EMC considerations for aerospace & defence
Author : David Wall | Chairman | Kemtron
17 March 2021
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The need to protect equipment in harsh environments and battlefield conditions has been responsible for driving forward advances in shielding technology, says David Wall, Chairman of EMI shielding & environmental sealing specialist, Kemtron.
RFI (radio frequency interference)/EMI (electromagnetic interference) shielding has always been important for the aerospace & defence sector, with security being the prime consideration. In particular, there are concerns around the susceptibility of some computing & telecommunications devices to emit electromagnetic radiation (EMR) in a manner that could be used to reconstruct intelligible data.
The TEMPEST (Telecommunications Electronics Material Protected from Emanating Spurious Transmissions) programme is a US NSA (National Security Agency) specification and NATO certification concerned with spying on information and communications systems via leaking emanations, including unintentional radio or electrical signals, sounds and vibrations. TEMPEST covers how to shield and protect equipment against such spying, also known as emission security (EMSEC), a subset of communications security (COMSEC). Protecting equipment is achieved with distance, shielding, filtering and masking.
Battlefield drivers include NEMP, or nuclear electromagnetic pulse, which is a burst of electromagnetic radiation created by nuclear explosions. The resulting rapidly changing electric and magnetic fields may couple with electrical and electronic systems to produce damaging current and voltage surges which could cause electronic equipment to fail. Physical drivers include harsh temperature environments, from -40c to + 200c, resistance to fuels, chemicals and agents such as NBC (nuclear, biological and chemical) wash down, as well as galvanic corrosion issues caused by salt fog in marine environments.
EMC, or electromagnetic compatibility, is the ability of equipment to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to other equipment in that environment. Electronics engineers are very familiar with this and will consider in their design good board layout, filtering, grounding and signal integrity to try and resolve EMI at its source.
However shielding of the enclosure is just as important and solves the problem of radiated emissions and susceptibility. Mating surfaces on an enclosure can look very flat, and you think there is full metal to metal contact, but in reality, in a mass production process, nothing can be that flat and gaps will exist. These gaps are slots and can become radiating antennas. This joint unevenness can be addressed by using more fixings to get good contact between the mating surfaces, and at lower frequencies, this can work.
For example, at 100MHz, to achieve 20db shielding, gaps of 150mm are acceptable; this reduces to 15mm at 40db. And at 1GHz, it’s impossible, as to achieve 40db the gap is now 1.5mm. These seams need to be filled with a conductive gasket, this will take up all the joint unevenness, and depending on the type of gasket, will also provide an environmental seal against dust and moisture.
Shielding is a mechanical fix for an electrical problem, and the enclosure design engineer should be aware of the types of gaskets available, and their different attributes, and ensure there is enough land area on the enclosure seams and doors to fit the gasket. In the quest to reduce size and weight of electronic equipment, there is a lot a pressure to reduce these land areas, which makes this more important than ever.
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Electrically conductive elastomers are the most preferred EMI shielding gaskets types in the aerospace, military & defence communications industry. The base Elastomer is silicone for normal environments and fluorosilicone for fuel and oil resistance. Both offer a wide temperature range of -40 to +160 and up to +200 for some grades. The conductive fillers available are silver plated aluminium, silver plated copper, pure nickel and nickel coated graphite.
Kemtron’s process capability is extrusion, moulding and conductive vulcanised jointing. Many extrusion profiles are available including cords, tubes, hollow D, solid D – and custom profiles are also available. Extrusions are supplied in continuous lengths, cut to length or made into large or small diameter O rings which are produced by joining extruded profiles with the same conductive compound as the base material; the join is heat vulcanised ensuring a strong conductive joint. Mouldings include sheet stock, O rings, custom shapes and gaskets. Sheets can be die cut to customers design and a conductive PSA can be applied to aid assembly.
Knitted wire mesh is the original gasket for RFI/EMI shielding and tends to be preferred in the military vehicle/communication cabin market. In applications where low frequencies interact with the magnetic field, such as HEMP (high altitude electromagnetic pulse), knitted mesh gaskets make it the best performer. At 10 kHz, H field (magnetic field), it is possible to achieve a shielding effectiveness of 62 dB; at higher frequencies, such as 10Ghz, attenuation’s of above 45 dB can be achieved.
Knitted mesh is very versatile as it can be knitted to any diameter from 1mm up to 25 mm as a solid mesh, or over any type of sponge rubber cord or tube, therefore fine-tuning its compressibility from hard to very soft. Rectangular sections are available and sponge or solid rubber with a self adhesive backing can be bonded along the length of the mesh to form an environmental seal and act as an assembly aid.
Custom gaskets can be made from knitted mesh by fabrication or bonding the mesh to the edge of die cut rubber shapes; compression stops can be bonded into the rubber carrier to protect the gasket from over compression.
Standard wire types available are Monel, TCS (tinned copper-clad steel), aluminium and stainless steel, giving the option of matching wire types with enclosure materials to give galvanic compatibility.
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