CRAC Class C (2025) — Questions 491–500

Contest scoring is done by cross-checking (matching) your log against the logs of the stations you contacted — only mutually confirmed QSOs count (A). Logs are not optional (B), you must not delete valid contacts (C), and VHF is no excuse to skip logging (D). 无线电系统基本原理 3 C H A P T E R Basic Principles of Radio Systems 本章从收发信机、天线、传输线到调制与卫星，建立无线电系统的整体认知。This chapter builds a systems-level understanding from transceivers, antennas and feed lines to modulation and amateur satellites. 收发信机与接收系统 Transceivers and Receiver Systems

A repeater using one antenna for both receive (uplink) and transmit (downlink) needs a duplexer — a pair of filters that let the receiver and transmitter share the same antenna while isolating them (A). A T/R switch is for half-duplex single- station use, a power divider splits power, and a circulator is directional but not the standard shared-antenna solution described.

A duplexer is essentially a set of cavity/coaxial filters: one passes the transmit frequency to the antenna while rejecting it from the receiver, and the other does the reverse, so the strong transmitted signal cannot overload or feed back into the receiver (A). It is not a switch (B), a mere matching network (C), or a one-way circulator path (D).

Third-order intermodulation products take the form 2F₁ − F₂ or F₁ + F₂ − F₃. Here the repeater's own downlink fT and an external signal fX mix to produce the uplink fR: 2fT − fX = fR ⇒ fX = 2fT − fR; and 2fX − fT = fR ⇒ fX = (fT + fR)/2. Both are the standard formulas, so A is correct.

Because no station is actually on the uplink yet the repeater keys up with voice-like noise, the likely cause is third-order intermodulation: two strong off-frequency signals mix in the repeater receiver to produce a product exactly on the uplink frequency, opening the squelch (A). "Definitely malicious" (B) is too strong; parasitic oscillation (C, D) would not mimic voice from external signals.

If the repeater's own downlink leaks back into its receiver (poor isolation between uplink and downlink), the receiver keeps seeing a "signal" and holds the transmitter on — a self-locking loop (A). This is a technical fault, not necessarily jamming (B), and power-supply issues (C, D) wouldn't cause a sustained key- up tied to the uplink path.

After the legitimate uplink stops, if the repeater keeps transmitting with voice-like noise, an external signal mixing with the repeater's own downlink can generate an intermod product on the uplink frequency that re-opens the squelch (A). As in U0842, this is intermod, not necessarily jamming (B) or power issues (C, D).

A headphone (耳机) blocks ambient room noise and couples the received audio directly to the ear, dramatically improving the ability to copy (抄收) weak SSB/CW in a noisy environment. Option A is correct. A low-pass filter (B) is an RF/audio component, not a listening device; a video display (C) and boom mic (D) do not help receiving.

NB = Noise Blanking (抑噪): it detects short, high-amplitude impulse spikes (e.g. from automobile ignition) and blanks them for a few microseconds. SQL = Squelch (静噪): it mutes the audio entirely when the signal is below a threshold. They are distinct functions. Option A states both correctly; B, C, D wrongly merge them or swap their definitions.

Spark-plug impulse noise is a burst of sharp spikes. The noise blanker (NB / 抑噪) is specifically designed to detect and clip these impulses before they reach the AGC and audio, preventing the AGC from pumping the desired SSB/CW signal down. Option A is correct. Lowering squelch (B) only opens the audio gate wider (more noise); tuning off (C) or RIT (D) does not remove impulse noise.