CRAC Class C (2025) — Questions 641–650

U0885. The figure shows the 14/21/29 MHz triband dipole commonly used by Chinese amateur radio operators for field communication. The functions of components A, B,

下图所示是我国业余无线电爱好者开展野外通信活动常用的 14/21/29MHz三频段偶极天线。图中部件A、B、C、D的作 用分别为：

U0886. The element of a resonant dipole must be at least half a wavelength long. If erection space forces you to shorten the element, inserting inductance into the element compensates the lost reactance and resonates the antenna at the desired frequency. To raise transmit efficiency, the best coil position depends on the site. The figure shows three loading schemes. Assuming the (grey) elements are equal in length, the transmit efficiency ranking of schemes A,

谐振偶极天线的振子长度最短也要半个波长。如果架设天线时 因条件受限而不得不将振子缩短，那么在振子之中串入电感可 以补偿失去的感抗，使天线谐振在所需频率下。为了提高发射 效率，应在振子的什么位置串入电感需根据架设条件择优确 定。下图给出三种加感方案。假设振子（灰色部分）均等长， 则A、B、C三种方案按发射效率可排列为：

U0887. For amateur services that must change bands frequently, operators sometimes parallel the dipoles of the used bands and feed them together, making a multi-band antenna operable with a single cable. The figure shows an example built from 7, 14 and 28 MHz half-wave elements. Which of the following analyses of its working principle is correct?

对于需要频繁改换频段的业余无线电业务，爱好者们有时会将 所用频段的偶极振子并联起来一同馈电，构成一种用一根电缆 即可操作的多频段天线。下图给出了一副由7、14和28MHz半 波长振子构成的多频段天线实例。关于其工作原理，以下分析 正确的是：

U0888. What is a Yagi antenna (八木天线)?

什么是八木天线？

U0889. A Yagi is a directional antenna made by adding directors and a reflector to a normal dipole (driven element). The visual method to determine the direction of maximum radiation of a full-size Yagi is:

八木天线是在普通偶极天线（主振子）的基础上通过添加引向 器和反射器而制成的一种定向天线。目视确定全尺寸八木天线 发射方向的方法是：

U0890. How is the front-to-back ratio (前后比) of a Yagi antenna defined?

八木天线的“前后比”是如何定义的？

U0891. A traveling-wave antenna radiates as RF current travels along a continuously matched long-wire structure. Common amateur traveling-wave antennas are the T2FD and the Beverage (贝伐列奇). Compared with standing-wave antennas, the characteristic of traveling-wave antennas is:

行波天线依靠射频电流在长导线组成的连续匹配回路中行进而 产生辐射。业余通信常用的行波天线有T2FD和贝伐列奇 （Beverage）。与驻波天线相比，行波天线的特点是：

U0892. Traveling-wave antennas commonly used in amateur radio, such as the T2FD and the Beverage (贝伐列奇), have a non- inductive resistor at the antenna termination. The correct statement about the function of this resistor is:

业余无线电通信中常用的行波天线，如T2FD和贝伐列奇 （Beverage），其天线终端都接有无感电阻。关于该电阻的作 用，正确说法是：

U0893. Xiaoming connects a coil at the midpoint of a 20 m wire and lifts the wire horizontally to a hanging position slightly above 20 m. The other end of the coil connects to another 20 m wire that drops vertically toward the ground. He connects the feed-line center conductor to the vertical wire and the feed- line shield to the ground plane. What is the polarization of this T-shaped antenna?

小明在一条20米长的导线中点连接一个线圈，并将导线水平提 升至稍高于20米的悬挂位置。线圈的另一端接有另一条长度为 20米并垂向地面的导线。他将馈线的芯线与垂直导线相连，将 馈线的屏蔽层连至地网。这种看似“T”形的天线，极化方式如 何？

U0894. To lower the radiation elevation angle, Xiaoming hangs half of the T2FD dipole vertically. He connects the folded-dipole feed end via a matching transformer to the cable center conductor, and connects the end with the absorbing resistor to the ground plane and the cable shield. The polarization of this antenna is:

为了降低辐射仰角，小明试着将T2FD天线的半边振子直立悬挂 起来。他将折合振子的馈电端通过匹配变压器连至电缆的芯 线，将带有吸收电阻的另一端与地网和电缆的屏蔽层相连。这 种天线的极化方式为：