CRAC Class C (2025) — Questions 801–810
U0983. In HF amateur bands, we sometimes hear on SSB a signal whose tone varies periodically, and within each period we can hear "tok-tok" or "clack-clack" sounds. Different stations send this signal with different tone-variation periods, ranging from several times per second to once every one or two seconds. The communication mode using such a signal may be:
在HF业余频段中，我们有时会在SSB方式下听到一种音调周期 性变化的信号，并可以在每个周期中听到"笃、笃"声或"咯、 咯"声。不同业余电台所发的这种信号，音调的变化周期可能不 同，从每秒数次至一两秒一次都有。使用这种信号的通信方式 可能是：
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SSTV (慢扫描电视) and FAX (facsimile) modulate a subcarrier whose audio tone sweeps up and down with a characteristic "tok-tok/clack-clack" per line/cycle, at rates from a few Hz to about 1–2 s per cycle — exactly the described sound. PSK31 is a steady tone, RTTY is two alternating tones, and FT4 is four fixed tones, so A is correct.
U0984. The spectrum of a CW signal appears as:
一个CW信号的频谱表现为：
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A CW (等幅电报 / continuous-wave) signal is essentially an unmodulated carrier keyed on/off, so on a waterfall/spectrum display it shows as a single vertical line whose brightness flickers with the Morse keying. A continuously-on carrier would be a fixed line (B), but real CW is keyed, hence "flickering" (A). Horizontal lines or a sine curve do not represent a CW spectrum.
U0985. The spectrum of an RTTY signal appears as:
一个RTTY信号的频谱表现为：
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RTTY (无线电传) is 2FSK: the carrier jumps between two audio tones (mark/space), so its spectrum shows two vertical lines. Because the tones turn on and off with the data, both lines flicker. Hence "two flickering vertical lines" (A) is correct; a single line (B) would be CW.
U0986. The spectrum of an SSB voice signal appears as:
一个SSB话音信号的频谱表现为：
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SSB (单边带) voice has no carrier and contains only the speech sideband, so its spectrum is a band of energy whose components (a cluster of vertical lines/bars) rise and fall asymmetrically with the syllables — one sideband only, hence asymmetric. A single flickering line (B) is CW-like; a fixed line (C) is an unmodulated carrier.
U0987. The spectrum of an AM voice signal appears as:
一个AM话音信号的频谱表现为：
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AM (调幅) voice keeps a constant carrier (a fixed central vertical line) plus two symmetric sidebands that carry the speech (a group of lines on each side that vary with the voice). That is exactly "a fixed vertical line flanked symmetrically by speech- varying lines" (A). A flickering single line (B) would be CW; fixed multiple lines (C) suggest an unmodulated multi-tone.
U0988. The spectrum of an FM voice signal appears as:
一个FM话音信号的频谱表现为：
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FM (调频) voice, being angle modulation, spreads energy across many sidebands (Bessel components) symmetrically around the carrier. The carrier (central line) amplitude varies with modulation (can even null at certain deviation), and the symmetric sideband lines vary with the speech — matching "a vertical line of varying height flanked symmetrically by speech- varying lines" (A). FM shows many lines, not a single flickering line (B).
U0989. The spectrum of a PSK31 signal appears as:
一个PSK31信号的频谱表现为：
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PSK31 (相移键控) uses a very slow 31.25 baud BPSK/QPSK keying, so its occupied bandwidth is only about 31 Hz — on a spectrum/waterfall it appears as a single, extremely narrow peak (A). It is not a wide symmetric sideband group (B) nor fixed lines (C); the narrowness is why PSK31 is so spectrum- efficient. 业余无线电卫星 Amateur Radio Satellites
U0563. In the amateur radio field, the abbreviation AMSAT refers to:
在业余无线电领域中，缩写AMSAT是指：
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AMSAT is the amateur radio satellite organization (originally Radio Amateur Satellite Corporation, now AMSAT) — a volunteer non-profit building and operating amateur satellites (A). It is not a satellite name or a generic class of satellites (B/C/D).
U0564. When describing amateur radio satellites, the abbreviation OSCAR (奥斯卡) is often used. The origin of this name is:
在描述业余无线电卫星时经常用到缩写OSCAR（奥斯卡），这 个名称的由来是：
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OSCAR stands for Orbiting Satellite Carrying Amateur Radio (A). It is an acronym, not a person's name (B, C) nor related to the film Academy Award (D).
U0565. The launch year, name, international OSCAR number, and transponder mode of China's first amateur satellite are, respectively:
我国发射的第一颗业余卫星的发射年份、名称、国际OSCAR系 列号和转发器模式分别为：
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China's first amateur satellite is Hope-1 (希望一号, XW-1), launched in 2009, assigned OSCAR number 68 (HO-68), using a V/U (J) mode transponder — uplink VHF, downlink UHF, mode J (A). The other years (B, C, D) and "no OSCAR number" (D) are wrong.