A waveguide of dimensions a = 15 mm and b = 7.5 mm is used as a high-pass filter. If the stop band attenuation required at 8 GHz is ~ 109.2 dB, whatis the length of the filter?
(assume conductor losses to be zero, approximate $$\pi = 3.14$$ and 1 Np ~ 8.69 dB) ($$log_{10}e = 0.4343$$)

A

100 mm

B

869 mm

C

86.9 mm

D

54.6 mm