How is antenna size determined?

• A. Antenna length is determined by the transmitter power
• B. Antenna length is matched to a fraction of the wavelength
• C. Antenna size is unrelated to frequency
• D. Antenna length scales with square of wavelength

Answer: (B)

Antenna size is driven by the wavelength of the signal it’s designed to radiate, not by transmitter power. To efficiently radiate, an antenna is typically made a simple fraction of the wavelength at the operating frequency—such as about half a wavelength for a dipole or a quarter wavelength for a monopole with a ground plane. When the length matches these fractions, the current distribution along the antenna supports a strong radiation pattern and a practical impedance, making power transfer from the transmitter into free space efficient. If the antenna is made much longer or shorter than these fractions, it becomes detuned, radiates poorly, and requires more complex matching. Transmitter power affects how much energy you push into the antenna and thermal considerations, not the fundamental resonant length. The idea that size scales with the square of the wavelength isn’t correct; the practical relationship is largely linear, via these fractional wavelengths, with adjustments for bandwidth and design optimizations.