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.
A helical antenna is used for satellite tracking because of its
circular polarization
A helix operating in its axial mode radiates a circularly polarised wave, and that is exactly what satellite work needs — option 1.
Why the helix polarises circularly. The current travels along a conductor wound as a spiral, so the radiating element itself turns through 360° every turn. The field it launches along the axis therefore rotates at the signal frequency, giving circular polarisation whose sense — right- or left-hand — is fixed by the direction of the winding. The axial mode holds when the circumference is about one wavelength:
\(C\approx\lambda\qquad\text{with pitch angle}\ 12^{\circ}\ \text{to}\ 14^{\circ}\)
Why circular polarisation matters for a satellite. The passage's own rule — "efficient operation requires that the receiving antenna be of the same polarization as the transmitting antenna" — is the problem. A satellite tumbles, spins or is stabilised in an orientation the ground station cannot control, so a linearly polarised link would suffer deep fades whenever the two axes drifted apart; at 90° the received signal would vanish entirely. A circularly polarised wave has no preferred axis, so the coupling is independent of relative rotation.
The second reason is the ionosphere. A linearly polarised wave passing through it suffers Faraday rotation, its plane of polarisation turning by an amount that depends on frequency and on the total electron content along the path — unpredictable and variable. Circular polarisation is immune, since rotating an already-rotating field changes nothing.
| Option | True of a helix? | Is it the reason? |
|---|---|---|
| Circular polarisation | ✓ | ✓ Yes |
| Elliptical polarisation | Only when imperfect | ✗ A defect, not a feature |
| Broad bandwidth | ✓ genuinely wide | ✗ True but not the tracking reason |
| Good front-to-back ratio | ✓ the ground plane helps | ✗ Secondary |
Options 3 and 4 are the interesting distractors because both are true statements about the helix — it does have a usefully wide bandwidth and a decent front-to-back ratio. But the question asks specifically why it is used for satellite tracking, and the passage itself links circular polarisation to that context.
Hence, the helical antenna is used for satellite tracking because of its circular polarization.
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 :
The discone antenna is
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 :
The discone antenna is
Which of the following antennas is best exerted from a waveguide ?
