Wavelength in vacuum (open air is almost the same)

    \[ \lambda _{0} = \frac{c}{f} \]

Frequency is 2400 MHz

    \[ = \frac{300}{2400} \]

Final resulting wavelength in cm

    \[ = 16.67 cm \]

Now for a PCB: Dk (Effective Dielectic constant)=Er (Epsilon r=Relative permitivity) =Eeff (Epsilon effective, recognizing it is not a solid homogeneous material but a mixture of materials (resin and glass)) FR4 (PCB material) the Eeff is roughly 4.2 Guide wavelength in a PCB

    \[ \lambda_{g}=\frac{c}{f\sqrt{\varepsilon_{eff}}} \]