Wikipedia

Log-periodic antenna: Difference between revisions

Browse history interactively
← Previous editNext edit →
Content deleted Content added
VisualWikitext
→Short wave broadcast antennas: Described drive method
→Basic concept: Shortened the paragraph on the differences between LPDA and Yagi antennas; they are different antennas that happen to look alike, no need to beat it to death
Line 17:
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.<ref name=d/> 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.<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 both consist of a number of dipole elements spaced outmounted 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., 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 athe separateoperating antennafrequency withacts theas a driven element, inwith the center,two aadjacent elements on either side as director and reflector, the shorter 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?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}}</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, Springer-Verlag London Ltd., London, 1996</ref> does not provide with broadband property for antennas.<ref>Y. Mushiake, “Constant-impedance antennas," ’’J. IECE Japan’’, 48, 4, pp. 580-584, April 1965. (in Japanese)</ref><ref>{{cite web|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; ''J. IEE Japan'', 69, 3, p. 88, March 1949. |publisher=Sm.rim.or.jp |accessdate=15 January 2014}}</ref> The broadband property of log-periodic antennas comes from its [[self-similarity]]. A planar 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|η<sub>0</sub>]]/2=188.4&nbsp;Ω at frequencies well within its bandwidth limits.<ref>{{cite web|url=http://www.sm.rim.or.jp/~ymushiak/sub.docu.1.htm##%% |title=Y. Mushiake, &#39;’’Origination of self-complementary structure and discovery of its constant-impedance property.&#39; ''J. IEE Japan'', 69, 3, p. 88, March 1949. (in Japanese) |publisher=Sm.rim.or.jp |accessdate=31 January 2014}}</ref><ref>{{cite web|url=http://www.sm.rim.or.jp/~ymushiak/sub.sca.htm|title=Y. Mushiake, &#39;’’ Infinite freedom."&#39; |publisher=Sm.rim.or.jp |accessdate=15 January 2014}}</ref><ref name="Rumsey Frequency">V. H. Rumsey, ‘’Frequency independent antennas’’, Academic Press, New York and London. 1966. [p. 55]</ref>
Retrieved from "https://en.wikipedia.org/wiki/Log-periodic_antenna"