What happens to beam diameter in the far field as frequency increases?

• A. Beam diameter becomes smaller
• B. Beam diameter becomes larger
• C. No change
• D. Not determinable

Answer: (A)

Diffraction governs how a beam spreads in the far field, and the amount of spread is tied to the wavelength. The far-field divergence for a circular aperture scales with λ/D, so as the wavelength gets shorter (which happens when frequency increases, since λ = c/f), the divergence shrinks. With a fixed distance, a smaller divergence angle means the beam diameter in the far field becomes smaller. In other words, increasing frequency reduces the far-field beam diameter (roughly proportionally to the change in wavelength; doubling the frequency halves the wavelength and about halves the beam spread).