CRAC Class B (2025) — Questions 691–700

In a loop antenna the radiated magnetic field lies along the loop's axis, i.e., perpendicular to the plane of the loop, while the electric field lies in the plane of the loop — so A is correct. B is wrong (a self-resonant loop's circumference is about a wavelength, not radius 1/4 λ); C is not the defining correct fact; D's bandwidth claim is not the accepted correct statement here.

A magnetically coupled tunable (magnetic) loop can be matched close to 1:1 SWR by sizing its elements (A); the tuning capacitor must withstand the high RF voltage across a small loop, so a high-voltage rating helps power handling (B); and a larger loop radiates more efficiently but tunes only to lower frequencies (C). D is false — the loop is not defined by losing half its energy, and while small it is not that lossy by principle.

The 100–200 km gap is the skip / dead zone where skywave has not returned but ground wave is weak. NVIS techniques close it: lower bands (A), a low horizontal dipole or sloper that radiates upward (B), and a ground reflector sending the main lobe overhead (C). A tiny 1 m loop (D) is too inefficient on HF to help. So A, B, C are correct.

An antenna tuner (ATU / 天调) transforms the antenna system's input impedance to the load impedance the transmitter needs (typically 50 Ω), so the transmitter sees a match and delivers full power. B is wrong — the tuner does not raise the antenna's own radiation efficiency; C is an incidental effect but A is the precise definition; D describes an antenna switch, not a tuner.

The tuner lets the transmitter see a matched load and thus deliver its rated RF power (A). It does NOT make the antenna itself resonant or raise its radiation efficiency (B false), nor does it remove SWR on the feed line between tuner and antenna (C false), and it does not primarily dissipate reflected power as heat (D false).

To keep the long feed line at 50 Ω and minimize mismatch loss, place the ATU right at the antenna so the mismatched run is as short as possible, then the SWR meter, then the 50 Ω feed line to the rig. Option A does this. B/C/D place the tuner at the shack, leaving a long mismatched feed line.

Efficiency is highest when the ATU is at the antenna (tower top, scheme 1), eliminating the mismatched feed-line run; next are schemes with the tuner at the tower base or with the meter in the shack (2, 3); worst is the tuner in the shack (scheme 4) leaving a long mismatched feed line. Option A matches this ranking.

EIRP (等效全向辐射功率, e.i.r.p.) = power delivered to the antenna × the antenna's absolute gain Gi. The 1 W limit is on EIRP, so the feed power must satisfy P_feed × Gi ≤ 1 W, i.e., P_feed ≤ 1/Gi — option A. B confuses EIRP with transmitter output; C/D confuse it with radiated power after losses.

EIRP = P_in × 10^(G_dBi/10). 3.85 dBd = 3.85 + 2.15 = 6.0 dBi; the linear factor is 10^(6.0/10) = 10^0.6 ≈ 3.98 ≈ 4. So EIRP ≈ 10 W × 4 = 40 W. Option A is correct.

ERP (有效辐射功率, e.r.p.) uses dipole-referenced gain: 5.15 dBi = 5.15 − 2.15 = 3.0 dBd; the linear factor is 10^(3/10) = 2. So ERP = 10 W × 2 = 20 W. Option A is correct. (EIRP would be ~32.7 W, but the question asks for ERP.) 传输线 Transmission Lines