Log-periodic antenna

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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. The antenna normally consists of a series of dipoles positioned along the antenna axis, spaced at intervals following a logarithmic function of the frequency. It is normal to drive alternating elements with 180° (π radians) of phase shift from one another. This is normally done by connecting individual elements to alternating wires of a balanced transmission line. LP antennas are widely used with television receivers, especially in the VHF band. LP's are also used for UHF, but have more recently been increasingly replaced by the Grey-Hoverman antenna and similar designs.

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.[1] This precedent governs patent litigation.[2]

In this connection, it should be strictly noted that the "log-periodic shape" does not provide with broadband property for antennas.[3][4] Actually, broadband property of so-called Log-periodic Antenna comes from self-complementary antenna that has always constant input impedance, 60π≒188.4 (Ω), independent of the frequency and its shape with Infinite freedom.[5][6][7]

This antenna design is used where a wide range of frequencies is needed while still having moderate gain and directionality. It is sometimes used for a (VHF/UHF) television antenna.[8]

Popular amateur radio variations[edit]

HB9CV[edit]

The HB9CV is a very popular two-element beam which can be considered[citation needed] as a log-periodic dipole array with only two elements.[9][10]

ZL special[edit]

This is a beam antenna which for a given boom length gives a higher gain than a Yagi antenna, it is a log-periodic[citation needed] where only two elements are driven with a series of parasitic elements (directors) in front of the smaller of the two driven elements.

The driven elements are folded dipoles which are linked by a length of balanced twin-lead feed line. Unlike the bidirectional W8JK designs, the ZL Special is essentially a unidirectional beam;[11] the polar plots of the ZL special and the HB9CV are heart shaped (cardioid), while the typical Yagi antenna has a large forward lobe and a smaller back lobe.

History of the ZL Special[edit]

A New Zealand amateur radio operator, George H. Pritchard (ZL3MH), brought the design to attention in 1949 (based on ideas of W5LHI and W0GZR), hence the "ZL" part of the name given the 3MH beam by Fred C. Judd (G2BCX), who further developed variants[12]

References[edit]

  1. ^ http://definitions.uslegal.com/b/blonder-tongue-doctrine/
  2. ^ http://www.ece.illinois.edu/mediacenter/alumninews/alumninews-sum04a.pdf
  3. ^ Y. Mushiake, “Constant-impedance antennas," ’’J. IECE Japan’’, 48, 4, pp. 580-584, April 1965. (in Japanese)
  4. ^ "Y. Mushiake, '’’ Log-periodic structure provides no broad-band property for antennas."' J. IEE Japan, 69, 3, p. 88, March 1949.". Sm.rim.or.jp. Retrieved 2014-01-15. 
  5. ^ "Y. Mushiake, '’’Origination of self-complementary structure and discovery of its constant-impedance property.' J. IEE Japan, 69, 3, p. 88, March 1949. (in Japanese)". Sm.rim.or.jp. Retrieved 2014-01-31. 
  6. ^ "Y. Mushiake, '’’ Infinite freedom."'". Sm.rim.or.jp. Retrieved 2014-01-15. 
  7. ^ V. H. Rumsey, ‘’Frequency independent antennas’’, Academic Press, New York and London. 1966. [p. 55]
  8. ^ http://www.radio-electronics.com/info/antennas/log_p/log_periodic.php
  9. ^ HB9CV Antenna Design Details
  10. ^ Online HB9CV Antenna Calculator
  11. ^ Orr, William I.; Stuart D. Cowan (W2LX) (1955). Handbook. Radio Publications, Inc. p. 156. 
  12. ^ Cebik, L. B. "The Poor Old ZL Special". Retrieved 6 June 2011. 

External links[edit]