The Satellite Station Had a Screw Loose

In the late 1960s, the LNAs (low noise amplifiers) used on satellite earth stations were very complicated. The dish was usually around 30m across, and they focused the signals onto a small sub-reflector on a tripod and then down a horn to the LNA cabin at the back of the dish. Three stages of parametric amplifier and a tunnel diode gave a total front-end gain of around 40dB with bandwidth 500Mhz and center frequency around 4Ghz. To achieve a decent noise level, these stages were packaged into a stainless-steel Dewar (vacuum flask) and cooled to around 15K by a cryogenic refrigerator using helium as the working fluid.

The LNAs were duplicated so that one could work while the other was serviced. The cryogenics needed regular overhauls to maintain the low temperature. Even a few minutes without power would cause a warm up and take several hours to sort out. The LNA team members on one station in the Arabian Gulf were mortified to occasionally see regular bursts of unwanted pulses on the LNA output along with the desired signals. Worst of all, the pulses came and went unpredictably, popping up and down on the spectrum analyzer, yet always on the same frequency. Complete and time-consuming overhauls of both LNA systems achieved nothing. It could never go on traffic until the problem was solved.

One of the team members had previously worked in avionics and suggested that perhaps the bursts of pulses might originate from a radar station. It didn't seem likely -- the dish was pointing up into the sky and looking across the Gulf. But this was the best hypothesis we had, so we looked into it. We quickly found that the nearby city airport radar worked on 2GHz, exactly half the observed frequency, and the repetition rate of the bursts of pulses did correspond to the rotation rate of the scanner. Yet, how were the signals getting into the system when the dish was pointing in the opposite direction? And why did the spurious signals come and go so unpredictably?

As we stood at the back of the dish and looked toward the airport, some 10 miles away, the penny began to drop. There was a steel sliding door on the LNA cabin. When it was slid open, the pulses appeared. Clearly, the open door allowed the radar pulses into the cabin. Yet, the sensitive gear was inside a stainless-steel flask, and everything was connected up with robust waveguide, so it still didn't make sense.

Finally, the LNA team stripped down every single piece of waveguide and found one flange joint that had been temporarily assembled without a soft metal gasket, and the bolts just finger tight. Obviously, the two mated pieces had then been picked up by someone else and bolted into place without further inspection. We fitted a gasket and tightened the bolts to the correct torque. That solved the problem. Many hours had been wasted by a simple oversight. Although all the drawings were meticulously checked and had specified the correct number of gaskets, bolts, nuts, washers etc., the policy was to "free-issue" bulk packs of such items rather than count them out as complete kits.

Given a complete kit, the extra leftover gasket might have given cause for concern and prevented the problem from occurring. The flipside of that policy is that if a gasket gets spoiled or a special nut is dropped into the desert sand, the job stops until a new one is sent from the UK. This is just one of the joys of working on an installation crew far from home. Of course, in aeronautical, aerospace, and nuclear engineering, to name but three, such strict controls on parts issued are routine and the cost of the supporting paperwork is an integral part of the safety culture.

This entry was submitted by Rod Hine and edited by Rob Spiegel.

Rod Hine, MA (Cantab) MIET MBCS, graduated from Churchill College in Cambridge, England. He has worked in satellite communications, meteorological telecomms, general automation, machine tools, and industrial control systems.

Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.

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