CRAC Class C (2025) — Questions 521–530

AT / TUNE = automatic antenna tuner (自动天线调谐): it matches the transmitter to the antenna by sensing the SWR and adjusting a matching network for the lowest SWR. Option A is correct. B is transmit AVC; C is ALC; D is AFC.

With separate RF/IF gain and audio gain, the best practice is: for a very weak signal maximize RF/IF gain (to keep the best SNR before the noise floor) and use audio gain only for comfort; for a very strong signal you can max audio gain but must reduce RF/IF gain (or use ATT) to avoid front-end overload, then set the other control for volume. Option A captures this. B reverses the weak-signal rule (max audio on weak signal just raises hiss); C/D's fixed-middle rules ignore the weak/strong distinction.

The defining feature of HF (short wave, 3–30 MHz) is skywave propagation — signals refract off the ionosphere (电离层) and return to Earth, enabling intercontinental communication. VHF/UHF are essentially line-of-sight. Option A is correct. B is false (HF fading is often severe); C is false (HF antennas are larger, e.g. 40 m dipole ≈ 20 m); D is false (HF channels are narrow, not broadband).

RF current induced on an audio lead is best suppressed by a ferrite choke (铁氧体磁环) slipped over the cable — it adds common-mode impedance at RF without affecting the audio. Option A is correct. Putting filters in the core wire (B/D) would distort the audio; a mic amp (C) does nothing for RFI. 收发信机面板控制 Transceiver Front-Panel Controls

RIT = Receive Incremental Tuning. It shifts only the receive frequency by a small amount (typically ±9.9 kHz) without moving the transmit (main) frequency, useful when the other station's signal is slightly off (A). XIT (B) is the transmit counterpart; SPLIT (C) is independent Tx/Rx; clearing memory (D) is unrelated.

XIT = Transmit Incremental Tuning. It offsets only the transmit frequency slightly while the main (receive) tuning stays put (A). It is the transmit counterpart of RIT (B); SPLIT (C) and memory clear (D) are different functions.

SPLIT means operating with receive and transmit on different frequencies (A). It is distinct from XIT/RIT (small offsets of one side) and from memory clear (D). Often the transmit VFO is offset to a separate frequency while receiving on another.

Split is primarily used to manage a pile-up: the DX station transmits on one frequency and asks callers to transmit a few kHz away so it can hear them amid the crowd (A). It is also used when correspondents are in different regions/bands and must operate on different frequencies to complete the contact (B). C (hiding conversation) is not a legitimate purpose, and D (can't make gear co-frequency) is a misuse rather than the intended scenario.

An SSB/CW signal that sounds off-pitch is slightly off your receive tuning; RIT lets you retune the receive frequency a few kHz without moving your transmit frequency, restoring normal pitch (A). Bandwidth (B) affects selectivity, squelch (C) gates the audio, and AGC speed (D) affects audio dynamics — none fix pitch.

CW-R (CW reverse) flips which sideband the receiver demodulates. Since CW is often received with a sideband plus BFO, reversing can move an adjacent interfering signal out of your passband, reducing/eliminating interference (A). It does not generate a cancelling sideband (B), double capacity (C), or prevent keying (D).