CRAC Class A (2025) — Questions 481–490
U0480. Stations
甲、乙业余电台相距10千米，分别使用左旋圆极化和半波长水 平偶极天线彼此对指进行UHF通联。现乙台改用右旋圆极化天 线，则改变前后的通信效果有什么不同？
Exam tips · 考试提示
Initially B is a linear dipole, which contains both circular components, so it couples to A's LHCP at −3 dB. After B becomes RHCP, it is opposite to A's LHCP → near-total rejection → much worse (A).
U0481. Stations
甲、乙业余电台相距10千米，分别使用左旋圆极化和半波长水 平偶极天线彼此对指进行UHF通联。现乙台改用左旋圆极化天 线，则改变前后的通信效果有什么不同？
Exam tips · 考试提示
Initially B is a linear dipole → −3 dB capture by A's LHCP. After B becomes LHCP matching A, the link is fully matched → improves (A).
U0482. Stations
甲、乙业余电台相距10千米，分别使用左旋圆极化和右旋圆极 化天线彼此对指进行UHF通联。现双方均改用半波长水平偶极 天线，则改变前后的通信效果有什么不同？
Exam tips · 考试提示
Initially opposite circular polarizations → near-total rejection (very poor). After both become horizontal dipoles (matched linear polarization) → big improvement (A).
U0483. Assume both receiving and transmitting antennas use half- wave dipoles. For short-distance communication between ground stations, the best polarization arrangement for the receiving and transmitting antennas should be:
假设接收和发射天线均使用半波长偶极天线，则在地面台站间 的近距离通联中，接收和发射天线的最佳极化方式应当安排 为：
Exam tips · 考试提示
For two ground stations the wave travels horizontally; the E- field must be perpendicular to the propagation direction (i.e., in the plane normal to the link line), and both ends must share the same polarization for best coupling (A). Orthogonal polarizations (B) or aligning the E-field along the link line (C/D) give poor coupling.
U0484. A right-hand polarized wave means, on any plane perpendicular to the direction of propagation, when viewed along the direction of propagation the electric-field vector rotates clockwise (to the right) with time — an elliptically or circularly polarized wave. If a ground amateur station observing an amateur satellite finds that the electric field of the radio wave arriving from the satellite rotates clockwise, then the polarization of that signal is:
右旋极化波是指在垂直于传播方向的任意平面上，沿传播方向 观察时，电场矢量为随时间向右（顺时针）旋转的椭圆或圆极 化波。如果地面上某业余电台在观测业余卫星时发现从卫星到 达该台的无线电波的电场是顺时针旋转的，则该信号的极化方 式为：
Exam tips · 考试提示
This is the classic "viewing from the opposite end" trap. RHCP is defined as clockwise when viewed along the propagation direction. The ground station views the arriving wave from the receiver's side — i.e., opposite to the propagation direction (the wave travels downward from satellite to ground). Clockwise as seen by the receiver equals counter-clockwise along propagation, which is left-hand. So the signal is left-hand (LHCP/LHEP) (A).
U0485. In line-of-sight communication, given the transmitting antenna is a left-hand circularly polarized antenna pointing at the receiving point, the best polarization for the receiving antenna is:
在视距通联中，已知发射天线为指向接收点的左旋圆极化天 线，接收天线的最佳极化方式为：
Exam tips · 考试提示
For best reception the receiving antenna should match the transmitting polarization: same sense (left-hand) circular, pointed back at the transmitter (A). Opposite sense (B) rejects the signal; fixed linear (C/D) loses 3 dB.
U0486. A satellite's downlink uses a right-hand circularly polarized antenna, flies from north to south, and the antenna always points at the Earth's south pole. If a ground amateur station uses a circularly polarized antenna to auto-track the satellite, the best polarization for that station's antenna should be:
某卫星下行链路采用右旋圆极化天线，从北向南飞行，天线始 终指向地球的南极。如果地面上某业余电台采用圆极化天线自 动跟踪该卫星，则该台所用天线的最佳极化方式应当为：
Exam tips · 考试提示
The satellite's antenna points south (downward toward Earth's south pole). As the satellite travels north→south and crosses overhead, the direction from satellite to the ground station flips (the wave arrives from the north before culmination, from the south after). Viewed along propagation, the apparent sense flips at culmination, so the ground station must switch from RHCP to LHCP (A). This is the same "viewing direction" principle as U0484.
U0487. It is known that a satellite's downlink transmitting antenna is a dipole pointing at the ground. Because the satellite keeps rotating, the polarization direction of the wave received by the ground station keeps changing. To avoid polarization issues interrupting reception, the receiving antenna can be:
已知某卫星下行信号的发射天线是指向地面的偶极天线。由于 卫星不断旋转，地面台站所收电波的极化方向就会不断变化。 为了不至极化问题致使接收中断，接收天线可以是：
Exam tips · 考试提示
A circularly polarized receiving antenna captures a linearly polarized (rotating) wave regardless of its instantaneous orientation (always ~−3 dB), so it will not drop out as the satellite tumbles (A). Fixed linear polarizations (B/C/D) would fade as the orientation changes relative to the antenna.
U0488. An amateur station using a half-wave vertical dipole for a contact receives an S4 signal strength at the other end. Now, with transmit power unchanged, the transmitting station switches to a Yagi antenna with 8.15 dBi gain (maximum radiation direction and polarization unchanged). What does the received signal strength at the other end become? [Hint: the receiver's S-meter increases by 6 dB per step from S1 to S9]
某业余电台使用半波垂直偶极天线通联时，对方所收信号的强 度为S4。现发射功率不变，发信方改用增益为8.15dBi的八木 天线（最大辐射方向和极化均不变），则对方所收信号的强度 变为：【提示：收信机信号强度指示从S1至S9每档增加6dB】
Exam tips · 考试提示
A half-wave vertical dipole has a gain of 2.15 dBi. The Yagi is 8.15 dBi, so the improvement is 8.15 − 2.15 = 6 dB, which is exactly one S-unit. S4 + 1 = S5 (A).
U0489. An amateur station transmitting with a half-wave vertical dipole yields an S4 received signal at the other end. Now, power unchanged, the transmitting station switches to a Yagi with 12 dBd gain (direction and polarization unchanged). What does the received strength become? [Hint: S-meter +6 dB per step]
某业余电台使用半波长垂直偶极天线发射时，对方所收信号的 强度为S4。现发射功率不变，发信方改用增益为12dBd的八木 天线（最大辐射方向和极化均不变），则对方所收信号的强度 变为：【提示：收信机信号强度指示从S1至S9每档增加6dB】
Exam tips · 考试提示
12 dBd = 12 + 2.15 = 14.15 dBi. A dipole is 2.15 dBi, so the improvement is 12 dB = two S-units. S4 + 2 = S6 (A).