CRAC Class A (2025) — Questions 561–570

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U0560. In radio communication, the "noise temperature" indicator is often used to describe the internal-noise magnitude of signal sources, amplifiers, receivers, or other equipment/system components. Taking a receiver as an example, the meaning of equivalent noise temperature Te is:

在无线电通信领域中，描述信号源、放大器或接收机等设备或 系统组件的内部噪声大小时常用"噪声温度"指标。以接收机为 例，等效噪声温度Te的意义是：

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等效噪声温度 (equivalent noise temperature, Te) represents the receiver's internal noise as the physical temperature a matched input resistor would need to have to generate the same thermal noise power (A). It is an input-referred equivalent, not an output heating temperature (B, C) nor an ambient operating temperature (D).

U0561. For an ideal amplifier or receiver that produces no internal noise, the noise figure Fn, noise figure NF, and noise temperature Te are respectively:

不产生任何内部噪声的理想放大器或接收机的噪声系数Fn、噪 声指数NF和噪声温度Te分别为：

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An ideal noiseless device adds no noise, so its input and output SNRs are equal: Fn = 1; NF = 10·log10(1) = 0 dB; and Te = 0 K (absolute zero) (A). Values of Fn = 0 (B, C, D) are impossible.

U0562. Suppose an antenna amplifier used for the amateur-satellite service operates at standard temperature (17 °C), with its input already connected to a well-matched antenna. If the internal noise produced by the amplifier is equivalent to the input thermal noise, then the amplifier's noise figure Fn, noise figure NF, and noise temperature Te are respectively:

假设一个用于卫星业余业务的天线放大器工作在标准温度 （17℃）下，其输入端已连接良好匹配的天线。如果放大器所 产生的内部噪声与输入的热噪声等效，则该放大器的噪声系数 Fn、噪声指数NF和噪声温度Te分别为：

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If the amplifier's added noise equals the input thermal noise, the total output noise is doubled relative to the ideal, so Fn = 2; NF = 10·log10(2) ≈ 3 dB; and Te = (Fn−1)·T0 ≈ 1×290 K = 290 K (A). The "standard temperature" reference is T0 ≈ 290 K (≈17 °C), which is why Te lands at 290 K. 业余无线电卫星 Amateur Radio Satellites

U0563. In the amateur radio field, the abbreviation AMSAT refers to:

在业余无线电领域中，缩写AMSAT是指：

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AMSAT is the amateur radio satellite organization (originally Radio Amateur Satellite Corporation, now AMSAT) — a volunteer non-profit building and operating amateur satellites (A). It is not a satellite name or a generic class of satellites (B/C/D).

U0564. When describing amateur radio satellites, the abbreviation OSCAR (奥斯卡) is often used. The origin of this name is:

在描述业余无线电卫星时经常用到缩写OSCAR（奥斯卡），这 个名称的由来是：

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OSCAR stands for Orbiting Satellite Carrying Amateur Radio (A). It is an acronym, not a person's name (B, C) nor related to the film Academy Award (D).

U0565. The launch year, name, international OSCAR number, and transponder mode of China's first amateur satellite are, respectively:

我国发射的第一颗业余卫星的发射年份、名称、国际OSCAR系 列号和转发器模式分别为：

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China's first amateur satellite is Hope-1 (希望一号, XW-1), launched in 2009, assigned OSCAR number 68 (HO-68), using a V/U (J) mode transponder — uplink VHF, downlink UHF, mode J (A). The other years (B, C, D) and "no OSCAR number" (D) are wrong.

U0566. The best time to conduct EME (Earth-Moon-Earth / moonbounce) communication experiments is:

开展EME通信试验的最佳时机是：

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EME path loss is enormous, so the best moment is when the Moon is closest to Earth — at perigee (A) — minimizing distance and thus path loss. Full moon (B) only affects illumination, not reflection gain; local weather (C) and sunspot count (D) are not the determining factors.

U0567. When installing a preamplifier for occasions requiring reception of extremely weak signals, such as EME communication, you should:

在EME通信等需要接收极微弱信号的场合，安装前置放大器时 应做到：

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For weakest-signal reception, the low-noise preamplifier (LNA) must sit right at the antenna feed to beat the feed-line noise, and cooling lowers its thermal noise (A). Putting it near the receiver (B, C) or using feedback for gain (D) worsens the noise figure.

U0568. For communication scenarios needing reception of VHF/UHF weak signals, such as moonbounce experiments, amateurs often add a low-noise preamplifier right at the feed point of their Yagi antenna. In contrast, HF amateurs rarely mount their preamplifier at the feed point of a Yagi or other receiving antenna. This is because:

对于需要接收VHF/UHF微弱信号的通信场景，比如月面反射实 验，爱好者们时常需要在所用八木天线的馈电端处就近加装低 噪声前置放大器。相比之下，HF通信爱好者却很少将他们所用 的前置放大器装在八木天线或其他收信天线的馈电点处。这是 因为：

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VHF/UHF external (galactic/atmospheric) noise is low, so receiver/feed-line noise dominates and a masthead LNA helps a lot (A). On HF the natural band noise is already high, so a front- end preamp barely improves the SNR — hence HF users seldom bother. B/C/D give wrong or trivial reasons.

U0569. Which of the following amateur bands is most suitable for meteor-scatter communication?

下列哪个业余波段最适合流星余迹散射通信？

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Meteor-scatter works best on the 6-meter band (A), where the ionized meteor trails efficiently reflect VHF signals at a favorable frequency/wavelength. Higher bands (2 m, 70 cm) and 10 m are less optimal for this mode.