Log Periodic Antenna / Aerial

- 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 of the type often used for television reception,

The log periodic antenna was initially developed by Dwight E. Isbell, Raymond DuHamel who published a paper in 1957 later additional variants were made by Paul Mayes. The concept of the log periodic antenna was patented by the University of Illinois in the USA.

Log periodic antenna applications

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. There are several areas where the antenna is used:

  • UHF Terrestrial TV:   The antenna is sometimes seen in the form of UHF terrestrial antenna applications. The television spectrum extends over a wide bandwidth - more than normal Yagi antennas can comfortably cover. Normally channels for a given area are located within a particular subset of the UHF television spectrum so that the channels are relatively close together. However there can be situations where the UHF channels may be located over a wide portion of the television spectrum. This may be because channels from different transmitters located across the television spectrum may be needed, or that spectrum restrictions can mean that the channels are spaced wide apart..
  • HF communications:   Log periodic antenna arrays are often used in applications where HF communications for diplomatic traffic. These antennas perform well because the diplomatic services may need to operate over a wide selection of frequencies in the HF bands, and it is often only feasible to have one antenna, for example on an embassy building.
  • EMC measurements:   EMC measurements require scans over a wide bands of frequencies. Log periodic antennas can be used in this application to enable operation and sensing of signals over the wide bandwidths needed. As such the log periodic antenna is key to the operation of many EMC tests and test environments.
  • Other applications:   There are many other applications where log periodic antennas can be used. Any applications where directivity and a wide bandwidth are needed are ideal applications for this form of RF antenna design.

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.

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.

A log periodic dipole array showing the way in which the element lengths shorten in the direction of the main beam,
Basic log periodic dipole array

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

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

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.

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.

In real terms a typical log periodic antenna might provide between 3 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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