What is 'dipole' in RF, and what is its typical impedance in free space?

• A. A loop radiator; impedance around 600 ohms.
• B. A coaxial radiator; impedance around 50 ohms.
• C. A simple two-conductor radiator; impedance around 73 ohms.
• D. A patch radiator; impedance around 300 ohms.

Answer: (C)

A dipole is the simplest RF radiator: two straight conductors fed at their center. When its total length is about half a wavelength, it resonates and the current distribution is strongest at the feed, dropping to zero at the ends. In free space, a thin center-fed half‑wave dipole presents an input impedance of about 73 ohms at resonance, which is largely resistive. That 73-ohm figure is the typical intrinsic impedance of this radiator, distinguishing it from other shapes like loops, patches, or coax-fed structures. If you’re using a 50-ohm feed line, you’d match it with a network or balun to bridge the 73-ohm dipole to 50 ohms.