Log Periodic Antenna Theory

- notes and overview about the theory of operation of a log periodic dipole array antenna..

The theory of operation of the log periodic dipole array can become complicated. However to give a comprehensible introduction to the log periodic theory some basic explanations are given below.

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.

Log periodic dipole array
Basic log periodic dipole array

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 α 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.

By Ian Poole

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