Log-periodic antenna

Lore & Background

John Dunlavy invented the log-periodic antenna in 1952 while working for the United States Air Force, but was not credited due to its 'Secret' classification. The University of Illinois at Urbana–Champaign patented the Isbell and Mayes–Carrel designs and licensed them exclusively to JFD Electronics. Channel Master and Blonder Tongue Labs ignored the patents, leading to lawsuits that the U.I. Foundation lost, evolving into the 1971 Blonder-Tongue Doctrine, which governs patent litigation. The log-periodic antenna is commonly used as a transmitting antenna in high-power shortwave broadcasting stations because its broad bandwidth allows a single antenna to transmit on multiple bands. Large log-periodic zig-zag designs with up to 16 sections cover 6 to 26 MHz, with power ratings up to 500 kW. An array of two such antennas driven in phase can achieve up to 17 dBi gain.

Reader's Guide

The log-periodic antenna's significance lies in its ability to maintain nearly constant radiation pattern, gain, and driving point impedance over a wide frequency range, making it valuable for applications requiring broad bandwidth such as rooftop terrestrial television reception and high-power shortwave broadcasting. In television use, it was common to combine a log-periodic design for VHF with a Yagi for UHF, with the Yagi side providing higher gain (10 to 14 dB) compared to the log-periodic's 6.5 dB. The LPDA's design parameters sigma and tau govern element spacing and length relationships. The broadband property arises from self-similarity, and planar log-periodic antennas can be made self-complementary, such as logarithmic spiral antennas or the log-periodic toothed design. The driving point impedance for the simplest self-complementary planar antenna is η0/2 = 188.4 Ω within its bandwidth limits.

Did You Know?

• The log-periodic antenna was invented by John Dunlavy in 1952 while working for the United States Air Force.
• Adding elements to an LPDA increases its frequency response (bandwidth), not its gain.
• The log-periodic zig-zag design can have up to 16 sections and handle power ratings up to 500 kW.
• The Blonder-Tongue Doctrine, established in 1971, governs patent litigation stemming from log-periodic antenna patents.

Design Philosophy and Self-Similarity

The log-periodic antenna takes its name from a core geometric principle: successive elements shrink in both length and spacing according to a logarithmic function of frequency. Two parameters govern this geometry—tau, which sets the ratio between consecutive element lengths, and sigma, which dictates the spacing intervals along the boom. Every element is a driven dipole, tied in parallel to the feedline with alternating phase, so no element is merely parasitic. At any given operating frequency, the dipole nearest its resonant wavelength (roughly twice its physical length) assumes the role of the active radiator, while its two neighbors behave as a director and reflector, mimicking a small Yagi–Uda section. Yet the full array contributes collectively, yielding slightly more gain than a single Yagi segment of identical dimensions. The IEEE notes that the log-periodic shape does not technically satisfy the formal broadband antenna definition; its wideband performance instead emerges from self-similarity. In planar variants, this self-similarity can be extended into self-complementarity, as in logarithmic spiral antennas, where Mushiake demonstrated a driving-point impedance of roughly 188.4 ohms across the operational band.

Visual Twins, Functional Opposites

At first glance, a log-periodic dipole array and a Yagi–Uda antenna look nearly identical: rows of metal rod elements strung along a central support boom. The distinction, however, is profound. In a Yagi, only one element—typically the second from the rear—is electrically driven; all others are parasitic, and adding more of them sharpens directionality and gain at a single frequency while bandwidth stays narrow. In an LPDA, every dipole is a driven element connected to the feedline in alternating phase, and adding elements extends the usable frequency range rather than boosting gain at one point. This trade-off made the LPDA ideal for applications demanding wide bandwidth, such as rooftop television reception across VHF and UHF channels. A popular hybrid design placed a Yagi for UHF in front of a larger LPDA for VHF, the two halves roughly equal in physical size. The Yagi side delivered 10 to 14 dB of gain, while the LPDA contributed about 6.5 dB, the extra UHF gain compensating for inherent signal challenges at those higher frequencies.

Invention, Secrecy, and Patent Warfare

John Dunlavy conceived the log-periodic antenna in 1952 while employed by the United States Air Force, yet he was never publicly credited because the work carried a "Secret" classification. The design's public life began through the University of Illinois at Urbana–Champaign, which held patents on related Isbell and Mayes–Carrel antenna configurations and licensed the package exclusively to JFD Electronics in New York. That exclusivity did not hold. Channel Master and Blonder Tongue Labs both ignored the patents and manufactured extensive lines of log-periodic-based antennas. The resulting legal battles, in which the U.I. Foundation ultimately lost, produced the 1971 Blonder-Tongue Doctrine—a precedent that continues to shape patent litigation strategy. The episode illustrates how military secrecy, academic patenting, and commercial ambition can collide, leaving the true inventor unacknowledged while the technology becomes ubiquitous in consumer and broadcast markets.

Power Broadcasting and the Zig-Zag Variant

Beyond television reception, the log-periodic antenna has found a demanding home in high-power shortwave broadcasting. Because a single LPDA can cover multiple frequency bands simultaneously, stations use it as a transmitting antenna to avoid the expense and complexity of separate antennas for each band. Large installations employ the log-periodic zig-zag configuration, with up to sixteen sections, typically spanning 6 to 26 MHz, though even larger structures have been built to reach down to 2 MHz. Power-handling ratings extend to 500 kW. A key structural difference from the standard LPDA is the feed arrangement: rather than driving elements in parallel off a central transmission line, the zig-zag design drives elements in series, connecting adjacent elements at their outer edges. This topology suits the enormous currents and voltages involved in kilowatt-class transmission while preserving the self-similar, broadband character that makes the log-periodic family so versatile.