CRAC Class A (2025) — Questions 491–500

Each end gains 6 dB (one S-unit) from its dipole (2.15 dBi) to the 8.15 dBi Yagi. Both link ends improve by one S-unit, so each signal goes from S4 to S4 + 1 + 1 = S6 (A).

As you move, the relative phase of the direct and reflected waves shifts, producing constructive/destructive interference — multipath fading (A). Doppler from walking is tiny; B/C/D are not the cause of the periodic signal swing.

Small position or frequency changes alter the multipath phase combination, possibly turning destructive interference into constructive (A). Path loss does not change much over a few steps (B); atmospheric and geomagnetic effects are not the cause here (C/D).

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.