Similar and Dissimilar Devices
Although the loop may be in the shape of a circle, distorting it into a somewhat different closed shape does not qualitatively 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. In a self-resonant loop antenna the most important characteristic, resonant frequency, is determined by the circumference of the loop. On the other hand, the main characteristics of a small loop antenna are rather determined by the area enclosed by the loop. 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.
Read more about this topic: Loop Antenna
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