CRAC Class C (2025) — Questions 551–560

Antenna gain (增益) is a directional measure: the radiation intensity (power density) in the main lobe relative to a reference antenna (isotropic for dBi, dipole for dBd), i.e. how much the antenna concentrates power in a given direction. A and B both express this. C is radiation efficiency (辐射效率), not gain; D is dissipation/loss ratio. Hence A, B correct.

dBi = gain relative to an isotropic (理想点源) radiator — a theoretical point source radiating equally in all directions. So dBi compares your antenna's main-lobe power density to that of an isotropic source, in dB. Option A is correct. B is the definition of dBd (relative to a half-wave dipole); C and D are other references.

The ideal isotropic radiator (理想点源天线) is a theoretical lossless point source that radiates equally in every direction — it cannot be built, but it is the reference for dBi gain. Option A describes it correctly. B/C/D invent false physical antennas; it is purely a mathematical reference.

dBd = gain relative to a half-wave dipole (半波长偶极天线), which itself has 2.15 dBi. So dBd compares your antenna's main-lobe density to that of a dipole, in dB. Option A is correct. B is dBi; C and D are other references.

A half-wave dipole has a gain of 0 dBd by definition; since 0 dBd = 2.15 dBi (the dipole is 2.15 dB stronger than an isotropic point source), its gain versus the ideal isotropic antenna is 2.15 dBi. Option A is correct. D (1.64) is the aperture efficiency factor, not the gain in dBi.

A bare "dB" with no stated reference (neither dBi nor dBd) is meaningless for comparing antennas, because gain must be relative to a defined reference antenna. Such a spec lacks reference value. Option A is correct. B is the dBi definition, C a dB-vs-vertical, D a front-to-back ratio — but the question says only "dB" with no basis, so none of those apply.

A vertical grounded antenna radiates equally in all compass bearings around it — its horizontal (azimuth) pattern is a circle, i.e. no horizontal directivity — which is why it is called omnidirectional (全向). Option A is correct. It does have vertical (elevation) directivity/nulls (B false); its patterns are not perfect circles in both planes (C false); and it is not truly isotropic (D false — only an ideal point source is isotropic).

A ground-plane antenna with a quarter-wavelength vertical element over a ground plane radiates its main lobe horizontally (near 0° elevation) and presents a feed-point impedance of about 50 Ω, which matches coaxial cable directly — so 1/4 wavelength is correct. A half-wave or 3/2-wavelength vertical has a high feed-point impedance and a different pattern; 1/8 wavelength is too short to be resonant.

A quarter-wave ground-plane antenna radiates omnidirectionally around the azimuth (H-plane, horizontal) and has its main lobe slightly above 0° elevation in the vertical (E-plane) cut — so A and B are correct. The figure-8 pattern belongs to a dipole's E-plane, not to a vertical ground-plane; C and D are wrong.

A rubber-duck is normally held vertically, so the vertical polarization component dominates (A). Because it is physically shorter than a full-size 1/4-wave element, it has lower gain on both transmit and receive (B). Helical loading does not make the radiated wave circularly polarized (C is wrong), and a cracked sheath does not instantly destroy the internal element (D is wrong).