CRAC Class B (2025) — Questions 701–710
U0905. Transmission lines are commonly used to connect the station and the antenna. The more common transmission lines are: (Choose all that apply.)
传输线通常用来连接电台与天线。比较常见的传输线有：
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Common transmission lines (传输线) are coaxial cable (同轴电 缆), parallel (open-wire) feed line (平行馈线), and waveguide (波 导). A corrugated hose (D) is not a transmission line. So A, B, C are correct.
U0906. Regarding the characteristic impedance of coaxial cable, which of the following statements is correct? (Choose all that apply.)
关于同轴电缆特性阻抗，以下说法正确的是：
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The coax characteristic impedance is Z0 = (60/√εr)·ln(D/d), where D is the outer-conductor inner diameter and d the inner- conductor outer diameter. Increasing D/d raises Z0 (A), and decreasing the dielectric constant εr raises Z0 (B). Shield density (C) and overall outer diameter (D) do not set Z0. So A and B are correct.
U0907. A 50 Ω coaxial cable is terminated at its far end with a 50 Ω load resistor. If you measure the near-end impedance at different frequencies with an antenna analyzer, the reading is:
一根特性阻抗为50欧的同轴电缆的末端接有一个50欧姆负载电 阻。如果用天线阻抗分析仪测量该电缆始端在不同频率下的阻 抗，则读数为：
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A transmission line terminated in its characteristic impedance (Z0 = 50 Ω) presents exactly Z0 at the input for any length and any frequency — the line is flat. So A is correct; B/C/D describe mismatched (open/short) terminations, not a 50 Ω load.
U0908. A 50 Ω coaxial cable is left open at its far end. If you measure the near-end impedance at different frequencies with an antenna analyzer, the reading is:
一根特性阻抗为50欧的同轴电缆的末端开路。如果用天线阻抗 分析仪测量该电缆始端在不同频率下的阻抗，则读数为：
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For an open-circuited line the input impedance is Z_in = −j·Z0·cot(βl). At a half-wavelength (even multiples of 1/4 λ) the open repeats → ∞; at a quarter-wavelength (odd multiples of 1/4 λ) it transforms to a short → ≈0. So A is correct; B has the values swapped; C/D are for terminated lines.
U0909. A 50 Ω coaxial cable is short-circuited at its far end. If you measure the near-end impedance at different frequencies with an antenna analyzer, the reading is:
一根特性阻抗为50欧的同轴电缆的末端短路。如果用天线阻抗 分析仪测量该电缆始端在不同频率下的阻抗，则读数为：
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For a short-circuited line Z_in = j·Z0·tan(βl). At a quarter- wavelength (odd multiples of 1/4 λ) it transforms to an open → ∞; at a half-wavelength (even multiples of 1/4 λ) it repeats the short → ≈0. So A is correct; B swaps the values; C/D are for terminated lines.
U0910. When measuring a mismatched antenna's impedance with an antenna analyzer, you will find the reading changes with the length of the jumper cable between the antenna and the meter. To ensure measurement accuracy, you can take the following measure:
在用天线阻抗分析仪测量某天线失配情况下的阻抗表现时，我 们会发现仪表的读数随天线和仪表间电缆跳线的长度而变。为 保证测量的准确性，可以采取如下措施：
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On a mismatched line the impedance repeats every half wavelength (transformation period λ/2). A jumper that is an integer number of wavelengths (here one full measuring wavelength) transforms the antenna impedance unchanged to the meter, removing length dependence. So A is correct; B/C/D address connectors/shielding but not the length-dependent transformation.
U0911. The electric field produced by a source (e.g., an antenna) creates a displacement current, exciting a varying magnetic field around it. That magnetic field in turn excites a varying electric field a little farther away. This cycle repeats, forming an electromagnetic wave traveling to the distance. Which of the following descriptions of electromagnetic-wave propagation is correct? (Choose all that apply.)
由场源（比如天线）产生的电场会形成位移电流，在其周边激 起变化的磁场。该磁场又会在稍远处再次激起变化的电场。这 个过程周而复始，形成传向远方的电磁波。关于电磁波的传 播，以下描述正确的是：
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Displacement current density is proportional to ∂E/∂t, hence to ω·E, so at a fixed field strength higher frequency means stronger displacement current and more radiation (A correct). Radiation also requires an open (non-closed) structure so the field can escape (B correct). C's wording (fixed energy → stronger field with frequency) is not the standard correct statement, and D is the opposite of B (radiation needs an open, not closed, system). So A and B are correct.
U0912. The two components of an electromagnetic wave are:
电磁波（electromagnetic waves）的两个组成部分是：
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An electromagnetic wave (电磁波) consists of mutually coupled electric and magnetic fields oscillating perpendicular to each other and to the direction of travel. Voltage/current (B), impedance/power (C), and the ionizing/non-ionizing classification (D) are not its two field components. So A is correct.
U0913. How fast do radio waves travel in a vacuum?
无线电波在真空中的传播速度有多快？
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Radio waves are electromagnetic waves and travel in vacuum at the speed of light c ≈ 3×10^8 m/s, independent of frequency. B confuses it with sound; C/D are wrong because in vacuum the speed is fixed and frequency-independent. So A is correct.
U0914. When an electromagnetic wave propagates in a vacuum, the distance it travels in each cycle is the same. This distance is called:
电磁波在真空中传播时，其各个周期的传播距离都是相等的。 该距离叫做：
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The distance an EM wave travels in one period is the wavelength (波长). Waveform (B) is the shape, wave speed (C) is c, beam (D) is the radiation pattern direction. So A is correct.