Electromagnetic interference (EMI) and its suppression are major issues in most current system designs. In Europe, essentially no electrical product can be sold without the CE mark—certification that it is emission-free. Troubleshooting a "leaky" design can be difficult and expensive, however. Fortunately, designers can build simple electromagnetic sensors that will detect emissions from the gigahertz range down to audio frequencies. These sensors are simple enough to be constructed by anyone seriously interested, yet effective enough to solve serious design problems.
An anecdote about how these sensors solved a near career-limiting EMI problem is a good place to start. It illustrates some fundamental principles of RF-leakage detection and leads into a description of how to make practical RF sensors.
To start a serious story with a little humor, what really solved the problem illustrates again the old engineering saying, "Even a blind pig gets an acorn now and then—if he keeps rooting around." As you'll see, rooting around was an essential element of the problem solution. The sensor described below has been used by the author and others since 1973. The concept is presently applied in circuit design environments, as well as under the pressures of a stopped production line.
The serious EMI problem that started this discussion occurred in a Signal Corps 8-GHz transmitter then completing its development. It was about done and the qualification unit was in final test. Then it failed to meet radiated emissions limits. As I recall, it was a 300-channel unit for analog voice multiplexing. The government specs were tight, as usual.
The unit had passed all of its functional requirements, such as noise-power ratio, stability, power output, noise, distortion, reliability, temperature extremes, humidity, and so on. Failing the EMI requirements at the last minute was a crisis. Delivery was due or overdue. It's hard to imagine a more stressful situation, especially when the problem is new, unexpected, and unusual. Failure to make deliveries is serious business.
The transmitter's internal construction was similar to most microwave equipment of the time. It had several rack-mounted RF subassemblies that were plumbed with miniature rigid coax connections. In this case, the vertical racks had been replaced by a 6-ft. tall, "EMI-tight," RF-gasketed cabinet with a swinging front door. The oscillator, multipliers, load isolators, amplifier equipment, etc., were all in the top 25% of the cabinet. The space below was completely empty. It was a 4-ft. tall, open unoccupied space.
For EMI test, the cabinet was placed in an 18- by 24-ft. screen room containing all the sensing and signal-generating equipment to measure both RF radiation and susceptibility. With the 8-GHz power amplifier operating, an escaped 8-GHz signal was detected outside and inside the closed cabinet door. Of course, that signal was found at greater intensity inside the cabinet. Recall that all of the RF-generating capability was in the equipment in the top 2 ft. of the cabinet. There should have been no leaking RF in those top 2 ft.
I should mention here that in screen rooms capable of meeting mil-spec requirements, there's a standard family of generators, receivers, sensors, and antennas. For the 8-GHz signal, a square trapezoidal horn is used as an antenna. It's about 7 by 7 in. at the open end. Inside this horn, at the small end of the trapezoid, a tiny loop antenna couples signals from the flared opening to a 50-Ω coaxial cable. That flexible cable is connected to a tunable RF voltmeter, scope, or other display, such as a spectrum analyzer. Thus, any signal received by the horn antenna is captured from its approximately 50-in.2 opening (7 by 7 in.) and concentrated to the small loop feeding the coax.
Reported symptoms were that the highest RF levels of leakage were at 8 GHz and at their maximum in the lower left rear of the cabinet—the empty space! Escaping radiation could not be traced to its source. Due to schedule urgency, I immediately got heavily involved. I personally held the trapezoidal antenna and moved it to every possible location trying to find a maximum signal level that would indicate some point of signal origination.
It didn't take long to realize that we were searching with wide-angle field glasses (no pun) where a microscope would be appropriate. With the standard mil-spec measuring horn sensor we had adequate sensitivity, but little spatial resolution. The size of the 7-in. horn prevented it from getting into local discrete areas of potential leakage. Clearly, what was needed was a sensor or pickup probe with more resolution, even if some sensitivity had to be sacrificed. After some discussions with the project engineer, I went off to try an idea that didn't engender much support when I first mentioned its concept.
There being no suitable RF sensors available, I went home that night and wound about 19 turns of 41-AWG enameled copper wire around the end of a shortened, round toothpick. I terminated the two end leads in a BNC connector and epoxied the assembly to a short length of 0.090-in.-diameter brass tube. The tube was soldered to the rear of a BNC connector (Fig. 1). The sensing coil, in diameter, was about 0.050 in.
If handled with care, the assembly was just strong enough. I had no idea what RF pickup characteristics this sensor would have. It was just something small and easy to try. Realizing it was a shot in the dark, I immediately proceeded with another parallel but somewhat different approach as a backup. I thought this second probe was likely to be useful at lower frequencies. That proved to be correct.