CRAC Class B (2025) — Questions 721–730
U0925. In free space, station
自由空间中的甲电台在145MHz联络相距100km的乙电台并获 得信号报告S8。如果双方改用频率435MHz，则信号报告变 为：【提示：收信机信号强度指示从S1至S9每级增加6dB】
Exam tips · 考试提示
435 MHz is 3× 145 MHz, adding ~9.5 dB loss = 9.5/6 ≈ 1.59 S- units. S8 − 1.59 ≈ S6.41, i.e., slightly above S6. Option A is correct.
U0926. In free space, station
自由空间中的甲电台在145MHz联络相距100km的乙电台并获 得信号报告S8。如果甲电台将发射功率减少到原来的1/4，则 信号报告变为：【提示：收信机信号强度指示从S1至S9每级增 加6dB】
Exam tips · 考试提示
1/4 power is −6 dB = exactly one S-unit. S8 − 1 = S7. Option A is correct.
U0927. An amateur station transmitting at 100 W gets an S8 report from the other station. Now it reduces power to 25 W. The other station's report should be: [Hint: receiver signal- strength indication S1 to S9, each step +6 dB]
某业余电台以100瓦发射功率工作时，对方报告信号强度S8。 现该台将发射功率降至25瓦，则对方给出的信号报告应为： 【提示：收信机信号强度指示S1至S9每级增加6dB】
Exam tips · 考试提示
100 W → 25 W is 1/4 power = −6 dB = one S-unit. S8 − 1 = S7. Option A is correct.
U0928. An amateur station transmitting at 80 W gets an S8 report. Now it drops to 5 W for a QRP experiment. The other station's report should be: [Hint: receiver signal-strength indication S1 to S9, each step +6 dB]
某业余电台以80瓦发射功率工作时，对方报告信号强度S8。现 该台将发射功率降至5瓦进行QRP实验，则对方给出的信号报告 应为：【提示：收信机信号强度指示S1至S9每级增加6dB】
Exam tips · 考试提示
80 W → 5 W is a factor of 16 = 4× quartering = 4 × (−6 dB) = −12 dB = two S-units. S8 − 2 = S6. Option A is correct.
U0929. Regarding the wave impedance Z of electromagnetic waves, which of the following descriptions is correct? (Choose all that apply.)
关于电磁波的波阻抗Z，以下描述正确的是：
Exam tips · 考试提示
Wave impedance Z = E/H. In the far field of free space Z approaches the constant ≈ 377 Ω (η0), so E and H are fixed in ratio and mutually derivable — A and B are correct. C confuses Z with feed-point impedance and D with reflection; neither is wave impedance. So A and B are correct.
U0930. In electromagnetic-field theory, the statement that intensity is inversely proportional to the square of the distance from the radiating source applies to:
在电磁场理论中，“强度与距辐射源的距离的平方成反比”的 说法适用于：
Exam tips · 考试提示
The inverse-square law for field intensity holds in the far field, conventionally defined as ≥10 wavelengths from the source. In the near field the relation does not hold, and it is not frequency- or band-specific. So A is correct.
U0931. Sometimes you need to compare the actual radiation of different omnidirectional vertical antennas at zero elevation using a field-strength meter in an open area. What should you watch out for in the measurement?
有时，我们需要在空旷场地用场强计测量比较不同型号的全向 垂直天线零仰角情况下的实际辐射效果。不过，测量时应注意 什么问题？
Exam tips · 考试提示
To measure true far-field (zero-elevation) radiation you must be in the far field, i.e., >10 λ from the antenna; otherwise near- field effects distort the reading. So A is correct; B/C/D are not the key precaution here.
U0932. A typical amateur-method for measuring antenna gain is shown.
业余条件下测试天线增益的典型方法如下图所示。用场强表或 接收机接收设置在远处同一地点，最大辐射方向朝向自己的半 波偶极天线（上）和待测天线（下）的信号。调整送至两副天 线的射频功率Po和P，使收到的场强相同。则，待测天线的增 益dBd值为：
Exam tips · 考试提示
With equal received field strength, a more efficient (higher- gain) antenna needs less power, so gain(dBd) = 10log(Po/P), where Po is the reference dipole power and P the test power. Option A is correct; B inverts the ratio; C/D are not logarithmic power ratios.
U0933. A typical amateur-method for measuring antenna gain is shown.
业余条件下测试天线增益的典型方法如下图所示。用场强表或 接收机接收设置在远处同一地点，最大辐射方向朝向自己的半 波偶极天线（上）和待测天线（下）的信号。调整送至两副天 线的射频功率Po和P，使收到的场强相同。则，待测天线的增 益dBi值为：
Exam tips · 考试提示
The measured value is gain relative to a dipole (dBd) = 10log(Po/P); converting to dBi adds 2.15 dB (since a dipole is 2.15 dBi). So dBi = 10log(Po/P) + 2.15 — option A. B inverts the ratio; C uses 20log wrongly; D is not a ratio.
U0934. If classified by propagation form, radio waves are roughly divided into:
若按传播形式分类，无线电波大体分为：
Exam tips · 考试提示
By propagation form (传播形式), radio waves are ground wave (地面波), sky wave (天波), space wave (空间波), and scatter wave (散射波). B classifies by frequency band, C by modulation, D by waveform — not by propagation. So A is correct.