The minimum frequency of a vertically directed radio wave which will penetrate a particular layer in the ionosphere; for example, all vertical radio waves with frequencies greater than the E-layer critical frequency will pass through the E layer. The critical frequency is also known as penetration frequency.
fc= [ sqrt(3)c2/pi*d ] * [ sqrt(ρ/Y) ] Where,
fc = Coincidence Frequency [Hz]
c = Speed of sound in air [ms-1]
d = Thickness of panel [m]
ρ = Density of panel [kgm-3]
Y = Young\'s modulus of panel [Nm-2]