CRAC Class C (2025) — Questions 611–620

Multipath causes delay spread and fading that can increase the BER of digital modes (A). B is too strong — multipath can still affect FM/data; C/D's linear rate claims are false.

Ghosting in VSB/ATV mainly comes from multipath: reflected paths arrive delayed and overlay a shifted copy of the picture (A). Additionally, a high VSWR at the transmit antenna makes part of the signal reflect back and forth along the feed line, producing a delayed echo that also appears as a ghost (B). Polarization errors (C) and repeater replay (D) are not the causes marked for this item.

The range of line-of-sight (视线传播) propagation of ultra-short waves (VHF/UHF) is limited by the radio horizon, which depends mainly on how high the antennas are above the local ground (the relative height), not on their absolute altitude above sea level. A higher antenna above ground sees farther over the Earth's curvature. Options B, C and D are secondary or irrelevant: absolute sea-level height matters far less than local terrain clearance, the height-to-wavelength ratio does not set the horizon limit, and antenna gain improves signal margin but not the geometric horizon.

Beyond-line-of-sight (超视距) VHF/UHF contacts over hundreds of km are most commonly caused by tropospheric ducting (大 气波导), where a temperature inversion traps radio waves in a waveguide-like layer that follows the Earth's curvature. Aircraft reflection (B) is rare and brief; rainfall (C) generally attenuates rather than enhances VHF/UHF; seasonal vegetation loss (D) has negligible effect at these bands. Therefore A is the standard correct explanation.

Sporadic E (突发E层) is the dominant cause of 6 m and 2 m signals reaching a thousand km or more: dense patches of ionized E-layer clouds reflect VHF signals far beyond the horizon, often with strong signals. Meteor scatter (B) gives only very brief, weak bursts and is not the main "regularly received" thousand-km mechanism; D-layer absorption (C) weakens signals rather than enabling DX; gray-line (D) is mainly an HF dawn/dusk effect. Hence A is correct.

Tropospheric ducting (大气波导) is created when a temperature inversion (逆温) — a layer of warm air above cooler air — forms in the lower atmosphere. This refractive-index gradient bends VHF/UHF rays downward, confining them in a duct. Sunspots/flares (B) affect HF via the ionosphere, not tropospheric ducts; hurricanes (C) and lightning plasma (D) are not the cause of normal ducting. Thus A is correct.

For a VHF signal to travel a thousand km, the usual mechanism is reflection from the sporadic E layer (突发E层) of the ionosphere, which can return 6 m/2 m signals over very long distances. Microwave relay (B), thunderstorm reflection (C) and cosmic-ray paths (D) are not realistic normal mechanisms for such reception. So A is correct.

Tropospheric scattering (对流层散射) — scattering of VHF/UHF waves by inhomogeneities in the troposphere — routinely yields contacts up to several hundred km (around 500 km here). D-layer (B) absorbs rather than refracts VHF usefully; F2 refraction (C) is an HF mechanism; Faraday rotation (D) is a polarization effect on satellite/satellite paths, not a 500 km terrestrial DX cause. Therefore A is correct. 天线系统与匹配 Antenna Systems and Matching

A resonant half-wave dipole has voltage/current standing waves along its elements (that's normal for a radiator). The feed line, when matched to the dipole's impedance, carries a traveling wave with no reflected power (A). Standing waves on the feed line (B/C) indicate a mismatch; pure traveling waves on the antenna (B/D) would mean no resonance.

The shortening factor (缩短系数) accounts for end effects that make a practical dipole slightly shorter than the free-space half-wave length. Its empirical value is about 0.95 (A) — i.e., you multiply the ideal length by 0.95. Values 1.05 (B), 0.707 (C), 1.414 (D) are unrelated here.