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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. 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. The log-periodic design looks very similar to the Yagi antenna, but is very different electrically. LPDA and Yagis are often combined in television antennas that cover both VHF and UHF.
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 each element is a function of the desired frequency response; for broadband reception 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 towards the transmitter. Sigma and tau are the key design elements of the LPDA design.
Every element in the log-periodic design is "active", connected electrically to the other elements. 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. Often the transmission line can be seen zig-zagging across the support boom holding the elements. A common design element 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. Many other forms using the transmission wire as the active element also exist.
Yagis and log-periodic 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, normally one close to the back of the array. The other dipoles on the boom are passive, acting as "directors" or "reflectors" depending on their position relative to the "driven element". This is often only visible by examining the wiring. A more obvious difference is the length of the dipoles; LPDA designs have shorter dipoles towards the front of the antenna, forming a triangular shape as seen from the top. Another visible difference is the spacing between the elements, which remains constant in the Yagi but becomes wider towards the rear of the LPDA.
It should be strictly noted that the "log-periodic shape" does not provide with broadband property for antennas. The broadband property of the log-periodic design 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.
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. This precedent governs patent litigation.
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 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
- This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).
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- Online HB9CV Antenna Calculator
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- LPDA Online Calculation
- Some thoughts on Log-Periodic Antennas