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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 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. Modern terrestrial television antennas are dedicated to UHF, and the bowtie array is more common today. == 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.〔(The Log-Periodic Dipole Array" )〕〔(Log Periodic Dipole Array (LPDA) )〕 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.〔("Log-periodic zig zag antenna" ), US Patent 3355740〕 Many other forms using the transmission wire as the active element also exist.〔(Photo Archive Of Antennas ), Illinois Historic Archive〕 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, 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. 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. Another visible difference is the spacing between the elements, which can be rather 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. It should be strictly noted that the "log-periodic shape" does not provide with broadband property for antennas.〔Y. Mushiake, “Constant-impedance antennas," ’’J. IECE Japan’’, 48, 4, pp. 580-584, April 1965. (in Japanese)〕〔(【引用サイトリンク】title=Y. Mushiake, '’’ Log-periodic structure provides no broad-band property for antennas."' ''J. IEE Japan'', 69, 3, p. 88, March 1949. )〕 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.〔(【引用サイトリンク】title=Y. Mushiake, '’’Origination of self-complementary structure and discovery of its constant-impedance property.' ''J. IEE Japan'', 69, 3, p. 88, March 1949. (in Japanese) )〕〔(【引用サイトリンク】title=Y. Mushiake, '’’ Infinite freedom."' )〕〔V. H. Rumsey, ‘’Frequency independent antennas’’, Academic Press, New York and London. 1966. (55 )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Log-periodic antenna」の詳細全文を読む スポンサード リンク
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