Radiation from a Dipole Questions and Answers

Answer: C

A wave travelling along the winding carries current whose direction rotates continuously around the axis, which is exactly what produces a rotating field vector, and the sense follows the handedness of the winding. No quadrature feed is needed, the ground plane merely improves the pattern, and the pitch is nearer thirteen degrees than forty-five. Achieving circular polarisation geometrically rather than electrically is what makes the helix so simple and so broadband.

Answer: D

All the linear wire antennas have an axial null, because the far field of a current element is proportional to the sine of the angle from the wire. The axial mode helix is the exception, radiating its maximum along the axis, which is what end fire means. The contrast is a useful reminder that the null is a property of a straight current distribution rather than of antennas in general.

Answer: A

Axial mode gain rises with the number of turns and the spacing, and ten turns at the usual pitch gives about 12 dBi, roughly twelve times the isotropic level. Doubling the number of turns adds about three decibels. A figure of 2 dBi would be a plain dipole and 30 dBi would require a large reflector. The helix occupies a useful middle ground: moderate gain, circular polarisation and very wide bandwidth in a simple structure.

Answer: B

The first three are right. The monopole radiates into a hemisphere rather than a sphere, so its directivity is double the dipole's, about 3.28, not equal to it. That doubling is the same fact as the halving of its radiation resistance, seen from the other side, and both follow from the image principle. Checking that the pattern and impedance consequences of an argument agree with one another is a good way to catch an error.

Answer: C

With the ground plane confining radiation to the upper hemisphere, the same power is spread over half the solid angle, so the intensity in any given direction is doubled. That is a three decibel improvement obtained purely by geometry. The plane radiates nothing itself, the current is unchanged, and efficiency in fact suffers over real ground. The result assumes a perfect plane; over poor earth much of the apparent advantage is lost to dissipation.

Answer: D

The Hertzian or elementary dipole is the theoretical building block from which the fields of any wire antenna are assembled by integration. Its uniform current is an idealisation that no physical antenna has, since current must vanish at an open end, but it makes the mathematics tractable. Folded, sleeve and biconical dipoles are all practical constructions with particular impedance or bandwidth properties rather than theoretical abstractions of any kind.

Answer: A

Taking the current as a filament along the axis with an assumed sinusoidal distribution reduces the problem to a single integral, and the result is accurate for wires thin compared with a wavelength. Thick elements need a numerical treatment. Mechanical rigidity is irrelevant, zero resistance is a separate idealisation affecting efficiency rather than pattern, and a uniform surrounding dielectric would change the wavelength but is not the assumption in question.

Answer: B

The inverse square term is the induction field, which stores and returns energy each cycle and dominates at a fraction of a wavelength from the element. The inverse cube term is the electrostatic field and the inverse first power term is the radiation field, the only one that carries net power to infinity. The supporting structure produces its own scattered field, which is an installation effect rather than one of the three theoretical terms of the exact solution.

Answer: C

The two terms are equal where the range equals the wavelength divided by two pi, that is about 0.16 wavelengths, and beyond that the radiation term dominates rapidly. This is why the reactive near field of a small antenna is so shallow. For a large aperture the far field boundary is set instead by the two D squared over lambda criterion, which is a phase flatness condition rather than a field term comparison, and the two criteria must never be confused with each other.

Answer: D

For a small antenna the boundary is where the radiation term dominates, at about a sixth of a wavelength; for a large one it is where the phase across the aperture is flat enough, at two D squared over lambda. Whichever is greater applies. Reciprocity makes transmit and receive identical, both criteria are used in practice as well as in theory, and both apply to any geometry. The distinction explains why a small antenna's far field starts almost immediately while a large one's may be kilometres away.