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{{short description|Multi-element, directional antenna useable over a wide band of frequencies}}
[[File:LPDA-Antenna.jpg|thumb|260px|Log-periodic antenna, 400–4000 MHz]]
{{Antennas|expanded=Common Types}}
A '''log-periodic antenna''' ('''LP'''), also known as a '''log-periodic array''' or '''log-periodic aerial''', is a multi-element, [[directional
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 [[
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 [[Antenna gain|gain]], while adding elements to
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 gain for adequate fringe reception.
== Basic concept ==
The LPDA normally consists of a series of [[dipole antenna|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]], known as ''d'' or ''sigma''. The length of the successive elements
Every element in the LPDA antenna is a [[driven element]], that is, connected electrically to the [[feedline]]. A parallel wire [[transmission line]] usually runs along the central boom, and each successive element is connected in ''opposite'' [[phase (waves)|phase]] to it. The feedline can often be seen zig-zagging across the support boom holding the elements.<ref name=d/> 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.<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>
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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 mounted along a support boom. The Yagi, however, has only a single [[driven element]] connected to the transmission line, usually the second one from the back of the array, the remaining elements are [[parasitic element|parasitic]]. 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,
It should be strictly noted that the log-periodic shape, according to the IEEE definition,<ref>
{{multiple image
| align = center
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| width1 = 200
| image2 = VHF UHF LP-antenna closeup.JPG
| caption2 = Log
| width2 = 150
| image3 = Log periodic VHF TV antenna 1963.jpg
| caption3 = 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
| width3 = 262
| image4 = Log-periodic monopole antenna.png
| caption4 = Wire
| width4 = 202
}}
== History ==
== Short wave broadcast antennas ==
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| header =
| image1 = Moosbrunn drehbare logarithmisch-periodische Antenne (2).JPG
| caption1 = Wire log
| width1 = 288
| image2 = LPA_antenna_suitable_for_short_wave_broadcast.png
| caption2 = Diagram of a zig-zag shortwave LPA antenna
| width2 = 180
}}
The log
== References ==
{{Reflist|30em}}
== Bibliography ==
* {{cite book |title= Antennas|author= John Daniel Kraus |publisher= McGraw-Hill College |year= 1988|edition = Subsequent|page=892 |isbn=978-0-070-35422-7}} $ 15-5: ''The Log-Periodic Antenna'', p. 703-708.
== Notes ==
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