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Electromagnetic Compatibility
By Fritz Burmeister

Posted October 27, 2006

Electromagnetic Compatibility (EMC) is not a household word, even in this era of pervasive use of technological jargon. EMC relates to a discipline or a branch of scientific investigation that clearly falls under the category of specialty engineering. The development of sophisticated electronic apparatus in recent decades has made it necessary to devote time and energy to this field in order to assure that electrical/electronic equipment operates in its intended function and in its assigned environment. In other words, in this modern day and age where computers and microprocessors are ubiquitous in everything that uses electrical power, compatibility in an electromagnetic environment has become an important part in the use of electronic gadgetry.

The military with its rapid advancements of modern electronic equipment, particularly during war times, recognized a need to seriously address the EMC issue long before equipment manufacturers did. Today, in a world of digitization and where even appliances are provided with a computer, national and international regulatory agencies have set up EMC requirements for equipment to be marketed. The increased use of the upper frequency spectrum and the super sensitivity of modern circuitry make it imperative to control the associated electromagnetic interference (EMI). Failing to do so, may result in a serious EMC problem. 

EMC problems manifest themselves in a variety of situations.  

-Household appliances, such as mixers or vacuum cleaners cause static in an AM radio or TV
-Fluorescent lights destabilize the sync circuits in video displays
-Lightning can cause damage in telephone equipment as well as in other electronic apparatus
-High power transmitters and radars interfere with car radios and other sensitive equipment
-Lab Tops interfere with aircraft electronics during take-off and landing
These are just a few examples of EMC problems with various consequences encountered in our daily lives. As can be seen, an EMC problem is always comprised of three elements, the source (culprit), the receptor (victim) and the path between them, which may be a wire (conductor) or just the intervening air space. Identification of these elements is a first step to the solution of the problem.  
To dig deeper into the details of this vast field of EMC which deals with the emissive as well as the immunity issues of electrical/electronic equipment, installed on aircraft, on board ship or on the ground, would be beyond the scope of this article. Books and papers on the subject are available to the reader who desires to study in greater depth the many aspects of EMC. 

The following two examples, however, further illustrate the existing need to control interference during the design phase of equipment to avoid later embarrassment or even tragedy. Once the box is installed, it is often difficult to fix the problem, and it is always costly.

In the waning months of WW II, German submarines suffered a hard blow due to a peculiar EMC phenomenon, according to the report of Karl Dönitz, commander of the U-Boats, in his book “Ten years and twenty days.” He related an incident that caused perplexity and the greatest of concern in the German submarine command. U-Boats had sunk Allied shipping, destined to relief England’s shortage of food and materials, during the many months of the raging “Battles of the Atlantic.” The Allies had finally gained the upper hand in the struggle to protect the convoys. Radars installed on aircraft enabled the search crew to detect U-Boats traveling on the surface during recharging of their batteries. The ensuing battle often ended with the loss of a submarine, since it had little chance to dive away in time. 

To alleviate this untenable situation, U-Boats were outfitted with a radar detection receiver. This device issued warning signals in the boat allowing ample time for it to slip beneath the surface. Unfortunately, the designers had neglected to subject the radar receiver to interference testing prior to installation on the boats. The receiver radiated an interference signal via its antenna, soon detected by Allied pilots. No longer did they have to turn on their radars, the boats would betray their own positions. Numerous losses of U-Boats was the result, and all because electromagnetic interference had gone uncontrolled. 

The second example to be cited here, has a personal touch. Not as tragic as the WW II story, but it was embarrassing as it was humorous. It happened during sea trials of the Boeing-built military hydrofoil boat. As the boat “flew” across the Puget Sound waters on its foils, the Navy captain had ordered crew members to observe equipment operation and report any failure, malfunction or false dial indication while the boat went through its maneuvers. All of a sudden the boat abruptly dropped on its hull without a prior command from the bridge! What happened? Everybody was dumbfounded. The captain immediately ordered everybody to remain at his station and report his last five minute activity. This revealed that the executive officer had flushed the toilet during that period !! But what did that have to do with landing the boat on its hull? An analysis team came up with the answer rather quickly. Flushing the toilet activated a pump who’s wiring was routed next to the input wiring of the computer, which controlled the flaps on the foils. The computer had reacted to an interference signal, generated by the pump and coupled into the computer input port. 

A typical EMC problem. Here the three elements were: the pump the source, the computer the victim and the path between them the closely coupled wiring. Due to the humorous aspects of this episode, the story spread within the EMC community of this country like wildfire. For a brief period of time, the toilet was in control of boat operation. 

These two incidents, occurring in a time span of over forty years, demonstrate that the advancement of modern technology did not eliminate EMI. It is just as important today as it was yesterday to make the effort of incorporating control measures early in the design phase to assure compatible system operation. Unfortunately, management is sometimes reluctant to allot the required time and money. 

Electromagnetic interference always stands in the way of electromagnetic compatibility, it is like a side effect in the operation of electronic gear, and as a colleague of mine aptly called it one time, it is “bad electricity!"


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