Radiation from a Dipole Questions and Answers

Answer: A

Rotational symmetry of the source about the wire forces the field to be independent of azimuth, and this holds for a wire of any length or thickness. It is why a vertical dipole covers all directions equally in azimuth. Radiation resistance is a single terminal number rather than a directional quantity, loss affects amplitude everywhere equally, and centre feeding influences the elevation pattern rather than the azimuthal symmetry.

Answer: B

The second is correct and the first is exactly wrong: the axial direction is a null and the broadside direction is the maximum. The pair is worth holding together because the confusion is so common and so consequential: a vertical dipole gives excellent coverage around the horizon and none at all straight overhead, which decides where it should be installed and what it can and cannot serve.

Answer: C

Real earth is a poor conductor, so the return current heats it and that loss appears in series with the radiation resistance, sometimes exceeding it. Radial systems, elevated counterpoises and ground screens are all attempts to carry that current in copper instead. The elevation pattern is altered, particularly at low angles, but not made uniform; resonance shifts only slightly; and the polarisation stays vertical though a small horizontal component appears at grazing incidence.

Answer: D

A counterpoise is a set of conductors raised above the earth to provide the return path for a monopole, avoiding the loss of a poor ground. Even a few elevated radials can outperform many buried ones. Mechanical counterweights, parasitic elements and capacitive top hats are all different devices, the last being a way of raising the current in a short whip rather than providing a return path. Managing the return current is half the design of any vertical antenna.

Answer: A

A top hat holds charge at the tip so the current no longer falls to zero at the top of the radiating section, raising the average current and therefore the radiation resistance and efficiency. It also reduces the loading inductance needed. It adds no useful height electrically, the return path is the ground system's job, and the pattern is little changed. It is the standard trick for making a physically short vertical perform closer to its potential.

Answer: B

Top loading means placing capacitance, as a hat, a set of wires or a folded top section, at the upper end of a short vertical, which raises the average current and so the radiation resistance. Base loading with a coil achieves resonance but does nothing for the current distribution, and is therefore inferior wherever top loading is mechanically possible. This distinction between resonating an antenna and making it radiate efficiently is one of the most practically useful in the subject.

Answer: C

A base coil cancels the reactance but the current still tapers from the coil to zero at the tip, so the average current and hence the radiation resistance remain small. Top loading changes the distribution itself. Base loading does reach resonance, needs a smaller rather than larger inductance than a top loaded design would, and does not affect polarisation. Where both are possible, loading nearer the centre or the top is always the better engineering.

Answer: D

When the helix diameter and pitch are small compared with the wavelength, it radiates broadside like a short dipole, the coiling being simply a way of fitting more conductor into a short space. The axial mode, which needs a circumference near one wavelength, is the circularly polarised end fire case and is a quite different animal. The rubber whip on a handheld radio is a normal mode helix, and its performance is simply that of a short and heavily loaded vertical.

Answer: A

Being electrically short, it has a small radiation resistance and a high quality factor, so it is inefficient and narrowband, which is exactly the Chu limit at work. Its pattern is the simple doughnut of a short vertical, with no excess gain and no side lobes, and its polarisation is essentially linear in this mode. Users accept the losses because the alternative, a quarter wave whip, is inconveniently long at the frequencies concerned.

Answer: B

Axial mode requires a circumference near one wavelength and a pitch of twelve to fourteen degrees, and it radiates a circularly polarised beam along the axis with a gain rising with the number of turns. It is broadband and easy to feed, which makes it a favourite for satellite ground stations. The normal mode is the small diameter broadside case. Feed position and core material are construction details rather than the defining difference.