Log Periodic PCB Antenna

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 is able to provide directivity and gain while being able to operate over a wide bandwidth.

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.

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.

Log periodic basics

The log periodic antenna can exist in a number of forms. The most common is the log periodic dipole array (LPDA). It 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 spacings 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.

A log periodic dipole array

It is possible to explain the operation of a log periodic array in straightforward terms. The feeder polarity is reversed between successive elements. Take the condition when this RF antenna is approximately in the middle of its operating range. When the signal meets the first few elements it will be found that they are spaced quite close together in terms of the operating wavelength. This means that the fields from these elements will cancel one another out as the feeder sense is reversed between the elements. Then as the signal progresses down the antenna a point is reached where the feeder reversal and the distance between the elements gives a total phase shift of about 360 degrees. At this point the effect which is seen is that of two phased dipoles. The region in which this occurs is called the active region of the RF antenna. Although the example of only two dipoles is given, in reality the active region can consist of more elements. The actual number depends upon the angle [greek letter alpha] and a design constant.

The elements outside the active region receive little direct power. Despite this it is found that the larger elements are resonant below the operational frequency and appear inductive. Those in front resonate above the operational frequency and are capacitive. These are exactly the same criteria that are found in the Yagi. Accordingly the element immediately behind the active region acts as a reflector and those in front act as directors. This means that the direction of maximum radiation is towards the feed point.

Feed arrangements

The log periodic dipole antenna presents a number of difficulties if it is to be fed properly. The feed impedance is dependent upon a number of factors. However it is possible to control this by altering the spacing, and hence the impedance for the feeder that connects each of the dipole elements together. Despite this the impedance varies with frequency, but this can be overcome to a large extent by making the longer elements out of a larger diameter rod. Even so the final feed impedance does not normally match to 50 ohms on its own. It is normal for a further form of impedance matching to be required. This may be in the form of a stub or even a transformer. The actual method employed will depend to a large degree on the application of the antenna and its frequency range.,/p>

Overview

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