A log-periodic antenna (LP), also known as a log-periodic array or aerial, is a multi-element, directional, narrow-beam antenna that operates over a wide band of frequencies, a broad bandwidth. A particular form of the log-periodic design, the log-periodic dipole array or LPDA, is often used in television antennas that work in the VHF band. An LPDA antenna consists of a number of half-wave dipole driven elements of gradually increasing length, each consisting of a pair of metal rods. The dipoles are mounted close together in a line, connected in parallel to the feedline with alternating phase. LPDAs look somewhat similar to the common Yagi antenna, but are very different designs. LPDA and Yag is are often combined in television antennas that cover both VHF and UHF.

Types of log period antenna

There are several formats in which the log periodic antenna can be realised. The exact type that is most applicable for any given application will depend upon the requirements.

The main types of log periodic array include:

• Zig zag log periodic array
• Trapezoidal log periodic
• Slot log periodic
• V log periodic
• Log periodic dipole array, LPDA

The type that is most widely used is the log periodic dipole array, LPDA, and that is described below:

Log periodic dipole array basics

The most common is the log periodic dipole array basically consists of a number of dipole elements. These diminish in size from the back towards the front. The main beam of this RF antenna coming from the smaller front. Not all the antenna is active at any given frequency. The active region, i.e. the sections of the antenna that are contributing to the transmission or reception vary with frequency. The element at the back of the array where the elements are the largest is a half wavelength at the lowest frequency of operation - the longest element acts as a half wave dipole at the lowest frequency. The element spacing also decrease towards the front of the array where the smallest elements are located. The upper frequency is a function of the length of the shortest element.

In operation, as the frequency changes, there is a smooth transition along the array of the elements that form the active region. To ensure that the phasing of the different elements is correct, the feed phase is reversed from one element to the next. There is also normally a sorted matching feeder stub attached to the end of the feeder furthest from the shortest element.