CRAC Class A (2025) — Questions 391–400

VOX stands for Voice Operated Transmit (声控 / 发信机声控). When enabled, the radio switches from receive to transmit automatically when it detects speech at the microphone, and returns to receive when speech stops — a hands-free alternative to the PTT (push-to-talk) switch. Option A is correct. B is AT/TUNE, C is ALC, D is PROC.

PTT (push-to-talk) requires a hand on the radio; VOX (voice- operated transmit) removes that need by keying the transmitter from speech, giving a hands-free mode. Option A states this correctly. B, C and D wrongly describe VOX as a Bluetooth link or as an audio-processing/compression function — it is neither; those are separate features (PROC/compression, Bluetooth pairing).

CTCSS (亚音调静噪 / 亚音) stands for Continuous Tone-Coded Squelch System. A low (sub-audible, 67–250.3 Hz) tone is superimposed on the voice; the receiver opens squelch only when it detects that specific tone, so stations on the same frequency using different tones do not open each other's audio. Option A is correct. B is DTMF, C is a description closer to DCS/selcall, and D is plain carrier squelch.

DTMF (双音多频) stands for Dual-Tone Multi-Frequency — the "touch tones" you hear when pressing keypad buttons. Each digit is made of one of four low tones plus one of four high tones (8 frequencies total), giving 16 combinations. It is used for remote control and sending digits, not for squelch gating. Option A is correct; B is CTCSS, C resembles DCS, D is plain squelch.

"亚音" is analog CTCSS (Continuous Tone-Coded Squelch System) and "数字亚音" is DCS (Digital Coded Squelch, a digital code sent in place of the analog tone) — both are sub-audible squelch gates. The menu items for them are CTCSS and DCS, so A and B are correct. DTMF (C) is an audible dual-tone control signal, not a squelch gate; SQL (D) is the basic squelch control itself, not the "sub-tone" technique described.

NFM = narrowband FM (窄带调频), the ~5 kHz deviation, 25/12.5 kHz channel format used for two-way communication including amateur VHF/UHF. WFM = wideband FM (宽带调频), the ~75 kHz deviation, ~200 kHz channel format used for broadcast FM (88–108 MHz). Option A is correct; the others invent unrelated meanings.

"全频偏/半频偏" sets the FM modulation deviation (调制频偏): full deviation = 5 kHz peak for a 25 kHz channel (NBFM), half deviation = 2.5 kHz peak for a 12.5 kHz channel. The setting matches the channel spacing/bandwidth, not the frequency stability or repeater use. Option A is correct; B, C, D misattribute it to drift, repeater type, or audio compression.

Severe distortion / poor readability of your transmitted FM signal can arise from: (A) low supply voltage, which lets the PA clip and distort; (B) multipath fading at your location, which at the receiving end produces distortion and dropouts (moving or changing frequency helps); and (C) an inaccurate transmit frequency (off-frequency carrier), which the receiver demodulates off-center and distorts. D is not a cause of distortion — an under-rated antenna causes mismatch/SWR problems, not modulation distortion. Hence A, B, C are correct.

To recall a favorite frequency instantly you store it in a memory channel (频道 / memory channel) and recall it with one key. Option A is correct. B/C concern CTCSS ("哑音静噪"), which is unrelated to frequency recall; D (scan) is for searching, not for instantly returning to a known frequency.

The negative (ground/return) lead of a mobile radio must go to a solid, low-resistance chassis ground — directly to the battery negative post or the engine grounding strap (接地带). This gives a clean RF and DC ground. Option A is correct. B is the antenna feed (must not be grounded), and C/D are unreliable grounds that can cause RFI and voltage drop.