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The microwave absorbers, usually for RF, for simplicity can be divided into two categories:
FOR ANECHOIC CHAMBERS, for antennas or open space. They work on the principle of gradual impedance transition, they are of pyramidal shape for anechoic chambers. They are often used to reduce the secondary lobes or unwanted couplings, for example on radar antennas, in this case they are more than 1 cm thick.
FOR ENCLOSED SPACE OR CAVITY, a common metal box acts as a cavity excited by secondary propagation modes, for very high frequencies (eg, microwave) or in large boxes, the RF circuit will tend also to self-oscillate. Since no box is identical, in size, shape, frequency of use and exciting mode, to another, it is very difficult to determine in advance what can be the propagation mode associated with that particular frequency.
The absorber inserted into the cavity is designed to attenuate the resonance peaks by changing the characteristics of the cavity and the phase of the reflected signal, moreover being made up of carbon it will tend to deteriorate the Q of the cavity in order to decrease the self-oscillations in resonance peaks. For optimum operation this absorber must be laid on a metal part (cover). As a general rule in an absorber as thickness is increased the minimum operating frequency is
lowered but increase costs, another important quality that a good absorber must have is a poor conductivity to prevent short circuits if it falls on the components. The MAS... and MAF-3A absorbers are only used in cavities (metal boxes) and placed on a metal plane, they are provided, for easy installation, of a self-adhesive film. Types MAF-10A and MAF-19, thanks to the big thickness and the fact that they are multilayer, are also suitable as absorber in open space, eg to reduce the lobes on the antennas, this two types have also high attenuation.
Click here to see our article published on VHF Communications 4-2004 about microwave absorbers