In the late sixties, I worked as a civilian technician in the navigation center of a missile tracking ship (the USNS Vandenberg, now an artifical reef off Key West). Part of our gear was an AN/SRN-9, the military forerunner of GPS. It had a helical antenna and pre-filter assembly that was mounted some 50+ feet above deck. One day the system just completely failed. It was determined that there was zero signal coming from the antenna assembly. It became my job to put on a safety harness and go out on this yardarm (scary!) to inspect the antenna for some sort of damage. Nothing visible from the outside, so I unmounted the unit and brought it down for troubleshooting on the bench.
The filter was inside what can be loosely described as two metal pie pans with a ring of closely-spaced screws and a rubber O-ring seal. We opened the can ... and then took the covers off the filter cavities themselves. Inside each cavity were adjustment capacitors mounted on threaded shafts that were tubes with flat flared bottoms ... the flat flared part acting as a capacitor to the inner wall of the cavity. The spacing was quite close. The interior was all silver-plated, as is standard at these frequencies (about 450 MHz if memory serves). Anyway, closer inspection revealed a bubble in the silver plating that was just big enough to touch the capacitor flare, thus shorting the cavity. That explained the total loss of signal. Further inspection revealed considerable corrosion inside the "sealed pie plate" housing. Turns out that the screws around the periphery were too long - just long enough to prevent pressure on the O-ring. Over time, salt spray had entered and caused the silver corrosion. We decided that we should replace the entire unit rather than risk another failure of this type. - Bill Whitlock, chief engineer, Jensen Transformers, www.jensen-transformers.com
The title of my paper is misleading since the tuning of the coupling screws do not effect the 3rd harmonic response of the filter. The characteristic of a 1/4 wave resonator repeats its function at 3/4 wave creating a reentry of the response at three times the fundamental response.
I used to run into similar problems when working for a defense contractor. You were often told to design an algorithm or part without being told what it would be used for. I actually first encountered this when I was in high school. I was studying physics and calculus and my father thought it would be a good exercise for me to determine the density of a flywheel he was using for a particular application. I never found out what the application was, by the way.
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