Antenna Fundamentals and Parameters Questions and Answers

Answer: C

Gain accounts for the power lost in the structure, so it is the directivity reduced by the radiation efficiency, and the two are equal only for a lossless antenna. A well made reflector may be ninety-nine per cent efficient so the distinction hardly matters, while a small electrically short antenna may be a few per cent efficient and the difference is enormous. Mismatch is usually accounted separately as realised gain, and aperture area enters the formula for directivity rather than this relation.

Answer: D

Radiation efficiency is the fraction of the accepted power that leaves as radiation rather than being dissipated in conductor and dielectric loss, and in circuit terms it is the radiation resistance divided by the sum of radiation and loss resistances. Referring to available rather than accepted power folds in the mismatch, which is a separate factor. Aperture illumination uniformity is aperture efficiency, a distinct quantity affecting directivity rather than loss.

Answer: A

Radiation resistance is a fictitious resistance which, carrying the same current, would dissipate the power the antenna actually radiates. For a half wave dipole it is about seventy-three ohms and for a short dipole only a few ohms, which is why short antennas are inefficient. Conductor loss is a separate resistance in series with it, the feed point resistance is the sum of the two, and ground loss is one particular contribution to the loss term in a monopole.

Answer: B

Radiation resistance falls as the square of the length in wavelengths, so for a very short element it becomes comparable with or smaller than the conductor and matching losses, and most of the input power is dissipated as heat. Its pattern is in fact very simple, close to that of an ideal dipole with a broad doughnut shape and no radiation along the axis at all, and its impedance is strongly capacitive rather than resistive. Efficiency, not pattern, is the limitation of an electrically small antenna.

Answer: C

A thin half wave dipole at resonance presents approximately 73 ohms with a small reactive part that vanishes when it is trimmed slightly short of a half wavelength. Fifty ohms is the cable standard rather than the dipole value, three hundred ohms is the folded dipole figure used with twin feeder, and 377 ohms is the intrinsic impedance of free space, which is a property of the medium and not of any antenna. The mismatch between 73 and 50 ohms is small enough to be tolerated in most installations.

Answer: D

A standing wave ratio of unity means no reflected wave, so all the power delivered to the feed enters the antenna. Total reflection gives an infinite ratio, as an open or short circuit termination does. A perfect match says nothing about efficiency, since a resistor terminating the cable would match perfectly and radiate nothing, and a purely reactive load reflects everything. Confusing a good match with a good antenna is one of the commonest errors in practice.

Answer: A

For a purely resistive load the ratio is simply the larger resistance over the smaller:

\[ S = \tfrac{75}{50} = 1.5 \]

giving 1.5, which corresponds to about four per cent of the incident power reflected and is entirely acceptable. By convention the ratio is always quoted as one or greater, so a value below one is a sign that the calculation has been inverted. This simple case is worth remembering because a dipole on a fifty ohm cable is exactly it.

Answer: B

Return loss states how far below the incident power the reflected power lies, so a large number in decibels means a good match; ten decibels corresponds to a standing wave ratio near two and twenty decibels to about 1.2. It is the same information as the standing wave ratio in a more convenient form for wideband plotting. Feeder loss, gain asymmetry and balun loss are all separate quantities, though each can disguise a poor match by absorbing the reflected power.

Answer: C

A reflected wave travels the length of the cable a second time, so it is attenuated twice over and the standing wave ratio measured at the transmitter is closer to unity than the one at the antenna. A very lossy cable can make a completely faulty antenna look acceptable. The cure is to measure at the antenna end or to correct for the known cable loss. Impedance transformation depends on length rather than loss, and instruments do not correct unless told the cable characteristics.

Answer: D

Bandwidth is the span over which some named parameter stays within limits, most often the standing wave ratio below two, but sometimes gain, beamwidth or polarisation purity. It is meaningless without saying which parameter and which limit. Thick elements, cages and folded arrangements all widen it. The other options describe no standard quantity: radiation exists at every frequency to some degree, and reactance appears everywhere except at resonance.