工程师故事：冻僵的发射机

什么原因导致发射机晚上失效，第二天又正常工作？

在我居住的阿拉斯加费班克市(Fairbanks,Alaska)拥有相当极端的气候。夏天的气温约有90℉(约32℃)，冬天则可能冷达零下64度(约-53℃)。阿拉斯加极端的气候与环境造就了极其独特的工程挑战，以及专业的工程师。除了通过PE测验以外，他们还必须研读一些有关极地工程的研究所课程，或者撰写一些有关极地工程学的论文。

1974年，我才刚从阿拉斯加大学的电子工程学系毕业，随即加入一项美国太空总署(NASA)的ATS-6实验卫星计划工作。该计划还包括将各种医疗保健、教育与新闻等电视节目透过卫星播送到整个阿拉斯加的大小乡镇中。我们在1970年代时所用的标准电视摄影棚就位于阿拉斯加大学费班克分校的一栋建筑物地下室中，并在建筑物上架设10英呎的S-频碟型天线进行传送与接收，远远看过去它就像是另一栋更高建筑物上长出的米老鼠耳朵。

每周二晚上，我们从费班克市接收有关阿拉斯加当地新闻等电视节目。有一晚，我们一如往常地启动卫星系统以便接收电视节目，但却……什么也没出现！难道是天线的接收机或发射机出了问题？我们将音讯连接到另一个卫星后，作业人员们又问我们看见讯号出现了吗？－答案当然是否定的。

这种天线虽然能因应轨道变化进行调整补偿，但当时我们通过远端控制来调整却一点儿也没反应。由于那是一个十分寒冷的夜晚，所以我们直觉认为可能是马达驱动方面出了问题──马达所用的润滑油在零下40时就会开始硬化。于是我和另一位工程师直奔天线端查看是否如同我们料想的一样，那么也许就可以手动来调整天线。在费班克市，每当寒冷的冬天来临时，连空气都会结冻，但那一晚却吹着凛冽的寒风，使得温度也持续变化。当我们走出来到屋顶上时，一阵强风吹落了我朋友的眼镜──但这位勇敢的阿拉斯加人仍坚持自己没事，于是我们继续执行任务。(所幸后来经过检查，他真的还好。)

由于马达仍正常运转，表示这部份并非问题所在。后来我们又检查了室外发射机部份，发现它看起来的确是故障了。其中一个散热片摸起来就像周边其它的金属一般的冷。我当时以为这个发射机已经故障，因此在它进行修复或更换以前，电视节目也只好停播了。那天晚上还有许多的节目都无法播送了。

下一页：被冻僵了？！

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到了第二天，我们却找不到任何问题。然而我们都能顺利地操作这套系统并正常播送电视节目。我们顿时明白这是由于夜晚极端寒冷，而使得发射机停止运作。我们只得把这台发射机送回厂检修一番。由于发射机设计者已预期到可能用于天寒地冻的环境，因而已为其增加了加热器，使晶体管维持在可正常运作的温度。

然而，天气会变得那么冷则是他们始料未及的。当然，在正常运作下，功率晶体管连接至散热片以利散热。但热源部份则仅来自同一散热片紧密连接的电阻器，并由温度传感器加以控制。当寒风吹去散热片上的热能比起电阻器增加热能的速度更快时，便导致晶体管停止运转。

被冻僵了

我们必须尽快解决这个问题。由于当时所进行的一年期计划刚好过了一半，而这种系统在制造上似乎短时间内也不至于发生什么改变。因此，我的解决之道是打造出一个绝缘盒来覆盖发射机，为其在寒冷的极地气候下运转时带来一点小小助益。之后，那年冬天我们没再遇过任何问题。等到气温高于-20℉(-28℃)时，我只需移除这个绝缘盒即可。

点击参考原文：Brrr! Transmitter failure in the Arctic chill

《电子工程专辑》网站版权所有，谢绝转载

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Brrr! Transmitter failure in the Arctic chill

Mark Espe

What caused the transmitter to fail at night but work the next day?

Fairbanks, Alaska has a very extreme climate. I've been in 90 degrees in the summer and -64 degrees in the winter. The Alaskan environment offers unique engineering challenges and Professional Engineers, in addition to passing the PE exam, must take a graduate level Arctic Engineering course or write a research paper on Arctic engineering.

In 1974 I was an electrical engineer freshly graduated from the University of Alaska. I worked on a NASA project for the ATS-6 experimental satellite. The project included broadcasting television to villages around Alaska for health, education, and news. The studio was a normal 1970s TV studio. It was at the University of Alaska Fairbanks in the basement of one building. The transmit and receive antennas were ten foot S-band dishes. They sat like Mickey Mouse ears on the top of another, taller building. On Tuesday nights we broadcast a show about Alaska news from Fairbanks. This particular night we turned on the satellite system to broadcast the show and ... nothing. Was the receiver or the transmitter the problem? We had an audio link on another satellite and our operator asked if anyone could see the signal - no one could.

The antennas could be adjusted to compensate for the orbital variations but our remote control adjustments had no effect. It was a cold, cold night so we thought the motor drive might be having problems; lubricants can get pretty stiff at forty below. With another engineer, I raced to the antennas to see if that was the problem. We might be able to adjust the antenna manually. When the winter cold settles in at Fairbanks, the air usually gets very still. But that night there was a very cold wind. When we came out on the roof the temperature change due to the wind shattered my friend's glasses. A tough Alaskan, he insisted he was ok and we went on. (Later on he was checked out and really was fine.)

The motor was working so that wasn't the problem. We checked the outdoor unit of the transmitter and it sure looked dead. There was a heatsink that felt as cold as the rest of the metal up there. I thought the unit had failed and we would be off the air until we could repair it or get a replacement. So much for our program that night.

The next day we couldn't find anything wrong. We were able to operate the system for another broadcast during the day. It dawned on us that the extreme cold that night must have made the transmitter unit stop working. We hauled it into the shop and had a look. The designers had anticipated cold weather and had provided heaters to keep the transistors at an operational temperature.

Unfortunately they didn't anticipate that much cold. Power transistors were attached to the heatsink to dissipate heat during normal operation, of course. But the heat sources were just power resistors bolted to the same heatsink and controlled by a temperature sensor. The wind pulled the heat off the heatsink faster than the resistors could add it so the transistors just stopped working.

We had to fix the problem soon. Since we were halfway through the one-year project, it didn't seem likely that any manufacturing changes would happen in time. My solution was to build an insulated box to cover the transmitter to give it a little extra help during the cold arctic winter. We had no more problems that winter. When it stayed above -20 I simply removed the insulated box.