CRAC Class C (2025) — Questions 811–820

U0566. The best time to conduct EME (Earth-Moon-Earth / moonbounce) communication experiments is:

开展EME通信试验的最佳时机是：

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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.

U0567. When installing a preamplifier for occasions requiring reception of extremely weak signals, such as EME communication, you should:

在EME通信等需要接收极微弱信号的场合，安装前置放大器时应做到：

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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.

U0568. For communication scenarios needing reception of VHF/UHF weak signals, such as moonbounce experiments, amateurs often add a low-noise preamplifier right at the feed point of their Yagi antenna. In contrast, HF amateurs rarely mount their preamplifier at the feed point of a Yagi or other receiving antenna. This is because:

对于需要接收VHF/UHF微弱信号的通信场景，比如月面反射实验，爱好者们时常需要在所用八木天线的馈电端处就近加装低噪声前置放大器。相比之下，HF通信爱好者却很少将他们所用的前置放大器装在八木天线或其他收信天线的馈电点处。这是因为：

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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.

U0569. Which of the following amateur bands is most suitable for meteor-scatter communication?

下列哪个业余波段最适合流星余迹散射通信？

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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.

U0570. Which of the following communication modes can be used for meteor-scatter and tropospheric-scatter communication experiments?

下列哪些通信方式可用于流星余迹和对流层散射通信实验？

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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

U0579. Electrostatic discharge (ESD) is a common electromagnetic phenomenon that accompanies daily life at all times. However, strong ESD can damage electronic equipment or even endanger human life and must be prevented. Which of the following are related to static electricity? (Choose all that apply.)

静电放电是一种常见电磁现象，时刻伴随日常生活。但是，较强的静电放电却足以损坏电子设备甚至危及人身，需要预防。以下所述与静电有关的是：

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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.

U0580. If a conductor is placed in an electrostatic field, the conductor will exhibit the following characteristics: (Choose all that apply.)

如果将导体置于静电场中，导体将呈现如下特点：

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Statement A is correct physics: at electrostatic equilibrium a conductor is equipotential, internal field is zero, and excess charge resides on the outer surface. The official answer key marks A and B. Study note: in real physics, surface charge concentrates where curvature is large (sharp points), so option D actually states the correct tip-discharge condition; the source key nonetheless marks B, so memorize the keyed answer.

U0581. Direct current (DC) is defined by the direction of charge motion never changing. Which of the following descriptions of direct current is correct? (Choose all that apply.)

直流电（DC）是以电荷的运动方向始终不变来定义的。关于直流电，以下描述正确的是：

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DC has fixed polarity terminals (A) and a battery supplies DC (B). C is false — 13.8 V can deliver dangerous short-circuit current; D is false — pulsating DC still contains an AC (ripple) component even though its average direction is fixed.

U0582. Alternating current (AC) is defined by the direction of charge motion alternating with time. Which of the following descriptions of alternating current is correct?

交流电（AC）是以电荷的运动方向随时间交替变化来定义的。关于交流电，以下描述正确的是：

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AC polarity alternates, so an AC source has no fixed +/− terminals (A). B is false — live (hot) and neutral must not be swapped; C is false — AC can be non-sinusoidal/multi- frequency; D is false — received RF is an AC signal.

U0583. Which of the following terms can be used to describe the number of times per second that alternating current changes polarity?

以下哪一个术语可以用来描述交流电每秒改变极性的次数？

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The number of polarity reversals per second is the frequency (频率), measured in hertz (A). Rate (B), wavelength (C), and pulse rate (D) describe other quantities.