Log-periodic antenna

Log-periodic antenna, 400–4000 MHz

A log-periodic antenna (LP), also known as a log-periodic array or log-periodic aerial, is a multi-element, directional, antenna designed to operate over a wide band of frequencies. It was invented by Dwight Isbell and Raymond DuHamel at the University of Illinois in 1958.

The most common form of log-periodic antenna is the log-periodic dipole array or LPDA, The LPDA consists of a number of half-wave dipole driven elements of gradually increasing length, each consisting of a pair of metal rods. The dipoles are mounted close together in a line, connected in parallel to the feedline with alternating phase. Electrically, it simulates a series of two or three-element Yagi antennas connected together, each set tuned to a different frequency.

LPDAs look somewhat similar to multi-element Yagi designs, but work in very different ways. Adding elements to a Yagi increases its directionality, or gain, while adding elements to a LPDA increases its frequency response, or bandwidth. Because both designs are linear, a widely used design for television reception combined a Yagi for UHF reception in front of a larger LDPA for VHF. These can be identified by the much smaller elements at the front, and often a V-shaped reflector between the two sections.

LPDA/Yagi combo antennas were very popular from the 1960s through the 1980s when television broadcasting moved largely to cable. The digital transition in the 2000s led to the retirement of the VHF frequencies for television use in most countries. Modern terrestrial television antennas are more often dedicated to UHF, and the bowtie array is more common today. In the United States, however, some stations remained on the VHF spectrum.

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.

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. 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. Many other forms using the transmission wire as the active element also exist.

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 elements, 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; Yagi's lack the zig-zag connection between the elements. 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 or non-existent. Another visible difference is the spacing between the elements, which is normally 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.

In general terms, the log-periodic design operates somewhat similar to a a series of three-element Yagis, where each set of three consecutive elements forms a separate antenna with the driven element in the center, a director in front and reflector behind. However, the system is somewhat more complex than that, and all the elements contribute to some degree, so the gain for any given frequency is higher than a Yagi of the same dimensions as any one section of the log-periodic. However, it should also be noted that a Yagi with the same number of elements as a log-periodic would have far higher gain, as all of those elements are improving the gain of a single driven element. In its common use as a television antenna, it was common to combine a log-periodic design for VHF with a Yagi for UHF, with both halves being roughly equal in size. This resulted in much higher gain for UHF, typically on the order of 10 to 14 dB on the Yagi side and 6.5 dB for the log-periodic. But this extra gain was needed anyway in order to make up for a number of problems with UHF signals.

It should be strictly noted that the "log-periodic shape" does not provide with broadband property for antennas. 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.

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Log-periodic antenna, 250–2400 MHz

Log periodic mounted for vertical polarization, covers 140-470 MHz

LP television antenna 1963. Covers 54-88 MHz and 174-218 MHz. Slanted elements were used because on the upper band they operate at the 3rd harmonic.

Wire Log-periodic monopole antenna.

History

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.

Short wave broadcast antennas

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Wire log periodic transmitting antenna at international shortwave broadcasting station, Moosbrunn, Austria. Covers 6.1 - 23 MHz

Diagram of shortwave LPA antenna, black shows metallic conductors, red shows insulating supports

The log periodic is commonly used in high power short wave broadcasting where it is desired to invest in only 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 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.

Notes

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

See Also

• Self-complementary antenna

External links

• Antenna-Theory.com Log-Periodic Tooth Antenna Page
• Blonder-Tongue Doctrine
• LPDA Online Calculation
• Some thoughts on Log-Periodic Antennas