In the far zone higher frequencies result in ______ beam diameters.

In the far zone higher frequencies result in ______ beam diameters.

• A. Larger
• B. Same
• C. Smaller
• D. Unchanged

Answer: (C)

In the far field, the beam size is determined by diffraction from the aperture. The spreading angle is roughly proportional to the wavelength over the aperture size (for a circular aperture, about 1.22 λ/D). Since wavelength is inversely related to frequency (λ = c/f), higher frequency means a shorter wavelength, which reduces the diffraction angle. At a fixed distance, the beam diameter is the spread angle times that distance, so increasing frequency makes the beam narrower. Put simply: higher frequency yields a smaller beam diameter in the far field.

In the far field, the beam size is determined by diffraction from the aperture. The spreading angle is roughly proportional to the wavelength over the aperture size (for a circular aperture, about 1.22 λ/D). Since wavelength is inversely related to frequency (λ = c/f), higher frequency means a shorter wavelength, which reduces the diffraction angle. At a fixed distance, the beam diameter is the spread angle times that distance, so increasing frequency makes the beam narrower. Put simply: higher frequency yields a smaller beam diameter in the far field.