| Radiocommunications Agency
|Shielding of areas and volumes|
Of course it is incorrect to talk about a shielded area, since shielding must always enclose a volume, but the title of this item uses the word ‘area’ because people often talk about shielding a particular area of a site.
In the rest of this item the phrase ‘shielded zone’ is used instead of shielded area or volume. A zone is a segregated part of a system or installation, and segregation is described here.
What this technique is used for
As we all know, electrical energy can travel in a conducted mode along any conductor, and we measure it using volts and amps. However, electrical energy can also travel in a radiated mode through the air and other insulators such as wood and plastic, and now we measure it using volts/meter and amps/metre (i.e. electric and magnetic field strengths).
Shielding is used to prevent internal energy from ‘leaking’ out by radiated routes and causing interference with other equipment. In this case the shielding would be said to be reducing emissions.
The shielding technique is also used to prevent external energy from ‘leaking in’ by radiated paths and causing interference. In this case the shielding would be said to be increasing immunity.
Because all conductors behave as ‘unintentional antennas’ and to some degree convert conducted energy to radiated energy (and vice-versa) the technique known as shielding helps to control both radiated and conducted emissions and immunity.
How this technique is used
Shielding reduces the amount of unwanted electrical energy travelling through a path in air, vacuum, or in an insulator such as plastic, by creating a significant change in the wave impedance of the path.
The wave impedance of air or vacuum is 377 ohms. Large changes in the wave impedance cause electric and magnetic fields to be reflected and the shielding material itself absorbs some of the remaining energy. Metal provides good shielding because its wave impedance can be made very low.
In installations, shielded zones always uses metal, sometimes as continuous sheets and sometimes in a mesh or grid, for example: ‘chickenwire mesh’; ‘weldmesh’, expanded or perforated metal, metal-coated woven and non-woven fabrics.
A shielded zone will have six shielded sides, and the base or floor of the shield should form part (or all) of that zone’s RF reference plane.
A degree of shielding can be achieved by placing items of equipment and their interconnecting cables as close as possible to the zone’s RF reference plane. But some shielding applications, and all high-performance shielding, requires the zone to be surrounded by a completely shielded volume (on all six of its sides, floor and ceiling).
Key issues in employing this technique
Buy or D-I-Y?
Some installation engineers design and/or build their own shielding. Applying shielding materials to existing walls, floors and ceilings is sometimes called ‘architectural shielding’. Sometimes stand-alone shielded rooms are constructed, from sheet or meshed metal. There are a few companies that offer architectural shielding services.
Metallised fabric can be used to create shielded ‘tents’, or for architectural shielding, although two separated layers will be needed for high performance and shielding effectiveness and even so its performance below 1MHz will be poor.
However, where a specified shielding effectiveness, or high performance and/or high frequency shielding is required, it is preferable to purchase a shielded room with a specified performance, from one of the many shielded room manufacturers.
Sometimes a standard room will suffice, but a custom room may be required, and most shielded room manufacturers will be able to design and manufacture them.
Apertures and conductor penetrations
Bulk metal that is more than 1mm thick provides excellent shielding at frequencies above 10kHz, but in practice theshield is seriously compromised by apertures – which can reduce its shielding effectiveness (SE) to zero. Apertures include doors, windows, air vents, gaps, seams, and joints. It does not matter how thin a gap or seam is – or how labyrinthine is the path that the radiated fields have to take to pass through it – their effect on the shielding depends on the largest dimension of the aperture that exists in the shield.
The degree to which apertures reduce the shielding of an enclosure depends on how many there are, their spacings and orientations, their maximum dimensions, and the wavelength of the signals to be shielded. Design techniques to control aperture ‘leakage’ include: minimising the number of apertures and their dimensions; using shielded windows and vent panels, and fitting a wide variety of types of conductive gaskets along seams and around the edges of metal doors.
All cables behave as ‘unintentional antennas’ – converting radiated fields to conducted voltages and currents, and vice-versa. So any cables that penetrate a shielding surface can carry unwanted field energy from one side of the shielding barrier to the other side, significantly reducing the shielding effectiveness that can be achieved.
Because of this, all conductors that penetrate a shield must either be filtered or shielded. When using filters it is very important that the metal body of the filter makes metal-to-metal contact with the shield barrier itself. The same applies to the shield when using a shielded conductor.
At frequencies above 10MHz the use of “360 degree bonding” techniques (sometimes called peripheral bonding) becomes more important, for both filters and cable shields.
Bonding to the RF Reference Plane
As was mentioned earlier, the base or floor of a shielded zone must be an RF reference plane. It may be only the RF reference plane for that zone or it might be a plane shared by a number of zones – or even for the whole installation.
A seamless metal floor or seamless shielded volume (e.g. a shielded room) makes the best RF reference plane, but more often the RF reference plane is a meshed common bonding network of some description. In the computer and telecomm’s industries some installation standards call it the ‘system reference potential plane’ (SRPP).
Most installations that have an RF reference plane use a meshed common bonding network (CBN) made from existing metalwork plus other ‘meshing’ conductors as needed to get the mesh size as small as is required. Meshed bonding systems are described in IEC 61000-5-2, where they are called MESH-CBNs or MESH-IBNs. (An IBN is an insulated bonding network, not generally recommended.)
The diagonal size of a mesh governs its shielding performance, with the highest frequency at which a meshed RF reference plane provides any shielding at all being given by the formula: F (in MHz) = 30/d (where d is the size of the largest diagonal in metres). There is a sketch of a meshed CBN in the Filters section.
Without a meshed CBN (or IBN) or sheet steel floor there is no RF reference plane for a zone and no sixth side for a shield. Consequently the shield will have a gaping aperture (due to missing a side) and will hardly provide any shielding at all to its zone.
In an installation, a shielded zone will comprise a great many different metal parts, and they must all be RF bonded together.
Low frequency shielding
Below 5kHz, ordinary sheet metals will be less effective at shielding magnetic fields: e.g. 1.5mm thick mild steel will only give 9dB shielding at 50Hz. High levels of shielding at low frequencies requires specialist materials and construction techniques.
The synergy of shielding and filtering
High-performance shielding at any frequency, or any useful degree of shielding at frequencies above 100MHz, requires both shielding and filtering, and also requires careful location and bonding of the filters to the shielding surface.