CRAC Class C (2025) — Questions 691–700

The attenuation of a ground wave (地波) is governed primarily by the wave frequency, the electrical conductivity of the ground over which it travels, and the distance covered. Higher frequencies are absorbed more strongly by the ground, and poor ground conductivity (e.g., dry soil) increases loss. Transmit power and antenna gain affect the signal level at the start but are not part of the propagation attenuation law itself, so B, C, and D are incorrect.

The ionosphere (电离层) is the ionized layer of the upper atmosphere that refracts (bends) high-frequency (HF, short- wave) radio waves back toward Earth, allowing long-distance "sky-wave" (天波) propagation around the globe. The troposphere, stratosphere, and magnetosphere do not provide this global reflection mechanism for HF.

The skip zone (静寂区 / silent zone) is the ring-shaped region around a transmitter where neither the ground wave (地波) nor the refracted sky wave (天波) reaches — the sky wave is refracted back to Earth beyond a certain distance (the skip distance, 越距). It is a direct consequence of HF ionospheric reflection geometry, not satellite coverage or obstacle shadowing.

Attenuation (衰减) is a steady, average reduction in signal power as it travels through a medium or circuit. Fading (衰落) is a time-varying fluctuation of the received amplitude caused by changing propagation paths (e.g., multipath or ionospheric changes). They are distinct: one is a constant loss, the other a random variation. Option A defines both correctly.

Multi-hop sky-wave propagation makes the ionosphere act like a rotating, scattering mirror: the polarization (极化) of the returning wave is randomized (A), and different frequency components fade independently — this is selective fading (选择 性衰落), matching B. Sidebands do not "reverse" (C), and broadcast audio intrusion (D) is ordinary interference, not a propagation effect. Hence only A and B are correct.

A 2FSK signal (such as RTTY, 无线电传) has two equal tones; in the ionosphere the two frequencies experience selective fading (选择性衰落) independently, so their received amplitudes vary randomly. Receiver AGC acts on the overall signal and would not make the two peaks differ; transmitter ALC and operating- point drift would affect both tones together, not split them. Hence A is correct.

Because ionospheric (sky-wave) reflection randomizes the polarization (极化) of the returned wave, there is no fixed optimum polarization to "arrange" between stations — it is unpredictable (A). This contrasts with line-of-sight (space- wave) links where matching polarization matters. Options B, C, D assume a deterministic polarization relationship that does not hold for sky-wave paths.

On a 2000 km sky-wave (天波) path the ionosphere randomizes polarization, so changing one antenna from horizontal to vertical does not give a predictable improvement or degradation (A). Because the returned wave's polarization is scrambled, "better," "worse," and "unchanged" are all unreliable predictions. Only A correctly expresses this uncertainty.

Ionospheric short-wave propagation is governed by solar and geomagnetic conditions (sunspots 太阳黑子, solar flares 太阳耀 斑, geomagnetic activity 地磁活动 — A), by time of day and season (B), and by the chosen frequency and path length (C). Weather such as clouds and air temperature (D) does not significantly affect ionospheric refraction. Thus A, B, C are correct.

The Maximum Usable Frequency (最高可用频率 / MUF) is the highest frequency that will still be refracted back to Earth between two points via the ionosphere (A). Because ionization is driven by sunlight, the MUF is typically low at night and high during the day (B). Per the official key, only A and B are marked correct; C and D are not part of the marked answer set.