What is the relationship between frequency and antenna size?

• A. Higher frequency usually means a larger antenna
• B. They are unrelated
• C. Antenna size increases with wavelength
• D. Higher frequency usually means a smaller antenna

Answer: (D)

Antenna size is tied to the wavelength of the signal. Wavelength is roughly the speed of light divided by frequency, so λ ≈ c/f. For efficient radiation, many antennas are designed to be about a half-wavelength long (a common dipole). When you increase frequency, the wavelength shortens, so the physical length you need becomes smaller. For example, at 100 MHz λ ≈ 3 m (half-wavelength ≈ 1.5 m); at 1 GHz λ ≈ 0.3 m (half-wavelength ≈ 0.15 m). So higher frequency usually means a smaller antenna. There are trade-offs with very small antennas, such as reduced efficiency and bandwidth, but the basic relationship is that antenna size scales inversely with frequency. The other options either state the opposite direction, claim no relation, or restate the size-wavelength idea in a less direct way.