CRAC Class C (2025) — Questions 701–710

The optimum working frequency for reliable DX is typically about 0.85 × MUF — just below the MUF so the signal is still refracted but not marginal. With MUF = 20 MHz, the best amateur band is 18 MHz (the 17 m band), which lies safely below 20 MHz. 21 MHz and 24 MHz exceed the MUF and will not be refracted; 14 MHz works but is well below optimum, giving a lower rate than 18 MHz.

The principal ionospheric layers affecting HF propagation, from highest to lowest altitude, are F2 (highest), F1, E, and D (lowest). The D layer (lowest) mostly absorbs, while F2 (highest) is the main reflector for long-distance HF. Options listing non-standard layer names (C1, C2, E2) or wrong ordering are incorrect.

By electron density, the same principal layers rank F2 (highest density, the strongest refractor), F1, E, and D (lowest). The F2 layer's high density is what allows it to refract the highest frequencies. The ordering by density coincides with the altitude ordering here, so F2, F1, E, D is correct.

The F2, F1, and E layers reflect (refract) HF sky waves (A), while the lowest D layer does not reflect but absorbs energy — especially at lower frequencies and by day (B). "D1/D2" and "C1/C2" are not the standard layer nomenclature for this effect, so C and D are not part of the marked answer. The official key marks A and B.

The gray line (灰线) is the terminator — the moving boundary on Earth between the sunlit hemisphere and the dark hemisphere. Along this line the D-layer absorption is minimal on both ends while the F-layer is still active, creating excellent low-band DX conditions. It is not the auroral zone (B), an MUF contour (C), or a great-circle path (D).

At the gray line (terminator) the D-layer absorption is low at both ends while the F-layer remains reflective, so a two-station path straddling the terminator can support reliable multi-hop sky-wave (天波) propagation on low bands (A). Options B, C, D describe geometries or advice contrary to the well-known gray- line effect.

Every great-circle path between two stations has a short path and a long path (the complementary bearing, 180° opposite). To work the long path (长路径) you aim your beam 180° away from the short-path bearing (A). The gray line and aurora directions (B, C, D) are unrelated to the long-path aiming rule.

Ionospheric propagation prediction requires the solar conditions (sunspot number 太阳黑子平均数, and a flux/geomagnetic index such as the A/K index — A and B), the great-circle geometry between the two stations (their locations — C), and the time of day/month (D), since ionization varies diurnally and seasonally. All four are mandatory inputs, so A, B, C, D are correct.

The 10.7 cm solar flux (F107, 太阳通量) normally ranges from about 65 (solar minimum) to roughly 300 (solar maximum). For prediction purposes the low end is taken near 50 and the high end near 300, so 300 is the correct maximum value.

The K-index (K指数) is a three-hourly geomagnetic-activity measure on a quasi-logarithmic scale from 0 (very quiet) to 9 (extreme storm). The A-index is the daily average converted to a linear scale. Thus the K-index range is 0–9.