Antenna Test Lab
English Russian
Once upon a time, when computers were large and programs were small, a mid-range university set up an antenna test laboratory. The lab was allocated a hectare of wasteland, where they built a hut, which housed several employees and their basic equipment. The hut was topped with a "radio-transparent shelter," which turned out not to be particularly transparent, and served as a storehouse of materials resistant to cold and condensation, as the shelter was not heated.
Soon, the meadow in front of the laboratory had recovered from the lab's construction. Here and there amidst the unkempt grass, ox-eye daisies whitened, blue bell-flowers bobbed, and sturdy buttercups yellowed. In the corner of the plot, the grass gradually gave way to sedge, and there grew several bushes of cotton grass. A wasp family nested in a segment of decimeter waveguide, discarded as unnecessary. People and their antennas did not interfere much with nature, although sometimes a team of students was sent to "reduce elevation angles", that is, chop out the willow bushes obstructing the path of the signal. One time, such a team caught a hedgehog, and, allegedly, they saw a viper on a heap of brushwood.
The local climate was not particularly conductive to field work. Summer was still tolerable, but who works in the summer? Students spend their summer break at home, employees go on vacation, and for a month or two no one scares the blue butterflies that fluttered over the drooping grass. From mid-September onward, the sky was covered by a gray cloud, which drizzled small specks of water. Sometimes they fell so slowly that they could be seen falling. The air cooled about 5 degrees Celsius a month, and by the end of the year the rain turned into snow, just as fine and unhurried. The snow cover lay until the middle of April, and the ground squelched underfoot for two more weeks. The meteorologists, armed with a heliograph and plenty of of free time, claimed a few hours of sunshine per month, 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, 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 fully develops at a distance of about 100 m from it. This distance of separation is hard to find indoors.
To measure the directivity diagram, the antenna was taken out, placed on a turntable, and connected to the receiver. The signal source was hung on a pole at the far corner of the field. But technological progress did not stand still, and while the graduate student was twirling the handle of the turntable, glancing between the mirror scale of the millivoltmeter and the gray sky, a LSI-11 computer was installed in the laboratory.
The computer was in a flat steel 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, punch and tape drive interfaces, and a special card that could send and receive 16 bits of data. The last card was the most interesting. A DAC was connected to the output signals, and an analog signal was sent to the oscilloscope. Thus, a graphical display was made, which managed to show, with an acceptable refresh rate, up to 1000 randomly spaced points on the screen. An ADC was connected to the input contacts, and large millivoltmeters with anti-parallax mirror scales became obsolete.
It turned out that one did not need to turn the antenna anymore either. It is possible to measure the field near the antenna, make some calculations. and find the far field. An experimental setup was built in the hut, which carried a small measuring antenna back and forth across the large, tested one. The measuring antenna sat on a carriage, propelled along the guides by a long steel screw. When the computer signaled to swap phases, the motor reversed and turned the screw in the desired direction. The work schedule was no longer dependent on the vagaries of the weather.
Measuring the near field turned out to be useful in and of itself. A slot antenna is a rectangular copper tube, a waveguide, with slots cut in one side. It was not always possible to make clean cuts, and burrs remained on the inside of the waveguide. If the measured field near the gap differed from the calculated one, the student took a plunger, dental hook, and sharpened hacksaw and eliminated the burrs. Sometimes one had to practice this practical and very useful kind of electrodynamics all day, without ever touching a pencil or paper.
This setup, assembled from found materials, was deservedly popular. One day, the department head came to see it. This time the installed slot antenna was was polished not only inside, but also outside. All lab junk was stowed away, students put on ties, and the head wore an ironed lab coat. A graduate student, the author of the program, took the seat at the console. At his left, the head of the lab took his pointer. Everything was ready for the reception of distinguished guests.
The student typed a long, intricate command line, 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 members to the success of the team and back. Finally, the command was finished, and the student pushed the big gray carriage return key.
A pair of relay contacts that controlled the direction of motion probably welded and remained closed. At the command from 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 went quiet. There was only a little echo in the ears, brown after-images of the stand moving in the darkness, and glowing round yellow screens of circuit analyzers.
However, the material damage was minor. The electronics remained 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 dared to appear there.
Satellite pictures shows that the hut with the shelter is still standing, a path leads to it from the gate, and the willow bushes were never fully cleared out. Dear meadow, what are you doing now?
