CRAC Class C (2025) — Questions 791–800

Amateur radio direction finding (业余无线电测向 / ARDF) is a popular sport where operators use directional antennas to locate hidden transmitters — ideal both for competitions and for hunting down QRM (interference, 干扰). The other techniques (Doppler sounding, echo location, PSD analysis) are more specialized research tools, not the standard amateur activity for this purpose.

ARDF (amateur radio direction finding, 业余无线电测向) fundamentally requires a directional antenna (定向天线) to determine bearings toward a hidden transmitter. RF bridges, noise bridges, and SWR bridges are antenna/impedance measurement tools and are not used for direction finding.

Under the Measures for the Administration of Amateur Radio Stations (《业余无线电台管理办法》), any transmitting station — including the hidden transmitters (隐蔽台) used in ARDF — is an amateur radio station (业余无线电台) and must be approved by the radio management authority (无线电管理机构) before establishment and operation. They are not license-exempt or outside radio management (B/C/D are incorrect).

The IARU (国际业余无线电联盟) ARDF rules specify that the 144 MHz beacon uses A2A (toneless AM with audio Morse) at 8–15 WPM, and its occupied bandwidth must not exceed 6 kHz. 12.5/25 kHz are FM-channel figures and 100 Hz is far too narrow for audio Morse sidebands, so 6 kHz (A) is correct.

In the far field (远场) the wave impedance (波阻抗) of free space (~377 Ω) is essentially constant, so the loop (magnetic) and whip (electric) combination gives a stable cardioid. In the near field (近场) the impedance varies with distance, unbalancing the E/H combination and degrading the null, which requires re- tuning the whip length. The correct explanation is A; B–D describe unrelated overload/heating effects.

A center-wound ferrite-rod (磁棒) antenna behaves magnetically like a small loop, so it can substitute for the shielded loop while being far smaller (A). However, an unshielded ferrite rod also picks up electric-field/capacitive signals near the transmitter, disturbing the null — a practical issue operators must manage (B). Options C and D incorrectly claim magnetic saturation acts like AGC/attenuator.

A two-element (driven + reflector) Yagi (八木天线) still provides usable unidirectional gain while being light and fast to swing in a race (A). Near the transmitter, finders rely on the antenna null (零点) rather than peak bearing, so having only two elements is acceptable (B). C (adding a director on the fly) and D (3- element worsening) are not the given rationale.

The HB9CV is a compact two-driven-element Yagi where the elements are fed with a ~225° phase shift to produce strong, symmetric unidirectional patterns (good for ARDF bearings) with a wide bandwidth and easy portability (A, B). It is usable for both direction finding and ordinary contacts, and its SWR is not inherently "high," so C and D are incorrect.

Two tones alternating at a steady rate on SSB is the classic audio signature of 2FSK RTTY (无线电传), where the mark and space tones shift back and forth (e.g., 170 Hz shift at 45.45 baud). PSK31 is a single-phase-reversing tone, FT4 is 4FSK (four tones), and SSTV/FAX have a continuously varying sweep — so RTTY (A) is the correct identification.

PSK31 (BPSK31/QPSK31, 相移键控) appears as a single steady audio tone whose phase reversals make it "flutter" or shimmer on a waterfall, without changing pitch. Both BPSK31 and QPSK31 share this single-tone signature (A, B). FT4 uses four distinct tones and SSTV/FAX sweep in frequency, so C and D do not match the "constant tone wobble."