that bandwidth and gain pull against each other: a structure whose dimensions vary continuously, like the cone, is inherently broadband but cannot concentrate energy, while a resonant, high-gain array works over a narrow band.
Hence, the discone is useful as a UHF receiving antenna.
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
Which of the following antennas is best exerted from a waveguide ?
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
Which of the following antennas is best exerted from a waveguide ?
