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A loop antenna is a radio antenna consisting of a loop (or loops) of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line. Within this physical description there are two very distinct antenna designs: the small loop (or ''magnetic loop'') with a size much smaller than a wavelength, and the resonant loop antenna with a circumference approximately equal to the wavelength. Small loops have a poor efficiency and are mainly used as receiving antennas at low frequencies. Except for car radios, almost every AM broadcast receiver sold has such an antenna built inside it or directly attached to it. These antennas are also used for radio direction finding. In amateur radio, loop antennas are often used for low profile operating where larger antennas would be inconvenient, unsightly, or banned.〔Low Profile Amateur Radio (A. Brogdon, W1AB)〕 Loop antennas are relatively easy to build.〔A Great Shortwave Loop http://antenadx.com.br/?page_id=99〕 A small loop antenna, also known as a magnetic loop, generally has a circumference of less than one tenth of a wavelength, in which case there will be a relatively constant current distribution along the conductor. As the frequency or the size is increased, a standing wave starts to develop in the current, and the antenna starts to acquire some of the characteristics of a resonant loop (but isn't resonant); these intermediate cases thus cannot be analyzed using the concepts developed for the small and resonant loop antennas described below. Resonant loop antennas are relatively large, governed by the intended wavelength of operation. Thus they are typically used at higher frequencies, especially VHF and UHF, where their size is manageable. They can be viewed as a folded dipole deformed into a different shape, and have rather similar characteristics such as a high radiation efficiency. ==Similar and dissimilar devices== Although a resonant loop may be in the shape of a circle, distorting it into a somewhat different closed shape does not greatly alter its characteristics. For instance, the quad antenna popular in amateur radio consists of a resonant loop (and usually additional parasitic elements) in a square shape so that it can be constructed of wire strung in between insulators. Or the loop can be completely collapsed into a line, in which case it is termed a folded dipole. In either case, the antenna's resonant frequency is determined by the circumference of the loop. On the other hand a small loop antenna is used at a far lower frequency compared to its size; its radiation resistance and efficiency are instead dependent on the area enclosed by the loop (and number of turns). For a given loop area, the length of the conductor (and thus its net loss resistance) is minimized in the case of a circle, making that shape optimum for small loops. Although it has a superficially similar appearance, the so-called halo antenna is not technically a loop since it possesses a break in the conductor opposite the feed point. Its characteristics are unlike that of either sort of loop antenna here described. Also outside the scope of this article is the use of coupling coils for inductive (magnetic) transmission systems including LF and HF (rather than UHF) RFID tags and readers. Although these do use radio frequencies, and involve the use of small loops (loosely described as "antennas" in the trade) which may be physically indistinguishable from the small loop antennas discussed here, such systems are not designed to transmit radio waves (electromagnetic waves). They are near field systems involving alternating magnetic fields only, and may be analyzed as poorly coupled transformer windings; their performance criteria are dissimilar to radio antennas as discussed here. 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「Loop antenna」の詳細全文を読む スポンサード リンク
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