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In telecommunication, a log-periodic antenna (LP, also known as a log-periodic array or log periodic beam antenna/aerial) is a broadband, multi-element, directional, narrow-beam antenna that has impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequency. The individual components are often dipoles, as in a log-periodic dipole array (LPDA). Log-periodic antennas are designed to be self-similar and are thus also fractal antenna arrays. 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. Lawsuits regarding the antenna patent which the UI Foundation lost, evolved into the Blonder-Tongue Doctrine. This precedent governs patent litigation.
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.
The lengths and spacings of the elements of a log-periodic antenna increase logarithmically from one end to the other. A plot of the input impedance as a function of logarithm of the excitation frequency shows a periodic variation.
Popular Amateur radio variations
This is a beam antenna which for a given boom length gives a higher gain than a Yagi antenna, it is a log-periodic 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; 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
A New Zealand ham, 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
- This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).
- HB9CV Antenna Design Details
- Online HB9CV Antenna Calculator
- Orr, William I.; Stuart D. Cowan (W2LX) (1955). Handbook. Radio Publications, Inc. p. 156.
- Cebik, L. B. "The Poor Old ZL Special". Retrieved 6 June 2011.
- Antenna-Theory.com Log-Periodic Tooth Antenna Page
- Blonder-Tongue Doctrine
- LPDA Online Calculation
- Some thoughts on Log-Periodic Antennas