The principal of this type of absorber is to mimic free space by ‘progressive impedance’ that is, to progressively absorb the radio wave/microwave through a resistive structure having a progressive shape, this converts the electromagnetic energy to heat energy (you can’t normally notice any heat).
The absorbing performance is relative to the wavelength, so, where the wavelength is long at lower frequencies the absorber sees less waves or only part of a wave and thus, the return loss performance is lower.
Where the wavelength is shorter at higher frequencies pyramidal absorbers work well and can achieve very high levels of up to -60dB of return loss anechoic performance.
These absorbers have 20% greater mass than four sided absorbers and so have a low frequency absorbing advantage, the payback is that the larger surfaces limit the microwave performance to about -40dB.
Hybrid Pyramidal absorbers are designed specifically for the use over ferrite tiles. The resistive doping is far less than normal pyramidal absorbers and is very specific.
Global EMC offer two different types of Hybrid pyramidal absorber:
GDSxxx-H3 - This is the doped foam type, used for many years and has proved to be fully compliant in CISPR16-4-1 3m, 5m and 10m EMC test chambers. This option is more cost effective.
Glue-less fixing systems are available
RLT450/RLT650 – This NEW product is a solid homogeneous resistive doped polystyrene system. This product has some major advantages:
Base board & 'plug-in' system
NOTE – All Global EMC pyramidal absorbers are fire resistant and zero halogen.
When larger pyramid absorbers are used it is not practical to have homogeneous doped foam pyramidal absorbers because the weight causes ‘drooping’.
Where the use of very long pyramids are required it is normal to have hollow rigid structure absorbers, these may have one or more than one resistive layer so as to gain both low and high frequency performance.
Hollow absorbers only work on progressive impedance so they can address low frequency and high frequency anechoic applications without using ferrite tiles.
The problem with Hollow absorbers is that to address the frequency of 30MHz (for EMC use) the pyramids need to be 2m long and are only useable in a very large chamber.