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What is a Log Periodic Antenna?

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  • Written By: G.W. Poulos
  • Edited By: Daniel Lindley
  • Last Modified Date: 11 September 2016
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A log periodic antenna is a specialized type of high frequency antenna. Unlike omnidirectional antennas, a log periodic antenna only receives in one direction, and unlike standard directional antennas, the typical television antennas seen on rooftops, these antennas can receive a broad range of frequencies. Typically, these antennas are constructed of a series of parallel metal tubes that are large in the back and get progressively smaller, forming a sort of triangle. These antennas are most often used for specialized applications; however, they are sometimes used as UHF and VHF television antennas.

Most high frequency antennas have a single pair of dipole antenna elements, a pair of metal tubes that serve as the active elements of the antenna, and a number of reflectors and directors that bounce signals on to the dipole. As the frequencies an antenna can receive are based on the physical dimensions of the dipole, most high frequency antennas are only capable of receiving signals in a narrow range. A log periodic antenna overcomes this shortcoming by employing a series of different sized dipole elements that vary in physical size and reception capabilities, according to a logarithm.

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The logarithm used in designing a log periodic antenna starts with the physical size needed for the highest frequency needed to be received, which will be the smallest set of dipole elements. A logarithm is established that determines the size of the second set of dipoles so that their minimum frequency reception slightly overlaps the maximum reception of the first set. This procedure is repeated, and each pair of dipole elements gets larger with each iteration, until the antenna is capable of receiving all frequencies desired for the application.

Different size dipole pairs are then oriented on a single axis so that they are parallel to one another, with the largest lower frequency dipole in the rear of the antenna and the smallest higher frequency dipole located in the front. As the phases of the received signals on one dipole can interfere with other dipoles, each dipole is wired 180 degrees out of phase with the next and the last. In this way, the dipoles will eventually reach a difference of 360 degrees and then be in line with one another, electrically speaking, increasing the overall gain of the antenna.

Log periodic antennas also experience problems with impedance, the amount of electrical resistance between the two elements of a single dipole. To solve this problem, it is common for the metal tubes of the dipoles to also become larger in diameter as they become longer, thus altering the dipole’s impedance. Another method employed to match the impedance is to install small matching transformers of different values to each pair of dipoles so that impedance is the same across all of the antenna’s active elements.

The result is an antenna that can only see signals in a single direction, like a Yagi antenna, has reception power comparable to an omnidirectional antenna, and is capable of receiving a much wider range of frequencies than either. Though sometimes used as television antennas, log periodic antennas are most often used by amateur radio operators desiring to operate over a broad spectrum of frequencies. These types of antennas have also been at the center of study for experimental transmission and reception of electrical power.

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