CRAC Class B (2025) — Questions 531–540
U0832. To facilitate time calculation, the Earth is divided into time zones; the theoretical division is 15 degrees per zone. The time zones that Beijing, Xi'an and Urumqi actually belong to are:
为了便于计算时间，将地球划分为若干个时区，其理论分区为 每区宽经度15度。北京、西安和乌鲁木齐实际所属的时区应 为：
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Although geographically Urumqi's longitude would suggest East 6, China uses a single legal time zone (Beijing Time, UTC+8) nationwide, so Beijing, Xi'an, and Urumqi all officially belong to East 8 (A). The longitude-deduced answers (B, C, D) ignore the legal uniform time zone.
U0833. Given Beijing time, the corresponding UTC time should be:
已知北京时间，相应的UTC时间应为：
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Beijing time = UTC+8, so UTC = Beijing time − 8 hours. When that subtraction yields a negative hour, you add 24 and step back one day (A). Adding 8 (C, D) is the reverse (Beijing-from- UTC) direction and is wrong here.
U0834. Given UTC time, the corresponding Beijing time should be:
已知UTC时间，相应的北京时间应为：
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Beijing time = UTC + 8. So take UTC hours, add 8; if the result exceeds 24, subtract 24 and advance the date by one day (A). Options B/C/D incorrectly operate on "Beijing time" or move the date backward.
U0835. Given an amateur station is in a West-N time zone (N an integer from 0 to 12), its local time should be ___ than Beijing time:
已知某业余电台处于西N时区（N为0-12间的整数），该台的 当地时间应比北京时间：
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Beijing is UTC+8; a West-N zone is UTC−N. The difference (Beijing − local) = 8 − (−N) = 8+N hours, meaning the local time is behind (later, 晚) Beijing by 8+N hours (A). East is ahead, west is behind.
U0836. Given an amateur station is in an East-N time zone (N an integer from 0 to 12), its local time should be ___ than Beijing time:
已知某业余电台处于东N时区（N为0-12间的整数），该台的 当地时间应比北京时间：
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Beijing is UTC+8; an East-N zone is UTC+N. The difference is (Beijing − local) = 8 − N hours. For the typical case N<8 the local time is behind Beijing ("晚", later) by 8−N hours, which is the form given in the official answer (A). The formula expresses the standard comparison with Beijing as reference.
U0837. If while participating in an amateur radio CW contest you copy "B4" or "WKD", it means:
如果在参加业余无线电CW竞赛时抄下"B4"或"WKD"则说明：
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In CW contesting, "B4" (before) and "WKD" (worked) are sent by a station to tell you it has already worked you — a duplicate. The correct response is to check your own log rather than re- log the contact (A). Re-calling (B), deleting records (C), or treating it as a friendly greeting (D) are wrong.
U0838. Why should everyone submit logs promptly when participating in contests?
为什么大家参加比赛都应及时提交日志？
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Contest scoring is done by cross-checking (matching) your log against the logs of the stations you contacted — only mutually confirmed QSOs count (A). Logs are not optional (B), you must not delete valid contacts (C), and VHF is no excuse to skip logging (D). 无线电系统基本原理 3 C H A P T E R Basic Principles of Radio Systems 本章从收发信机、天线、传输线到调制与卫星，建立无线电系统的整体认知。This chapter builds a systems-level understanding from transceivers, antennas and feed lines to modulation and amateur satellites. 收发信机与接收系统 Transceivers and Receiver Systems
U0839. When an amateur repeater shares one antenna for uplink and downlink, a ___ must be inserted between the receiver, transmitter, and antenna:
业余中继台上下行共用一副天线时，需要在接收机、发信机和 天线之间插入一个：
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A repeater using one antenna for both receive (uplink) and transmit (downlink) needs a duplexer — a pair of filters that let the receiver and transmitter share the same antenna while isolating them (A). A T/R switch is for half-duplex single- station use, a power divider splits power, and a circulator is directional but not the standard shared-antenna solution described.
U0840. When an amateur repeater shares one antenna for uplink and downlink, a duplexer is inserted between the receiver, transmitter, and antenna. Its basic construction and function are:
业余中继台上下行共用一副天线时，需要在接收机、发射机与 天线之间插入一个双工器。其基本构造和作用为：
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A duplexer is essentially a set of cavity/coaxial filters: one passes the transmit frequency to the antenna while rejecting it from the receiver, and the other does the reverse, so the strong transmitted signal cannot overload or feed back into the receiver (A). It is not a switch (B), a mere matching network (C), or a one-way circulator path (D).
U0841. Before erecting an amateur repeater you must check whether there are nearby transmitters on other frequencies whose signals are strong enough to produce third-order intermodulation with the repeater's downlink signal inside the repeater's receiver, with the intermod product falling within the repeater's uplink frequency range. If the repeater's uplink and downlink frequencies are fR and fT respectively, the frequency fX that may cause such third-order intermodulation interference can be calculated as:
在架设业余中继台之前必须调查台址附近有没有其他频率的发 射机，其信号强到可以在中继台的接收机中与中继台的下行信 号产生三阶互调，并且互调产物落入中继台的上行频率范围 内。如果中继台的上、下行频率分别为fR和fT，则可能造成这 种三阶互调干扰的频率fX可以计算为：
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Third-order intermodulation products take the form 2F₁ − F₂ or F₁ + F₂ − F₃. Here the repeater's own downlink fT and an external signal fX mix to produce the uplink fR: 2fT − fX = fR ⇒ fX = 2fT − fR; and 2fX − fT = fR ⇒ fX = (fT + fR)/2. Both are the standard formulas, so A is correct.