G4NSJ – Log Periodic antenna aerial


Recently, several people have asked me about the log periodic antenna. It’s an interesting piece of kit and, once understood, it’s not too difficult to grasp the basic operation of the aerial. However, it’s going to take me a while to write a decent and easy to understand explanation. So, please bear with me.

The Log Periodic Antenna:

A log periodic antenna is a multi element directional antenna which operates over a wide range of frequencies. It was invented in 1952 by John Dunlavy. It consists of a series of dipole elements arranged in a specific geometric pattern. A Yagi aerial has a driven element (a dipole) a reflector and several directors. In a log periodic, all the elements are driven. Each one is a dipole.

The antenna works by taking advantage of the properties of the dipole elements. Each element is a half-wave resonator for a particular frequency. The dipole elements are arranged in a geometric pattern where the length of each element decreases as you move toward the end of the antenna. This dipole arrangement creates a series of resonant frequencies that overlap.  As the frequency changes, different elements become resonant which allows the antenna to operate over a wide range of frequencies.

As a signal is received or transmitted by the log periodic antenna, it is captured by the dipole elements that are resonant for that frequency. The signal is then combined and sent to the receiver or transmitter through the feedline.

One of the advantages of the log periodic antenna is its directional properties, which allow it to focus the signal in a specific direction. One of the key advantages of the LPA is its wideband performance, which means it can operate over a broad frequency range with a constant gain and impedance.