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Beware of hidden antennas!
http://www.edn.com/electronics-blogs/the-emc-blog/4439771/Beware-of-hidden-antennas-

There are many causes of radiated emissions and the product designer needs to be able to identify the “hidden antennas” that cause this.

An important concept to grasp is the electrical dimension of an electromagnetic radiating structure. EMC engineers often call any radiator of electromagnetic energy an “antenna,” whether it is an actual antenna or another radiator, such as a cable or circuit-board trace.

Figure 1 – An example where two PC bards form a “hidden dipole”. I’m using a Rohde & Schwarz RT-ZD10 1 GHz differential probe to measure the voltage between the connector ground shells of two boards. If the boards are bonded together well enough, the voltage should read zero.

These dipole structures could include:

• Cables (I/O or power)
• PC boards with attached cables
• Seams/slots in shielded enclosures as they approach 1/2 λ
• Apertures in enclosures as they approach 1/2 λ
• Poorly bonded sheet metal (of enclosures)
• Internal interconnect cables
• Peripheral equipment connected to the equipment under test (EUT)

For example, as a cable or slot approaches 1/2-wavelength (or a multiple) at the frequency of concern, it becomes an efficient transmitting or receiving antenna for interference. To reduce the antenna currents (common-mode currents) on these structures, the solution might be to install ferrite chokes or filters on cables and seal up slots in enclosure seams.

As the structures approach the resonant length of a dipole, their antenna properties become more efficient.