Dipole Antenna

Overview

A half-wave dipole is two equal lengths of wire fed at the centre. Its total length is close to a half wavelength at the frequency of interest, which makes it resonant and presents an impedance near 73 ohms at the feed point in free space — close enough to 50 ohms that most radios need no matching at all.

In practice the wire is not in free space. Height above ground, nearby objects, and the insulation on the wire all change the resonant frequency, which is why a dipole is always cut slightly long and then trimmed while watching an SWR meter or analyser. A dipole at a half wavelength above ground has a broad, low-angle pattern that is excellent for distance work.

Bands and modes

Band coverage

Modes

Safety note

Using it well

• Cut the wire long, then trim in small steps while watching SWR at the band edges.
• Get the feed point as high as you reasonably can before adding any matching.
• Fit a 1:1 current balun at the feed point to stop shield current.
• Mark the trimmed length on the wire so future adjustments start from a known point.