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 is a non-resonant antenna ?
Rhombic
The rhombic is the standard example of a non-resonant, travelling-wave antenna — option 2.
The distinction between the two families.
| Resonant (standing wave) | Non-resonant (travelling wave) | |
|---|---|---|
| Termination | Open at the far end | Terminated in its characteristic impedance |
| Current distribution | Standing wave — forward and reflected | Travelling wave — forward only |
| Length | Tied to \(\lambda/2\) or \(\lambda/4\) | Several wavelengths, not critical |
| Bandwidth | Narrow | Wide |
| Pattern | Bidirectional | Unidirectional |
How the rhombic works. Four long wires form a diamond, fed at one apex and terminated at the other in a resistance equal to the line's characteristic impedance — typically 600 to 800 Ω. That termination absorbs whatever reaches it, so no reflected wave returns and no standing wave forms. The current is a pure travelling wave, and the radiation from the four legs adds in one direction only, giving a strongly unidirectional beam with gains of 10 to 15 dB.
What it costs and what it buys. Roughly a third of the input power is dissipated in the terminating resistor and never radiated — a real efficiency penalty. In exchange, the antenna works over a wide frequency range without retuning, which is exactly what long-distance HF communication needs, since the usable frequency changes with the time of day and the ionosphere. Rhombics were the backbone of intercontinental HF point-to-point links.
Why the other three are resonant. The Marconi is a quarter-wave grounded antenna — the passage says so, and a quarter wave is a resonant length. The Yagi-Uda is built from half-wave elements, with a reflector slightly longer and a director slightly shorter, all resonant and therefore narrowband. The discone is the awkward case: it is genuinely broadband, but it achieves that through a gradually tapering structure rather than by termination, so it is not a travelling-wave antenna in the technical sense — the rhombic is the intended answer.
Hence, the non-resonant antenna is the rhombic.
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 of the following antennas is best exerted from a waveguide ?
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
