World Library  
Flag as Inappropriate
Email this Article

Log periodic antenna

Article Id: WHEBN0000041332
Reproduction Date:

Title: Log periodic antenna  
Author: World Heritage Encyclopedia
Language: English
Subject: Self-complementary antenna, Fractal antenna, AWX antenna, Folded inverted conformal antenna, Sterba antenna
Publisher: World Heritage Encyclopedia

Log periodic antenna

Log-periodic antenna, 250–2400 MHz
Log-periodic antenna, 400–4000 MHz

A log-periodic antenna (LP), also known as a log-periodic array or aerial, is a multi-element, directional, narrow-beam antenna that operates over a broad band of frequencies. This type of antenna is defined as having its structure prescribed by opening angles. A particular form of the log-periodic design, the log-periodic dipole array or LPDA, is often used in television antennas that work in the VHF band.[1] LPDA antennas look somewhat similar to the Yagi antenna, but are very different designs. LPDA and Yagis are often combined in television antennas that cover both VHF and UHF.

Basic concept

The LPDA normally consists of a series of dipoles known as "elements" positioned along a support boom lying along the antenna axis. The elements are spaced at intervals following a logarithmic function of the frequency, known as d or sigma. The length of the elements correspond to resonance at different frequencies within the antenna's overall bandwidth. This leads to a series of ever-shorter dipoles towards the "front" of the antenna. The relationship between the lengths is a function known as tau. The ever-decreasing lengths makes the LPDA look, when viewed from the top, like a triangle or arrow with the tip pointed in the direction of the peak radiation pattern. Sigma and tau are the key design elements of the LPDA design.[2][3]

Every element in the LPDA design is "active", that is, connected electrically to the feedline along with the other elements, though at any one frequency most of the elements draw little current from it. Each successive element is connected in opposite phase to the active connection running as a transmission line along the boom. For that reason, that transmission line can often be seen zig-zagging across the support boom holding the elements.[3] One common design ploy is to use two booms that also acts as the transmission line, mounting the dipoles on the alternate booms. Other forms of the log-periodic design replace the dipoles with the transmission line itself, forming the log-periodic zig-zag antenna.[4] Many other forms using the transmission wire as the active element also exist.[5]

The Yagi and the LPDA designs look very similar at first glance, as both consist of a number of dipole elements spaced out along a support boom. The Yagi, however, has only a single dipole connected to the transmission line, usually the second one from the back of the array. The other dipoles on the boom are passive, with their two sides shorted, acting as "directors" or "reflectors" depending on their slightly different lengths and position relative to the "driven element". The difference between the LPDA and Yagi becomes obvious when examining their electrical connections. Another clear difference is the length of the dipoles; LPDA designs have much shorter dipoles towards the front of the antenna, forming a triangular shape as seen from the top, whereas the difference in lengths of Yagi elements is less noticeable. Another visible difference is the spacing between the elements, which can be rather constant in the Yagi but becomes exponentially wider along the LPDA. Although both directional, the LPDA is intended to achieve a very wide bandwidth, whereas the Yagi has a very narrow bandwidth but achieves greater gain.

It should be strictly noted that the "log-periodic shape" does not provide with broadband property for antennas.[6][7] The broadband property of log-periodic antennas comes from its self-similarity. Y. Mushiake found, for what he termed "the simplest self-complementary planar antenna," a driving point impedance of η0/2=188.4Ω at frequencies well within its bandwidth limits.[8][9][10]


The log periodic antenna was invented by Dwight E. Isbell, Raymond DuHamel and variants by Paul Mayes. The University of Illinois at Urbana-Champaign had patented the Isbell and Mayes-Carrel antennas and licensed the design as a package exclusively to JFD electronics in New York. Channel Master and Blonder-Tongue ignored the patents and produced a wide range of antennas based on this design. Lawsuits regarding the antenna patent which the UI Foundation lost, evolved into the Blonder-Tongue Doctrine.[11] This precedent governs patent litigation.[12]

Short wave broadcast antennas

Short wave LPA antenna, black shows metallic conductors, red shows insulating support members

The log periodic is commonly used in high power short wave broadcasting[13] where it is desired to invest in only in a single antenna to cover transmissions over multiple bands. The log-periodic zig-zag design with up to 16 zig zag sections has been used. These large antennas are typically designed to cover 6 to 26 MHz but even larger ones have been built which operate as low as 2 MHz. Power ratings are available up to 500 KW. The antenna is supported by two taller and two shorter masts and is fed from the small end. The antenna shown here would have about 14 dBi gain. An antenna array consisting of two such antennas, one above the other and driven in phase has a gain of up to 17 dBi. Being log-periodic, the antenna's main characteristics (radiation pattern, gain, driving point impedance) are almost constant over its entire frequency range, with the match to a 300 ohm feed line achieving a standing wave ratio of better than 2:1 over that range.


  1. ^ "Log periodic antenna",
  2. ^ The Log-Periodic Dipole Array"
  3. ^ a b Log Periodic Dipole Array (LPDA)
  4. ^ "Log-periodic zig zag antenna", US Patent 3355740
  5. ^ Photo Archive Of Antennas, Illinois Historic Archive
  6. ^ Y. Mushiake, “Constant-impedance antennas," ’’J. IECE Japan’’, 48, 4, pp. 580-584, April 1965. (in Japanese)
  7. ^ , 69, 3, p. 88, March 1949."J. IEE Japan"Y. Mushiake, '’’ Log-periodic structure provides no broad-band property for antennas."' . Retrieved 2014-01-15. 
  8. ^ , 69, 3, p. 88, March 1949. (in Japanese)"J. IEE Japan"Y. Mushiake, '’’Origination of self-complementary structure and discovery of its constant-impedance property.' . Retrieved 2014-01-31. 
  9. ^ "Y. Mushiake, '’’ Infinite freedom."'". Retrieved 2014-01-15. 
  10. ^ V. H. Rumsey, ‘’Frequency independent antennas’’, Academic Press, New York and London. 1966. [p. 55]
  11. ^
  12. ^
  13. ^

External links

  • Log-Periodic Tooth Antenna Page
  • Blonder-Tongue Doctrine
  • LPDA Online Calculation
  • Some thoughts on Log-Periodic Antennas
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.