A center-fed half-wave dipole resonates at 100 MHz. What is its approximate free-space length?

A center-fed half-wave dipole resonates at 100 MHz. What is its approximate free-space length?

• A. 3.0 m
• B. 1.5 m
• C. 0.75 m
• D. 6.0 m

Answer: (B)

A center-fed half-wave dipole resonates when its total length is about half the wavelength of the RF signal. The wavelength is found from lambda = c / f, with c ≈ 3 x 10^8 m/s and f = 100 MHz, giving lambda ≈ 3 m. Half of that is about 1.5 m, which is the total length. Since it’s center-fed, each arm is roughly a quarter-wavelength, about 0.75 m each. So the approximate free-space length is 1.5 m. The other options correspond to full wavelength (3.0 m), a quarter-wavelength (0.75 m), or two wavelengths (6.0 m), which don’t match the half-wave resonance.

A center-fed half-wave dipole resonates when its total length is about half the wavelength of the RF signal. The wavelength is found from lambda = c / f, with c ≈ 3 x 10^8 m/s and f = 100 MHz, giving lambda ≈ 3 m. Half of that is about 1.5 m, which is the total length. Since it’s center-fed, each arm is roughly a quarter-wavelength, about 0.75 m each. So the approximate free-space length is 1.5 m. The other options correspond to full wavelength (3.0 m), a quarter-wavelength (0.75 m), or two wavelengths (6.0 m), which don’t match the half-wave resonance.