What is a Log-periodic Antenna in Amateur Radio?

What is a Log-periodic Antenna in Amateur Radio?

Log-periodic antennas are typically used for VHF and UHF frequencies. There are radio amateurs who have built it for 20-6m as well! It is also known as a log-periodic array and is a multi-element directional antenna designed to operate over a wide band of frequencies. Log-periodic dipole array is a common type of log-periodic antenna which consists of several half-wave dipole driven elements. They are of gradually increasing length and mounted close together in a line and connected in parallel to the feedline with alternating phase. Thus it simulates a series of Yagi-Uda antennas tuned to different frequencies, connected together. But the Yagi-Uda antenna has only a single driven element while all elements in a log-periodic dipole array are driven elements. Other elements in a Yagi-Uda antenna are parasitic, with one reflector and several directors. While Yag-Uda antenna has a very narrow bandwidth, log-periodic dipole array has a large bandwidth.

While adding elements to a Yagi-Uda antenna increases its directivity and gain, adding elements to a log-periodic dipole array increases its frequency response or bandwidth. Log-periodic antenna has the same radiation resistance, SWR, gain and front-to-back ratio across the frequency range for which it is meant. Elements of a log-periodic dipole array are pairs of metal rods fixed on a boom. Spacing of the elements follow a logarithmic function of the frequency. Length of successive elements and spacing between them gradually decrease along the boom.