For 'beam' type directivity, an external half-wave hertzian dipole with reflector and director(s) would also increase radiation strength.
(The active element is actually 2 quarter-waves back-to-back).
The half-wave (λ/2) is measured across total 'width' of dipole.
A single-element quarter wavelength is onmi-directional, perpendicular to the antenna 'wire' (doughnut shape radiation field).
As c is (3*(10^8))m or (300*(10^6))m and your'e working in MHz (f.MHz*(10^6), simply divide
300 by n.MHz to get λ (Full Wave length) length in metres.
Then (λ/2) or (λ/4) for half or quarter-wave dimensions.