Log periodic antenna

- overview, summary, tutorial about the log periodic antenna or aerial used for wideband RF antenna applications.

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.

The log periodic antenna is used in a number of applications where a wide bandwidth is required along with directivity and a modest level of gain. It is sometimes used on the HF portion of the spectrum where operation is required on a number of frequencies to enable communication to be maintained. It is also used at VHF and UHF for a variety of applications, including some uses as a television antenna.

Log periodic array capabilities

The log periodic antenna was originally designed at the University of Illinois in the USA in 1955.

This type of RF antenna design is directional and is normally capable of operating over a frequency range of about 2:1. 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.

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 will be described here.

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. 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 element spacing also decrease towards the front of the array where the smallest elements are located. 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.

Log periodic dipole array
Basic log periodic dipole array

Log periodic performance

The log periodic antenna is a particularly useful design when modest levels of gain are required, combined with wideband operation. A typical example of this type of RF antenna design will provide between 4 and 6 dB gain over a bandwidth of 2:1 while retaining an SWR level of better than 1.3:1. With this level of performance it is ideal for many applications, although a log periodic antenna will be much larger than a Yagi that will produce equivalent gain. However the Yagi is unable to operate over such a wide bandwidth.

By Ian Poole

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