Log-periodic antenna: Difference between revisions

A '''log-periodic antenna''' ('''LP'''), also known as a '''log-periodic array''' or '''log-periodic aerial''', is{{dubious|date=June 2018}} a multi-element, [[directional antenna|directional]] [[antenna (radio)|antenna]] designed to operate over a wide band of [[Frequency|frequencies]]. It was invented by [[DwightJohn Isbell]] and Raymond DuHamel at the [[University of Illinois at Urbana–Champaign|University of Illinois]]Dunlavy in 19581952.

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 element]]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]] with alternating [[phase (waves)|phase]]. Electrically, it simulates a series of two- or three-element [[YagiYagi–Uda antenna]]s connected together, each set tuned to a different frequency.

LPDA antennas look somewhat similar to [[Yagi antenna]]santennas, 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 [[Antenna gain|gain]], while adding elements to aan LPDA increases its frequency response, or [[bandwidth (signal processing)|bandwidth]].

One large application for LPDAs is in rooftop terrestrial [[television antenna]]s, since they must have large bandwidth to cover the wide television bands of roughly 54–88 and 174–216 MHz in the [[Very high frequency|VHF]] and 470–890 MHz in the [[Ultra high frequency|UHF]] while also having high [[antenna gain|gain]] for adequate fringe reception. One widely used design for television reception combined a Yagi for UHF reception in front of a larger LPDA for VHF.

The LPDA normally consists of a series of [[dipole antenna|half wave dipole]]s known as "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]], known as ''d'' or ''sigma''. The length of the successive elements correspond to resonance at different frequencies withinand the antenna'sspacing overallbetween bandwidth.them Thisgradually leadsdecrease to a series of ever-shorter dipoles towardsalong the "front" of the antennaboom. 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.<ref>[http://www.salsburg.com/Log-Periodic.pdf The Log-Periodic Dipole Array"]</ref><ref name=d>[{{Cite web|url=http://www.ewh.ieee.org/soc/es/Nov1998/13/LPARRAY/LPDA.HTM|archive-url=https://archive.today/20141005152354/http://www.ewh.ieee.org/soc/es/Nov1998/13/LPARRAY/LPDA.HTM|url-status=dead|archive-date=October 5, 2014|title=Log Periodic Dipole Array (LPDA)|website=[[IEEE]]}}</ref> The [[radiation pattern]] of the antenna is unidirectional, with the [[main lobe]] along the axis of the boom, off the end with the shortest elements. Each dipole element is [[resonance|resonant]] at a [[wavelength]] approximately equal to twice its length. The [[bandwidth (signal processing)|bandwidth]] of the antenna, the [[frequency]] range over which it has near-maximum [[antenna gain|gain]], is approximately between the [[resonant frequency|resonant frequencies]] of the longest and shortest elements.

Every element in the LPDA designantenna is "active"a [[driven element]], that is, connected electrically to the [[feedline]]. along withA theparallel otherwire elements,[[transmission thoughline]] atusually anyruns one frequency most ofalong the elementscentral drawboom, little current from it.and Eacheach successive element is connected in ''opposite'' [[phase (waves)|phase]] to the active connection running as a transmission line along the boomit. For thatThe reason, that transmission linefeedline can often be seen zig-zagging across the support boom holding the elements.<ref name=d/> One Another common designconstruction ploymethod 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.<ref>[http://www.google.ca/patents/US3355740 "Log-periodic zig zag antenna"], US Patent 3355740</ref> Many other forms using the transmission wire as the active element also exist.<ref>[http://www.ece.illinois.edu/about/history/antenna/photos.html Photo Archive Of Antennas], Illinois Historic Archive</ref>

The [[Yagi–Uda antenna|Yagi]] and the LPDA designs look very similar at first glance, as they both consist of a number of dipole elements spaced outmounted along a support boom. The Yagi, however, has only a single dipole[[driven element]] connected to the transmission line, usually the second one from the back of the array., Thethe otherremaining dipoles on the boomelements are [[passiveparasitic element|parasitic]]s, 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 seendiffers from the top, whereas the differenceLPDA 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 hashaving a very narrow bandwidth but achieves greater [[antenna gain|gain]].

In general terms, at any given frequency the log-periodic design operates somewhat similar to a series of three-element Yagis,Yagi whereantenna; eachthe setdipole ofelement threeclosest consecutiveto elementsresonant formsat the operating frequency acts as a separatedriven antennaelement, with the driventwo elementadjacent inelements on either side as director and reflector to increase the centergain, athe directorshorter element in front acting as a director and reflectorthe 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, 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&nbsp;dB on the Yagi side and 6.5&nbsp;dB for the log-periodic.<ref>{{cite book |url=https://books.google.ca/books?{{GBurl|id=jDCs1Ckne_EC&|pg=PA177}} |page=178 |title= Computational Electromagnetics for RF and Microwave Engineering |first=David |last=Davidson |publisher=Cambridge University Press |date=2010|isbn=978-1-139-49281-2}}</ref> But this extra gain was needed anyway in order to make up for a number of problems with [[UHF television broadcasting#UHF vs VHF|UHF signals]].

It should be strictly noted that the log-periodic shape, according to the IEEE definition,<ref>“"''Log-periodic antenna'' Any one of a class of antennas having a structural geometry such that its impedance and radiation characteristics repeat periodically as the logarithm of frequency.”" (see ''The new IEEE Standard Dictionary of Electrical and Electronics Terms'', 1993 ⓒ IEEE.) </ref><ref>“"''Log-periodic antenna'' Any one of a class of antennas having a structural geometry such that its impedance and radiation characteristics repeat periodically as the logarithm of frequency.”" (see Acknowledgments, and footnote in page 1), ''Self-Complementary Antennas―Principle of Self-Complementarity for Constant Impedance''―, by Y.Mushiake, MushiakeYasuto, Springer-Verlag London Ltd., London, 1996.</ref> does not providealign with broadband property for antennas.<ref>Y. Mushiake, “ConstantYasuto, "Constant-impedance antennas,", ’’J''J. IECE Japan’’Japan'', 48, 4, pp. 580-584, April 1965. (in Japanese).</ref><ref>{{cite webjournal|last=Mushiake|first=Yasuto|url=http://www.sm.rim.or.jp/~ymushiak/sub.non-const.htm|title=Y. Mushiake, &#39;’’ Log-periodic structure provides no broad-band property for antennas."&#39; ''|journal=J. IEE Japan'', |volume=69, |issue=3, p. |page=88, |date=March 1949. |publisher=Sm.rim.or.jp |accessdateaccess-date=15 January 2014}}</ref> The broadband property of log-periodic antennas comes from its [[self-similarity]]. A planerplanar log-periodic antenna can also be made [[self-complementary antenna|self-complementary]], such as logarithmic [[spiral antenna]]s (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 [[Impedance of free space|η₀]]/2=188.4&nbsp;Ω at frequencies well within its bandwidth limits.<ref>{{cite webjournal|last=Mushiake|first=Yasuto|url=http://www.sm.rim.or.jp/~ymushiak/sub.docu.1.htm##%% |title=Y. Mushiake, &#39;’’OriginationOrigination of self-complementary structure and discovery of its constant-impedance property.&#39; ''|journal=J. IEE Japan'', |volume=69, |issue=3, p. |page=88, |date=March 1949. (in Japanese)|lang=ja |publisher=Sm.rim.or.jp |accessdateaccess-date=31 January 2014}}</ref><ref>{{cite web|last=Mushiake|first=Yasuto|url=http://www.sm.rim.or.jp/~ymushiak/sub.sca.htm|title=Y. Mushiake, &#39;’’ Infinite freedom."&#39; |publisher=Sm.rim.or.jp |accessdateaccess-date=15 January 2014}}</ref><ref name="Rumsey Frequency">Rumsey, V. H. Rumsey, ‘’Frequency''Frequency independent antennas’’antennas'', Academic Press, New York and London. 1966. [p. 55]</ref>

| caption2 = Log -periodic mounted for vertical polarization, covers 140&ndash;470&nbsp;MHz

| caption3 = LP television antenna 1963. Covers 54&ndash;88&nbsp;MHz and 174&ndash;218&nbsp;MHz. Slanted elements were used because on the upper band they operate at the 3rdthird harmonic.

| caption4 = Wire Loglog-periodic monopole antenna.

| caption1 = Wire log -periodic transmitting antenna at international shortwave broadcasting station, Moosbrunn, Austria. Covers 6.1&ndash;23&nbsp;MHz.

| caption2 = Diagram of a zig-zag shortwave LPA antenna,. blackBlack shows metallic conductors,; red shows insulating supports.

The log -periodic is commonly used as a transmitting antenna in high power [[International broadcasting|short waveshortwave broadcasting]]<ref>http{{Cite web|url=https://www.antenna.be/art1.html|title=Antennas for the Shortwave Broadcaster|website=www.antenna.be}}</ref> wherestations itbecause isits desiredbroad tobandwidth invest in onlyallows a single antenna to covertransmit transmissionson overfrequencies in multiple [[Shortwave bands#International broadcast bands|bands]]. 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&nbsp;MHz but even larger ones have been built which operate as low as 2&nbsp;MHz. Power ratings are available up to 500&nbsp;kW. PowerInstead fromof the elements being driven in parallel, attached to a balancedcentral transmission line, isthe appliedelements betweenare thedriven twoin setsseries, ofadjacent zig-zagselements connected nearat the groundouter edges. The antenna shown here would have about 14&nbsp;dBi [[antenna gain|gain]]. An antenna [[Antennaantenna array (electromagnetic)|array]] consisting of two such antennas, one above the other and driven in phase has a gain of up to 17&nbsp;dBi. Being log-periodic, the antenna's main characteristics ([[radiation pattern]], gain, [[Antenna (radio)#Impedance|driving point impedance]]) are almost constant over its entire frequency range, with the match to a 300&nbsp;Ω feed line achieving a [[standing wave ratio]] of better than 2:1 over that range.