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 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.

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.

 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.

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.

 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).

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.

 The rhombic is the standard example of a non-resonant, travelling-wave antenna — option 2.

The distinction between the two families.

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.

 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.

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.