CRAC Class B (2025) — Questions 481–490

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U0787. FT8 signals are commonly found on HF and VHF amateur bands. The advantages of this mode are: (Choose all that apply.)

FT8信号常见于HF和VHF业余频段。这种方式的优点是：

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FT8's strengths are its extreme weak-signal sensitivity (works at QRP for DX, A) and its ability to reveal fleeting propagation such as ducts, meteor trails, and tropospheric scatter (B). C is wrong (FT8 bandwidth is ~50 Hz, not 3 kHz, and it is efficient, not redundant), and D is wrong (FT8 uses 8FSK, not 50 bps / Baudot).

U0788. When answering a station's FT8 call, how should we use the waterfall display provided by the logging software (e.g., WSJT-X) to select an appropriate transmit frequency?

回应某个电台的FT8呼叫时，我们应如何使用通联软件（例如 WSJT-X）提供的瀑布图选出适当的发射频率？

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In FT8 you answer on the frequency where the other station is listening (its "rx" offset), choosing a clear spot in its monitoring time slot to avoid colliding with other responders. Picking randomly (B), using its calling slot (C), or transmitting on its own call frequency (D) would cause overlap.

U0789. FT4 is a communication mode similar to FT8. Its characteristics are: (Choose all that apply.)

FT4是与FT8类似的一种通信方式。其特点是：

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FT4 is a faster (≈2× FT8 cycle) contest-oriented 4FSK mode; its throughput is comparable to RTTY while occupying a narrower bandwidth (A, B). C is false (FT4 can still catch meteor/scatter bursts, just less sensitive than FT8), and D is false (FT4 uses 4FSK, not Baudot).

U0790. When preparing for FT8, FT4, or JT65 contacts, we need to feed the audio baseband output of the logging software (e.g., WSJT-X) into the transmitter and route the receiver's output back to the software through a sound card. Some worthwhile setup tips are: (Choose all that apply.)

在准备FT8、FT4或JT65联络时，我们需要将通联软件（例如 WSJT-X）输出的音频基带馈入发信机，并将收信机的输出通 过声卡送回该软件。值得借鉴的一些设置经验是：

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All four are sound digital-mode setup practices: balancing tx audio to prevent ALC/clipping (A), balancing rx audio for clean decoding (B), using CAT for reliable PTT/band control (C), and pre-configuring the logger to avoid manual mistakes (D).

U0372. Yaesu System Fusion, abbreviated YSF, is a communication mode integrating digital voice and data transmission. Its characteristics are: (Choose all that apply.)

Yaesu System Fusion简称YSF，是数字语音和数据传输相互 融合的一种通信方式，特点是：

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YSF (Yaesu System Fusion) is a widely used amateur digital voice+data mode, works through repeaters, and uses C4FM modulation (A, B, C). D is false: YSF is C4FM/FDMA based, not TDMA (TDMA is used by some other systems such as DMR).

U0373. The Icom D-STAR System provides a data communication mode, whose characteristics are: (Choose all that apply.)

Icom D-STAR System提供了一种数据通信方式，特点是：

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D-STAR is a widely used amateur digital voice/data system: it works via repeaters (B), supports Internet linking and personal hotspots (C), and carries GPS position reports (D). All four statements are correct.

U0374. FreeDV is a digital voice mode. Its characteristics are: (Choose all that apply.)

FreeDV是一种数字语音方式。其特点是：

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FreeDV is an open-source, narrowband HF digital voice mode that lets hams experiment with DSP and build their own gear (A, B, C). D is false: FreeDV is designed for SSB transceivers, not FM handhelds, and "especially beautiful audio" is not a characteristic.

U0375. Digital amateur television (DATV) is a mode applying digital audio/video compression coding to real-time image communication. When conducting DATV contacts via a satellite transponder, amateurs generally choose the DVB-S2 protocol; the modulation may be: (Choose all that apply.)

数字业余电视（DATV）是将数字音视频压缩编码实验用于实 时图像通信的一种方式。通过卫星转发器进行DATV联络时， 爱好者们普遍选用DVB-S2协议，调制方式可以是：

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DVB-S2 (used for satellite DATV) supports several modulation schemes: QPSK, 8PSK, 16APSK and 32APSK. All four are valid DVB-S2 modulations, so all are correct.

U0376. When using the DRM protocol for digital slow-scan television (DSSTV) communication, amateurs always tend to transmit the DSSTV baseband (generated within the audio range) via single sideband rather than FM phone. The main reason is:

使用DRM协议进行数字慢扫描电视（DSSTV）通信时，爱好 者总是倾向于通过单边带而非调频话方式来传输在音频范围内 生成的DSSTV基带。主要原因是：

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SSB linearly passes the audio-range DSSTV baseband without FM's threshold/nonlinearity, preserving the digital signal and aiding error control in fading. B conflates cause (FM's threshold does widen bandwidth but the stated main reason is A's baseband fidelity); C/D mischaracterize FM's effect on quality.

U0791. An amateur radio station transmits in FM with an RF output power of 10 W, and its power-supply efficiency is about 80%. If the station transmits continuously for 10 seconds, the average transmit power during this period:

一部业余无线电台以FM方式发射时的射频输出功率为10瓦， 电源效率约为80%。若该台连续发话10秒钟，则此期间的平均 发射功率：

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FM is a constant-envelope mode, so the RF output stays at 10 W continuously while transmitting; the 80% power-supply efficiency only affects how much DC power is drawn (≈12.5 W), not the RF output. Average RF power ≈ 10 W.