CRAC Class C (2025) — Questions 541–550

For diversity to work, the two antennas must sample decorrelated signals: space them at least ~1 wavelength apart (A) and use orthogonal polarization (e.g., one horizontal, one vertical) (B) so that when one fades the other is strong. Preamps (C) and preselectors (D) are optional/conditional, not the key requirement; C/D overstate necessity and are not the marked answers.

To run a USB SDR dongle you need a PC plus the SDR control software (e.g., SDR#, HDSDR), the mode decoder (CW/FT8), and a virtual-audio-cable/bridge to route the SDR's audio into the decoder (A). You do not need to write code (B, C) or attach a scope/spectrum analyzer (D); off-the-shelf software suffices.

A transceiver (收发信机) and a linear power amplifier (线性功率 放大器) connect via: the RF interface (射频接口) carrying the signal, the PTT control interface (PTT控制接口) to key the amplifier, and the ALC interface (ALC接口, automatic level control) for feedback (A, B, C). AGC (automatic gain control, D) is an internal receiver function, not an inter-unit link.

The ALC (automatic level control, 自动电平控制) line feeds back from the amplifier to the transceiver to limit drive and prevent over-driving. Over-driving creates heavy intermodulation (互调) and splatter that interferes with adjacent frequencies (A). B/C/D describe other functions (harmonic filtering, equalization, PAPR) not performed by ALC.

To suppress a harmonic leaving the amplifier, you place a filter that attenuates the harmonic frequency in the output path — between the amplifier's output and the antenna (A). A notch (陷波器) at the harmonic band rejects that specific spurious frequency. B/D put the filter in the input (wrong direction); C notches the operating band (would hurt your own signal).

For a tube linear amp, the standard tune-up is: set "Tune" (loading/plate tuning) for minimum plate (屏极) current at resonance, then adjust "Load" for maximum RF output power, iterating a few times (A). Maximum plate current (B) indicates mistuning; adjusting for SWR at the antenna (C/D) is not the proper internal tune-up procedure.

The large RF choke (射频扼流圈) to ground at the output provides a DC ground path. Its safety role is: if the plate- coupling capacitor fails short, the exposed high DC voltage is shorted to ground, blowing the AC fuse and protecting the operator (A). B's lightning/QRN claim, C's tuning role, and D's broadcast bypass are not its purpose.

The interlock switch (联锁开关) is a safety device: the moment the cover is removed it cuts the mains (市电) input, de- energizing the high-voltage power supply so a technician cannot touch live HV (A). It does not target RF input (B), filament (C), or the cooling fan (D). 天线的作用 Antenna Functions

An antenna (天线) is a transducer: it converts the guided RF current from the transmitter into radiating electromagnetic waves (radio waves). Option A is correct. Gain (B) is a directional focusing effect, not power amplification; an antenna does not generate power. C is a receiver/detector function; D is wrong.

A receiving antenna is the reciprocal of a transmitting one: it intercepts passing radio waves and converts them back into a tiny RF voltage delivered to the receiver input. Option A is correct. Gain (B) focuses, it does not "amplify" free-space waves; audio (C) is produced later by the receiver, not the antenna; D is wrong.