CRAC Class B (2025) — Questions 271–280

A linear transponder (线性转发器) receives a whole uplink band, translates it in frequency (shift, or shift + inversion for some modes), and retransmits it without demodulating — preserving each signal's modulation (A). Options B/C/D all involve demodulation and re-modulation, which describe a "repeater" type, not a linear transponder.

The defining feature of a linear transponder is that it passes the whole band through without altering modulation, so CW, SSB, FM, digital, etc. can all be relayed (A). B is true of the hardware but not the distinctive system feature asked; C describes demodulate-and-remodulate (not linear); D wrongly limits it to amplitude modes and misstates the principle.

Periodic fast fading of a satellite downlink is typically caused by the satellite (or its antenna) tumbling, so the polarization rotates and no longer matches your fixed antenna (A). The ionosphere (B) causes slower/scintillation effects, not this mechanical periodic fade; C and D are not the cause.

To predict a satellite's position the software must have that satellite's Keplerian elements (TLE/orbital parameters); without them it cannot compute the orbit (A). Magnetic parameters (B), sunspot number (C) and F10.7 flux (D) relate to propagation, not to geometric orbit prediction.

Because an LEO satellite only sees a small area at a time, global messaging is achieved by store-and-forward: the satellite records your uplink and repeats it on a later pass over another station (A). B/C/D do not overcome the limited footprint of a single pass.

A linear transponder has a fixed total power budget; an excessively strong uplink "captures" the transponder (AGC reduces gain), lowering the downlink power available to everyone else and degrading their communications — so overpowering must be opposed (A). B and C are false; D is partly true (waste/pollution) but misses the core harm to other users, and A is the complete correct statement.

The practical rule is to adjust your uplink so your downlink signal is roughly equal to the satellite's beacon — strong enough to be copied, but not stronger (A). Telemetry (B) usually shows whole-transponder status, not your level; audio distortion (C) is a receiver issue; asking others (D) is unreliable. A is the standard method.

At VHF/UHF the path to a satellite is essentially line-of-sight through space, so the dominant loss is free-space path loss (which grows with distance and frequency) (A). The ionosphere neither absorbs nor reflects VHF/UHF significantly (B, C are false), and ground absorption (D) is not the main factor for the space link.

The letter designations (Mode A, B, K, JA, JD, S, T…) specify which amateur bands are used for the uplink and downlink and what modulation the transponder supports (e.g., Mode A = 2 m uplink / 10 m downlink) (A). They are not country prefixes (B), power classes (C), or orbital data (D).

The Doppler effect (多普勒效应 / Doppler shift) is a frequency offset produced by the relative motion (changing distance) between source and observer (A). B/C/D describe amplitude fading from propagation or polarization, not the Doppler frequency shift.