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Spiral antenna
In microwave systems, a spiral antenna is a type of RF antenna. It is shaped as a two-arm spiral, or more arms may be used.〔Richard C. Johnson, Henry Jasik, Antenna Engineering Handbook, Second Edition, ISBN 0-07-032291-0, 1961, Chapter 14-2〕 Spiral antennas were first described in 1956. Spiral antennas belong to the class of frequency independent antennas which operate over a wide range of frequencies. Polarization, radiation pattern and impedance of such antennas remain unchanged over large bandwidth.〔Paul E. Mayes, “Frequency-Independent Antennas and Broad-Band Derivatives Thereof”, Proceedings of the IEEE, Volume: 80 , Issue: 1, Digital Object Identifier: 10.1109/5.119570, Publication Year: 1992, Page(s): 103 - 112〕 Such antennas are inherently circularly polarized with low gain. Array of spiral antennas can be used to increase the gain. Spiral antennas are reduced size antennas with its windings making it an extremely small structure. Lossy cavities are usually placed at the back to eliminate back lobes because a unidirectional pattern is usually preferred in such antennas. Spiral antennas are classified into different types; archimedean spiral, square spiral and star spiral etc. Archimedean spiral is the most popular configuration.
==Working==
These antennas operate in 3W way; traveling wave, fast wave and leaky wave. The traveling wave, formed on spiral arms, allows for broadband performance, fast wave due to mutual coupling phenomenon occurring between arms of spiral and leaky wave leaks the energy during propagation through the spiral arms to produce radiation. Ring theory (band theory) explains the working principle of spiral antenna. The theory states that spiral antenna radiates from a region called active region where the circumference of spiral equals wavelength.〔A. Mehta, D. Mirshekar-Syahkal and H. Nakano , “Beam adaptive single arm rectangular spiral antenna with switches”, Microwaves, Antennas and Propagation, IEE Proceedings - Volume: 153 , Issue: 1, Digital Object Identifier: 10.1049/ip-map:20050045, Publication Year: 2006 , Page(s): 13 – 18〕
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