An antenna is placed in dielectric medium of dielectric constant 6.25. If the maximum size of that antenna is 5.0 mm, then it can radiate a signal of minimum frequency of __________GHz. 

Wavelength, $\lambda =4l=4\times 5\times {10}^{-3}=20\times {10}^{-3}m$

 $v=\frac{C}{\sqrt{{\mu }_r{\in }_r}}or\lambda f=\frac{C}{\sqrt{{\mu }_r{\in }_r}}$

$f=\frac{C}{\lambda \sqrt{{\mu }_r{\in }_r}}=\frac{3\times {10}^8}{20\times {10}^{-3}\times \sqrt{6.25}}[\because {\mu }_r=1fornon-magneticmedium]$

$=\frac{3\times {10}^8}{20\times {10}^{-3}\times 2.5}=6\times {10}^{9}Hz=6GHz$