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A log-periodic antenna (LP), also known as a log-periodic array or log-periodic aerial, is a multi-element,
directional antenna A directional antenna or beam antenna is an antenna that radiates or receives greater radio wave power in specific directions. Directional antennas can radiate radio waves in beams, when greater concentration of radiation in a certain directio ...
designed to operate over a wide band of
frequencies Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
. It was invented by John Dunlavy in 1952. 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 In radio and telecommunications a dipole antenna or doublet is one of the two simplest and most widely used antenna types, types of antenna; the other is the monopole antenna, monopole. The dipole is any one of a class of antennas producin ...
driven element In an antenna array made of multiple Electrical conductivity, conductive elements (typically metal rods), a driven element or active element (also called driven radiator or active radiator) is electrically connected to the radio receiver, rec ...
s 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 A radio transmitter or receiver is connected to an antenna which emits or receives the radio waves. The antenna feed system or antenna feed is the cable or conductor, and other associated equipment, which connects the transmitter or receiver w ...
with alternating
phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform *Phase space, a mathematica ...
. Electrically, it simulates a series of two- or three-element
Yagi–Uda antenna A Yagi–Uda antenna, or simply Yagi antenna, is a directional antenna consisting of two or more parallel Antenna (radio)#Resonant antennas, resonant antenna elements in an Antenna array#Types, end-fire array; these elements are most often metal ...
s connected together, each set tuned to a different frequency. LPDA antennas look somewhat similar to Yagi antennas, in that they both consist of dipole rod elements mounted in a line along a support boom, but they work in very different ways. Adding elements to a Yagi increases its directionality, or gain, while adding elements to an LPDA increases its frequency response, or
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
. One large application for LPDAs is in rooftop terrestrial
television antenna A television antenna, also called a television aerial (in British English), is an antenna specifically designed for use with a television receiver (TV) to receive terrestrial over-the-air (OTA) broadcast television signals from a television s ...
s, since they may require large bandwidth to cover various frequencies in the VHF and/or
UHF Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter ...
bands. One widely used design for television reception combined a Yagi for UHF reception in front of a larger LPDA for VHF.


Basic concept

The LPDA normally consists of a series of half wave dipole "elements" each consisting of a pair of metal rods, positioned along a support boom lying along the antenna axis. The elements are spaced at intervals following a logarithmic function of the
frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
, known as ''d'' or ''sigma''. The length of the successive elements and the spacing between them gradually decrease along the boom. The relationship between the lengths is a function known as ''tau''. ''Sigma'' and ''tau'' are the key design elements of the LPDA design. The
radiation pattern In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: " ...
of the antenna is unidirectional, with the
main lobe In a antenna (electronics), radio antennas, the main lobe or main beam is the region of the radiation pattern containing the highest Power (physics), power or exhibiting the greatest field strength. The radiation pattern of most antennas shows ...
along the axis of the boom, off the end with the shortest elements. Each dipole element is
resonant Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
at a
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
approximately equal to twice its length. The
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
of the antenna, the
frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
range over which it has near-maximum gain, is approximately between the resonant frequencies of the longest and shortest elements. Every element in the LPDA antenna is a
driven element In an antenna array made of multiple Electrical conductivity, conductive elements (typically metal rods), a driven element or active element (also called driven radiator or active radiator) is electrically connected to the radio receiver, rec ...
, that is, connected electrically to the
feedline A radio transmitter or receiver is connected to an antenna which emits or receives the radio waves. The antenna feed system or antenna feed is the cable or conductor, and other associated equipment, which connects the transmitter or receiver w ...
. A parallel wire
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
usually runs along the central boom, and each successive element is connected in ''opposite''
phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform *Phase space, a mathematica ...
to it. The feedline can often be seen zig-zagging across the support boom holding the elements. Another common construction method is to use two parallel central support 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 they both consist of a number of dipole elements mounted along a support boom. The Yagi, however, has only a single
driven element In an antenna array made of multiple Electrical conductivity, conductive elements (typically metal rods), a driven element or active element (also called driven radiator or active radiator) is electrically connected to the radio receiver, rec ...
connected to the transmission line, usually the second one from the back of the array, the remaining elements are
parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...
. The Yagi antenna differs from the LPDA in having a very narrow bandwidth. In general terms, at any given frequency the log-periodic design operates somewhat similar to a three-element Yagi antenna; the dipole element closest to resonant at the operating frequency acts as a driven element, with the two adjacent elements on either side as director and reflector to increase the gain, the shorter element in front acting as a director and the longer element behind as a reflector. 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, 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 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. The log-periodic shape, according to the IEEE definition, does not align with broadband property for antennas. The broadband property of log-periodic antennas comes from its
self-similarity In mathematics, a self-similar object is exactly or approximately similar to a part of itself (i.e., the whole has the same shape as one or more of the parts). Many objects in the real world, such as coastlines, are statistically self-similar ...
. A planar log-periodic antenna can also be made self-complementary, such as logarithmic spiral antennas (which are not classified as log-periodic ''per se'' but among the frequency independent antennas that are also self-similar) or the log-periodic toothed design. Y. Mushiake found, for what he termed "the simplest self-complementary planar antenna," a driving point impedance of η0/2=188.4 Ω at frequencies well within its bandwidth limits.Rumsey, V. H., ''Frequency independent antennas'', Academic Press, New York and London. 1966. . 55/ref>


History

John Dunlavy invented the log-periodic antenna in 1952 while working for the United States Air Force but was not credited with it due to its "Secret" classification. The
University of Illinois at Urbana–Champaign The University of Illinois Urbana-Champaign (UIUC, U of I, Illinois, or University of Illinois) is a public land-grant research university in the Champaign–Urbana metropolitan area, Illinois, United States. Established in 1867, it is the f ...
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 Labs Blonder Tongue Laboratories Inc. is a global communication equipment supplier for TV broadcasters, cable system operators, lodging video, internet systems and institutional systems. A cable television pioneer, the company was founded in 1950. Ba ...
ignored the patents and produced a wide range of antennas based on that design. Lawsuits regarding the antenna patent, which the U.I. Foundation lost, evolved into the 1971 Blonder-Tongue Doctrine. This precedent governs patent litigation.


Short wave broadcast antennas

The log-periodic is commonly used as a transmitting antenna in high power
shortwave broadcasting Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (app ...
stations because its broad bandwidth allows a single antenna to transmit on frequencies in multiple
bands Bands may refer to: * Bands (song), song by American rapper Comethazine * Bands (neckwear), form of formal neckwear * Bands (Italian Army irregulars) Bands () was an Italian military term for Irregular military, irregular forces, composed of nati ...
. The log-periodic zig-zag design with up to 16 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. Instead of the elements being driven in parallel, attached to a central transmission line, the elements are driven in series, adjacent elements connected at the outer edges. The antenna shown here would have about 14 dBi gain. An
antenna array An antenna array (or array antenna) is a set of multiple connected antenna (radio), antennas which work together as a single antenna, to transmit or receive radio waves. The individual antennas (called ''elements'') are usually connected to a s ...
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 In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: " ...
, gain, driving point impedance) are almost constant over its entire frequency range, with the match to a 300 Ω feed line achieving a
standing wave ratio In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the tran ...
of better than 2:1 over that range.


References


Bibliography

* $ 15-5: ''The Log-Periodic Antenna'', p. 703-708.


Notes


See also

* Self-complementary antenna


External links


Antenna-Theory.com Log-Periodic Tooth Antenna PageBlonder-Tongue DoctrineLPDA Online Calculation


* ttps://www.allaboutcircuits.com/technical-articles/wi-fi-antennas-part-1-fundamentals/ All About Circuits - The Fundamentals of Wi-Fi Antennasbr>Electronics Point - Forum / Antenna Maker ProEE Power
{{Antenna Types Radio frequency antenna types Antennas (radio)