CRAC Class B (2025) — Questions 751–760
U0955. The average period of sunspot activity is about:
太阳黑子活动的平均周期约为：
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The solar (sunspot) cycle (太阳黑子活动周期) averages about 11.2 years. Each successive cycle differs in intensity (some are strong, some weak), so the correct statement is A: ~11.2 years with varying activity. The 38-year figure refers to the longer Gleissberg modulation, not the basic cycle.
U0956. The strength of sunspot activity is described by the "sunspot number (SSN)." The general rule is:
太阳黑子活动的强弱是用"太阳黑子平均数（SSN）"来描述 的。其一般规律为：
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A higher sunspot number (SSN, 太阳黑子平均数) means a more strongly ionized ionosphere and a higher MUF, which opens the higher HF bands (10 m, 6 m) for long-distance (DX) work. So larger SSN favors short-wave DX (A). Smaller SSN (B) suppresses it; C and D overstate the exceptions.
U0957. A solar flare can trigger a Sudden Ionospheric Disturbance (SID). Its effect on short-wave communication is:
太阳耀斑可以引发电离层扰动（SID）。其对短波通信的影响 是：
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A Sudden Ionospheric Disturbance (电离层扰动 / SID) dramatically increases D-layer absorption on the sunlit side, and the effect is strongest at lower frequencies (the D layer absorbs low bands like 80 m/160 m most severely, while higher bands such as 15 m/10 m are less affected). So A is correct; the disturbance is a daytime (not night) effect.
U0958. Amateur operators often use a parameter abbreviated F107 when predicting HF propagation. Its meaning is:
业余无线电爱好者在预测HF传播时经常用到一个缩写为F107的 参数。其意义是：
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F107 (太阳10.7cm波长射电辐射通量指数) is the solar flux at a wavelength of 10.7 cm (a microwave proxy for solar UV/EUV output that drives ionospheric ionization). It is a primary input to HF propagation models. It has nothing to do with 107 MHz broadcasts, 10.7 MHz attenuation, or a frequency ratio.
U0959. Amateur operators often use the parameter abbreviated F107 when predicting HF propagation; its value is generally in the range 50–300. If this value increases, then:
业余无线电爱好者在预测HF传播时经常使用缩写为F107的参 数，其值大体都在50-300的范围内。如果该数值增大，则：
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A higher F107 means stronger solar (quiet-Sun) radio flux, more ionizing UV/EUV, and a denser ionosphere — which raises the MUF and improves F-layer reflection of DX signals (A). Option D states the opposite, and B/C confuse the 10.7 cm index with actual 10.7/107 MHz radio propagation.
U0960. China's Radio Frequency Allocation Regulations (《无线电频率 划分规定》) state that the 122.25–123 GHz amateur band is allocated to the amateur service but excludes the amateur- satellite service. This is because:
我国《无线电频率划分规定》指出，122.25-123GHz业余频段 用于业余业务，但是不包括卫星业余业务。这是因为：
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The Radio Frequency Allocation Regulations of the PRC (《无线 电频率划分规定》) allocate 122.25–123 GHz to the amateur service (业余业务) but not the amateur-satellite service (卫星业 余业务) because the molecular-absorption bands of the atmosphere near 60, 120, and 183 GHz cause severe attenuation that prevents reliable ground-to-space links. The other options cite nonexistent cosmic-ray or equipment limitations.
U0961. Among the following bands, the one most severely affected by rainfall is:
以下频段受降雨影响最为严重的是：
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Rain attenuation increases sharply with frequency; the EHF (极 高频 / millimeter-wave) band around 30–300 GHz is the most rain-affected because raindrop size becomes comparable to the wavelength. HF, LF, and even UHF are far less affected. Hence EHF (millimeter wave) is correct.
U0962. According to ITU recommendations, vegetation absorbs radio waves to some extent. The general rule is:
根据ITU的建议，植被在一定程度上吸收无线电波。大体规律 是：
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Per the ITU (国际电信联盟) recommendations, foliage/vegetation absorption of radio waves increases with frequency (higher frequencies, especially VHF/UHF and above, are absorbed more by leaves and moisture). So "the higher the frequency, the greater the absorption" (A) is correct; frequency is the governing factor, not species/season alone. 发射机与调制 Transmitters and Modulation
U0501. When testing a self-made amateur radio transmitting device, before it passes inspection by a radio testing organization and obtains a radio station license, you should connect (or connect after series-connecting the necessary instruments) to the device's antenna port:
自制业余无线电发射设备，在经无线电检测机构检验合格并取 得电台执照之前，应在调测时在设备的天线端口连接（或在串 联必要的仪表之后连接）：
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During bench testing of a transmitter (especially a homebrew one) that is not yet licensed, the output must be terminated in a dummy load (假负载) so that no radio-frequency signal is radiated into the air, which would cause unlawful interference. A dummy load is a non-radiating, properly matched resistor. Real antennas (B, C, D) would radiate and are not permitted for unlicensed/uncertified testing. This aligns with the principle that an amateur radio station (业余无线电台) may only radiate after passing inspection and being licensed.
U0502. In a radio transmitter, the role of the modulator is:
在无线电发射机中，调制器的作用是：
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Modulation (调制) means impressing the information signal onto an RF carrier. The modulator (调制器) uses the baseband/modulating signal to vary the carrier's amplitude (AM), frequency (FM) or phase (PM). Option B describes a PA biasing function, C is antenna matching (impedance matching), and D is a spectrum/bandwidth control function — none define modulation. So A is correct.