Difference between revisions of "Antenna Test Lab"

Revision as of 16:36, 20 September 2017

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Once upon a time, when computers were large and programs were small, in a mid-range university, there was an antenna laboratory. A hectare of wasteland was allocated for the needs of the laboratory, and a hut was built, which housed several employees and their simple equipment. The hut was topped with a "radio-transparent shelter," which was not particularly transparent, and served as a storehouse of materials that were not afraid of cold and condensation, since the shelter was not heated.

The meadow in front of the laboratory had recovered from the lab's construction. Here and there amidst the unkempt grass, chamomiles whitened, bells bobbed and buttercups yellowed. In the corner of the site, the grass gradually gave way to sedge, and several bushes of cotton grass grew. Wasps nested in a segment of decimeter waveguide disposed for being unnecessary. People and their antennas did not interfere much with nature, although sometimes a team of students was sent to "reduce the corners of closure", that is, cut out the willow bushes obstructing the signal. One time, such a team caught a hedgehog, and, allegedly, they once saw a viper on a heap of brushwood.

The local climate was not particularly conducive to field work. Summer was still tolerable, but who works in the summer? Students spent their summer break at home, employees went on vacation, and for a month or two no one scared the blue butterflies that fluttered over the drooping grass. From mid-September onward, the sky was covered by a gray cloud, which drizzled small drops of water. Sometimes they fell so slowly that they could be seen in the air. The weather cooled about 5 degrees a month, and by the end of the year the rain was replaced by snow, just as small and unhurried. The snow lay until the middle of April, and the ground squelched underfoot for another two weeks. The meteorologists, armed with a heliograph and plenty of of free time, found a few hours of sunshine, but common folks considered this finding just experimental error. On such matters, climatologist Köppen said tersely: "Dfb", while the laboratory staff, looking at the work plan and rain outside the window, used stronger expressions.

The main task of the laboratory was to measure the parameters of microwave antennas. The directivity diagram of a 1 meter antenna in the X-band is fully formed at a distance of about 100 m from it. You cannot achieve this indoors.

To measure the radiation pattern, the antenna was taken out, placed on a turntable, turned on, and sent a signal from a source that was hung on a pole at the far corner of the site. But the progress did not stand still, and while the student-graduate student twirled the handle of the turntable, glancing at the mirror-scale of the millivoltmeter, then at the gray sky, a computer was installed in the laboratory, the Electronika-60.

The computer was in a flat iron box with 3 white fangs: power, timer, and console monitor. Inside the box was a processor running 500K operations per second, 56K of memory, a typewriter interface, a punch and tape drive interface, and a special card that could read and output 16 bits of data. The last device was the most interesting. A DAC was connected to the output signals, and an analog signal was applied to the oscilloscope. Thus, a graphical display was made, which managed to show with an acceptable refresh rate, 1000 randomly spaced points on the screen. An ADC was connected to the input contacts, and large mirror millivoltmeters became obsolete.

It turned out that you do not need to turn the antenna anymore either. It is enough to measure the field near the antenna, calculate a little and find a far field. An experimental setup was built in the hut, which carried a small measuring antenna back and forth across the large examined one. The measuring antenna sat on a carriage, which was propelled along the guides by a long steel screw. When the computer signaled, swapped two phases, and the motor turned the screw in the desired direction. The work plan was no longer dependent on the vagaries of the weather.

Measuring the near field turned out to be useful in and of itself. The slit antenna is a rectangular copper tube, with cut gaps in one side. It was not always possible to make clean cuts, and burrs remained on the inside of the waveguide. If the field at the gap differed from the calculated one, the student took a plunger, a dental hook, a sharpened hacksaw and eliminated the burrs. Sometimes one had to practice this practical and very useful kind of electrodynamics all day, without touching either a pencil or paper.

This setup, assembled from found materials, was deservedly popular. One day, the department head came to see it. A slot antenna was installed on the stand, and was polished not only inside, but also outside. Extra junk was stowed away, students put on ties, and the head wore a ironed lab coat. The spot behind the console was taken by the graduate student, the author of the program. At his left, there was a head of the lab with a pointer. Everything was ready for the reception of distinguished guests.

The student typed a long, intricate command, trying to show all the capabilities of the system at once. Taking advantage of the pause, the head built logical connections from the success of individual employees to the success of the laboratory and back. Finally, the command was finished, and the student pushed the big gray carriage return key.

The pair of relay contacts that controlled the direction of motion probably welded and remained closed. At the command of the computer, the other phase connected to the same point. There was a deafening shot, for a moment the laboratory was flooded with the white-green light of vaporized copper, and everything was quiet. There was only a little echo in the ears, while the after-images of the stand moved in the darkness, and the round yellow screens of circuit analyzers glowed.

However, the material damage was not great. The electronics was unharmed, the relay was replaced by a stronger one, the black soot of copper oxide was wiped off, and the stand was once again ready for work. For many years it served reliably, aiding education and the industry. However the management no longer appeared there.

From the satellite pictures, it is clear that the hut with the shelter is still standing, a path leads to it from the gate, and the willow bushes never did get fully cleared out. Dear field, what are you doing now?