A baseband signal of 3 . 5MHz frequency is modulated with a carrier...

The wavelength of the carrier wave is,

${\lambda}_{c} = \dfrac{c}{{ν}_{c}} = \dfrac{3 \times {10}^{8}}{3 . 5 \times {10}^{9}} = \dfrac{3}{35}m$.

The minimum size of the antenna required is $\dfrac{1}{4}th$ of the wavelength of the carrier. Therefore,

$l = \dfrac{1}{4} \times \dfrac{3}{35} = 21 . 4mm$