Antenna Fundamentals and Parameters Questions and Answers

Answer: B

Front to back ratio compares the radiation in the main beam direction with that in the opposite direction, and is quoted in decibels. A Yagi may reach twenty decibels and a reflector far more. Comparison with the largest side lobe is the side lobe level, forward gain referred to input power is simply gain, and the ratio of beamwidths describes the shape of the beam rather than any rejection. A high front to back ratio matters where interference arrives from behind the antenna.

Answer: C

Side lobe level is the ratio, in decibels, of the peak of the largest side lobe to the peak of the main lobe, and a uniformly illuminated aperture gives about minus thirteen decibels. Tapering the illumination lowers the side lobes at the cost of a wider main beam and a lower aperture efficiency. The count and position of the lobes follow from the aperture size, and the total side lobe power relates to beam efficiency, which is a different specification used in radiometry.

Answer: D

Radiation intensity is watts per steradian, and being a per solid angle quantity it does not fall off with distance, which makes it the natural variable for describing a pattern. Power per unit area is power density, which does obey the inverse square law, accepted power is an input quantity, and field strength at a reference distance is a related but different measure. Directivity is defined as the ratio of radiation intensity in a direction to its average over all directions.

Answer: A

Directivity compares the intensity in the strongest direction with the intensity an isotropic radiator of the same total power would give in every direction. It is a property of the pattern alone and takes no account of losses. Radiated over accepted power is the radiation efficiency, comparison with a dipole gives gain in dBd, and the aperture relation is a formula for directivity rather than its definition. Gain is directivity multiplied by efficiency.

Answer: B

Directivity is four pi divided by the beam solid angle:

\[ D = \tfrac{4\pi}{0.04} \approx 314 \]

so about 314, or roughly 25 decibels. The relation shows directly that concentrating radiation into a smaller solid angle raises directivity in exact proportion, and it is the cleanest statement of what an antenna does: it redistributes a fixed radiated power over a smaller range of directions rather than creating power.

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.