Electromagnetic coupling into rectangular rack-and-panel connectors

Measurement of the surface transfer impedance of both standard and EMI rack-and-panel connectors from 1 kHz to 100 MHz showed that the standard rack-and-panel connector provides essentially no electromagnetic shielding because it contains no positive mechanism for maintaining electrical contact between the plug and receptacle. Supplementary grounding devices reduce the transfer impedance to desirable levels for frequencies below a few tens of kHz. Electromagnetic shielding at high frequencies is minimal. Spring fingers around the periphery of the plugs were very effective for maintaining electrical contact between the plug and receptacle and preventing electromagnetic coupling through the plug/receptacle interface. Control of the electromagnetic coupling through the plug/backshell interface was essential for good high frequency shielding performance. Gaskets installed in the plug/backshell interface were electromagnetically effective but cumbersome to install. At 100 MHz, the surface transfer impedance was dominated by the bonding impedance between the receptacle and the chassis. When spring fingers and a gasketted backshell were used, the transfer impedance of the rack-and-panel connector slowly increased from a fraction of a milliohm at low frequencies to several milliohms at high frequencies. This is about equivalent to the electromagnetic shielding expected of a quality circular connector and a good backshell.