An unbiased data sheet
Anechoic absorbers for the absorbing of radio wave and microwave energies can also be known a RAM (radar absorbing material)
The inside of a shielded chamber will reflect any radio wave/microwaves this causes gross measurement distortion hence the requirement for absorbers.
The pyramidal shape of the absorber presents a small amount of mass (at the point) and progressively increases the mass as the wave moves towards the base.
The technology is known as ‘progressive impedance’
The substrate material is commonly polyurethane foam impregnated with a resistive carbon solution which is then kiln dried.
The doping of the foam can be ‘taper doped’ or ‘double dipped’ so the doping is often not homogeneous in the true sense.
The outer aspects are normally spray painted with a UL 94-V0 fire rated latex paint
The finished product has to be ‘fire safe’. An American naval standard NRL8093 parts I, II & III are most commonly used but there are other oxygen index methods that can be applied.
The performance (return loss) of a pyramidal absorber is mainly due to the relationship between the length of the pyramid and the wavelength.
Example: for say 760MHz: 300/760=0.395 (395mm)
Wavelength can be known as Lambda and uses
the symbol λ
the following is a guide:
There are special doped pyramid absorbers that can give -60dB at 10 x wave length
The above figures are in normal incidence.
The decay in performance with off normal incidence (where the wave hits the absorber at an angle) range from no effect at 20o off normal up to 50% performance loss at 70o off normal
