CRAC Class C (2025) — Questions 431–440

U0783. The modulation method of PSK31 is:

PSK31的调制方式为：

Exam tips · 考试提示

PSK31 uses binary phase-shift keying (BPSK31) by default and offers quadrature PSK (QPSK31) as an alternative. 8FSK is FT8, 8ASK/64QAM are unrelated higher-order schemes.

U0784. What is FT8?

什么是FT8？

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FT8 is a weak-signal, low-power, low-data-rate FSK mode in the WSJT-X family, designed to complete contacts even at very low SNR. It is not an SNR enhancement trick (B), not satellite audio/video (C), and not a high-power/high-rate mode (D).

U0785. The modulation method of FT8 is:

FT8的调制方式为：

Exam tips · 考试提示

FT8 transmits one of 8 tones (8-frequency-shift keying, 8FSK) per symbol, with continuous phase to keep the signal clean. 8ASK/8PSK/8QAM are not used by FT8.

U0786. Regarding the digital communication mode JT65, which of the following descriptions is correct: (Choose all that apply.)

关于数字通信模式JT65，以下描述正确的是：

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JT65 is a very weak-signal mode: it works at QRP (A) and each transmission lasts ~50 s, so it is slow (B). C is false (it is designed for low power, not kilowatts), and D is false (50 s/message means roughly one message per minute, not 50).

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？

Exam tips · 考试提示

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：

Exam tips · 考试提示

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.