CRAC Class C (2025) — Questions 531–540

QSK / full break-in means the transceiver switches to receive between individual CW elements (dots/dashes), so you can hear the other station — or your own keying — in the infinitesimally short gaps (A). It does not require manual switching (B), speed up the keyer (C), or raise power (D).

Selectable IF bandwidth lets you match the filter to the mode (narrow for CW, wide for AM), rejecting out-of-band noise and adjacent stations to improve SNR (A). The other options describe multi-receiver architectures, not the benefit of bandwidth choice itself.

Matching bandwidth to mode: CW is narrow (100 Hz or 400 Hz) (A); SSB needs ~2.7 kHz (B); AM is widest among these voice modes at ~6 kHz (C). FT8, however, has a very small necessary bandwidth (~50 Hz) and is best received with the narrowest filter (100 Hz), not 400 Hz, so D is incorrect. The official key marks A, B, C.

Matching the receive bandwidth to the mode's necessary bandwidth excludes the maximum amount of out-of-band noise and interference, giving the optimal SNR (A). Input impedance (B), dynamic range (C), and band visibility (D) are not the reasons for bandwidth matching.

A receive preamplifier (LNA) must sit as close to the antenna as possible — between the antenna and the receiver — so it boosts the weak signal before feed-line and receiver noise are added (A). Placing it after the PA (B), at the tuner (C), or at audio output (D) would not improve receive sensitivity and could damage it.

A low-pass filter that suppresses spurious/harmonic emissions belongs in the RF transmit path, between the transmitter and the antenna, so unwanted products are removed before they radiate (A). It has nothing to do with the receiver (B), power supply (C), or microphone (D).

A higher-order LC filter has a steeper roll-off, so it rejects harmonics (e.g., 2nd harmonic into the doubled band) more strongly (A). Lower order (B) is weaker; order does not reduce loss (C is false — higher order usually adds a little loss) and does not by itself raise power handling (D).

A quarter-wave coax stub presents different reactance depending on whether it is shorted or open at the end. A 40 m quarter-wave shorted stub is a high-impedance (parallel- resonant) trap at 40 m/15 m (harmonics) and passes those while shorting 20 m/10 m (A); a 40 m quarter-wave open stub is the complement, passing 20 m/10 m and shorting 40 m/15 m (B). The half-wave versions (C, D) do not match these standard behaviors, so the key marks A and B.

The RX ANT jack lets you feed a dedicated receive-only antenna (e.g., a magnetic loop or Beverage) into the receiver, bypassing the transmit antenna for better receive (A). It is not for a second receiver (B), nor for connecting the transmit antenna (C, D).

A built-in sub-receiver is typically used for true split (independent VFO/antenna on a different frequency) (A) and for diversity reception (same frequency, separate antenna, combined to beat fading) (B). I/Q output (C) and IF output (D) are different features (panadapter/SDC interfaces), not the sub-receiver's purpose.