CRAC Class A (2025) — Questions 571–580

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.

Dust/moisture change an insulator's (dielectric's) properties, causing dielectric loss (介质损耗) — energy dissipated in the dielectric under RF (A). Eddy-current (B) and hysteresis (C) losses belong to magnetic cores; magnetic-reluctance loss (D) is not the effect described.

All four are correct: semiconductors' conductivity is controllable (A), they are intrinsic vs impurity (P/N) types (B), the P-N junction has a built-in EMF and rectifying (one-way) behavior (C), and silicon/germanium build transistors and ICs (D).

Bipolar (A), gallium-nitride (B), and LDMOS (C) transistors are all solid-state semiconductor devices. A vacuum-tube triode (D) is not semiconductor — it is an electron-tube device.

Static-related phenomena include charge buildup on wind- blown guy wires (A), thundercloud charge (B), and atmospheric/static noise QRN heard on receive (C). RF energy fed to the antenna while transmitting (D) is intentional radio emission, not static electricity.