People often say RF is black magic and it sometimes feels that way. I remember one evening I was called down to the production floor to help troubleshoot a technical problem found during swing shift. There was a product going through final test and it would only pass if held at a certain angle. At first I was doubtful that this was the case, but I held it in my hands, watched the performance on the network analyzer, rotated the unit, and saw the performance degrade. First we suspected the VNA cables, but a golden unit was solid regardless of its orientation. Then we performed the standard “shake while listening for something rattling test” but couldn’t hear anything—plus the repeatability seemed to suggest it wasn’t due to FOD. X-ray imaging didn’t yield any clues. Eventually, we had to send it off to de-lid, found nothing wrong, and after real-seal the performance was stable. The best theory we had was that the problem was due to flux improperly cleaned from a feedthrough.
It was this type of problem that drew me to RF engineering in college. Circuits that only worked when you placed a finger in a certain spot. The gain reduced by the microscope light. While it felt like black magic we all knew that in reality it was physics too complicated to be fully modeled. To this day, I still find these problems fun until all of a sudden a revenue commitment is missed.