A broadband antenna with almost frequency-independent characteristics within its designed band of operation. To achieve this, the structural dimensions increase in proportion to the distance from the origin of the structure, thus giving an approximately constant number of active elements within logarithmically changing frequency bands (hence the name log-periodic).

A common form is composed of dipoles arranged along a two-conductor tapered transmission line and is fed at one end of the transmission line. The dipoles are typically short in length and close together near the feed and become longer and more widely separated further from the feed. The radius of the dipoles should also increase with distance from the feed, but this is usually implemented in a series of steps. Where the elements of the dipoles on one side are arranged along the same transmission line, the antenna operates in broadside mode (see antenna array); where they are arranged on alternate transmission lines, it operates in endfire mode.