CRAC Class A (2025) — Questions 381–390

A short, thick DC cable minimizes resistance, reducing the voltage drop when the PA draws high current during transmit, so the rig gets stable voltage. B/C/D are unrelated or absurd.

Simplex = both stations transmit and receive on the same frequency, taking turns (PTT switches one at a time). Two PTT handhelds on the same frequency is simplex. A repeater link (D) is semi-duplex; phones (B/C) are full duplex.

True duplex = simultaneous two-way transmission. A ground station using a separate 430 MHz receiver and 144 MHz transmitter to a satellite receives and transmits at the same time → full duplex. B (QSK) is very fast T/R switching (effectively full break-in but still alternating, not simultaneous), while C is semi-duplex and D is simplex.

Semi-duplex = one end full duplex (the repeater receives and transmits at once on different frequencies) and the other end simplex (the handheld, PTT-switched). That is exactly A. B is QSK, C is simplex, D is full duplex.

PTT = Push-To-Talk: pressing the key grounds a line that switches the rig from receive/standby to transmit. B is speech processing, C is a preamp, D is automatic antenna tuning (ATU) — none is PTT.

SQL is the universal front-panel/Menu abbreviation for Squelch (静噪). Squelch mutes the receiver's audio output when the received signal-to-noise ratio falls below a set threshold, so the operator is not subjected to constant background hiss. Option A states exactly this. The other choices describe unrelated functions: B is speech compression (PROC), C is the preamplifier (PRE), and D is automatic antenna tuning (AT/TUNE).

Squelch (静噪) is a receive-side gate: it suppresses the audio path whenever no usable signal is present, so the operator hears silence instead of receiver hiss. Option A captures this precisely. B and C describe noise filtering/processing of the actual audio, which squelch does not do; D describes automatic gain control (AGC), a different function.

Because squelch mutes the audio whenever the signal is below its threshold, a very weak station that rides near or below the threshold will be cut off or hard to hear. Disabling squelch (or setting the threshold to 0) removes that gate so the audio path passes even the weakest signal. B only raises loudness, not sensitivity; C and D concern VOX (transmit voice control), which is irrelevant to receiving.

The "squelch tail" is a well-known artifact of carrier/noise- operated squelch on FM. The circuit senses the disappearance of the signal by monitoring the high-frequency (quieting) noise at the discriminator output; the sense-and-switch action has a finite time constant, so for a few tens of milliseconds after the carrier drops the audio path is still open and the harsh receiver hiss leaks through. A describes this correctly. B/C attribute it to the transmitter (it is a receiver-side effect), and D confuses it with AGC.

VOL is the universal symbol for Volume (音量), i.e. the audio output level control. Option A is correct. B describes speech compression (PROC), C the preamplifier (PRE), and D automatic antenna tuning (AT/TUNE) — all unrelated to VOL.