Mumbai University > Electronics and Telecommunication > Sem 5 > RF Modeling and Antennas

Marks: 6M

Year: May 2015

The log periodic antenna or aerial is in many areas to provide a wideband RF antenna solution with gain and directivity over that offered by a dipole. One of the major drawbacks with many RF antennas is that they have a relatively small bandwidth. This is particularly true of the Yagi beam antenna. One design named the log periodic antenna is able to provide directivity and gain while being able to operate over a wide bandwidth. In particular the log periodic dipole array is the most widely used version of this antenna family.

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.

Log periodic performance

a. The main log periodic antenna performance differentiator is the wide bandwidth it possesses.

b. This type of RF antenna design is normally capable of operating over a frequency range of about 2:1 while still being able to provide a usable level of forward gain over a dipole.

c. It has many similarities to the more familiar Yagi because it exhibits forward gain and has a significant front to back ratio. In addition to this the radiation pattern of this RF antenna design stays broadly the same over the whole of the operating band as do parameters like the radiation resistance and the standing wave ratio. However it offers less gain for its size than does the more conventional Yagi.