CRAC Class B (2025) — Questions 851–860

EME path loss is enormous, so the best moment is when the Moon is closest to Earth — at perigee (A) — minimizing distance and thus path loss. Full moon (B) only affects illumination, not reflection gain; local weather (C) and sunspot count (D) are not the determining factors.

For weakest-signal reception, the low-noise preamplifier (LNA) must sit right at the antenna feed to beat the feed-line noise, and cooling lowers its thermal noise (A). Putting it near the receiver (B, C) or using feedback for gain (D) worsens the noise figure.

VHF/UHF external (galactic/atmospheric) noise is low, so receiver/feed-line noise dominates and a masthead LNA helps a lot (A). On HF the natural band noise is already high, so a front- end preamp barely improves the SNR — hence HF users seldom bother. B/C/D give wrong or trivial reasons.

Meteor-scatter works best on the 6-meter band (A), where the ionized meteor trails efficiently reflect VHF signals at a favorable frequency/wavelength. Higher bands (2 m, 70 cm) and 10 m are less optimal for this mode.

FT4 is a weak-signal, short-transmission WSJT mode well suited to the brief, fading reflections of meteor- and tropospheric- scatter (A). APRS (B) is a position-reporting system, QPSK31 (C) is not a standard weak-signal mode here, and MT63 (D) is a robust but not scatter-optimized mode; the official key marks A. 与业余无线电有关的电学基础 4 C H A P T E R Electrical Fundamentals Related to Amateur Radio 本章复习导体、欧姆定律、电路元件、晶体管放大、数字逻辑与测量仪表等必备电学基础。This chapter reviews the essential electrical fundamentals: conductors, Ohm's law, circuit components, transistor amplifiers, digital logic, and test instruments. 导体与电流 Conductors and Current

A conductor has many free electrons/ions and conducts under voltage (A), and most metals (silver, copper, aluminum) are good conductors (B). C is false — superconductivity occurs at very low, not high, temperature; D is false — piezoelectricity is a crystal property, not typical of "metals" as conductors.

Ionized gas conducts (A), electrolyte solutions conduct via ions (B), and graphite (a form of carbon) conducts (C). Mica (D) is an insulator/dielectric, not a conductor.

All four are correct: insulators bind charges tightly and show high resistance (A); they are dielectrics usable in capacitors like ceramic/polystyrene (B); excessive field causes breakdown into conduction (C); and heating reduces insulation, so temperature matters (D).

Engineering plastics (A), phenolic resin (B), and silicon dioxide (C, e.g., glass/quartz) are insulators. Tin dioxide (D, SnO₂) is a semiconductor/conductive oxide, not an insulator, so it is excluded.

Breakdown is dielectric failure into conduction: an overvolted capacitor shorting (A), an arc across a tuner capacitor (B), and a neon lamp ionizing/glowing under its breakdown voltage (C) all illustrate it. D is ordinary incandescent heating, not breakdown, so it is excluded per the key.