Spiral antenna

Two-arm log 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.^[1] Spiral antennas were first described in 1956.^[2] 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.^[3] 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 3 ways: 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.^[4]

Design

Different design parameters are to be considered while designing a square spiral antenna. The parameters include spacing between the turns $s$, width of arm $w$, inner radius $r_{1}$ and outer radius $r_{2}$. The inner radius is measured from center of the spiral to center of the first turn while the outer radius is measured from center of the spiral to center of the outermost turn. Other than these design parameters, spiral antennas have lowest ($f_{{{\text{low}}}}=c/2\pi r_{2})$ and highest $(f_{{{\text{high}}}}=c/2\pi r_{1})$ operating frequencies. Here $c$ corresponds to speed of light. In an $(r,\theta )$ coordinate system, the spiral grows along the $r$-axis and $\theta$-axis simultaneously. All spirals satisfy $r=a+b\theta$ equation where $a$ corresponds to growth factor and $b$ corresponds to multiplication factor.

Different designs of spiral antenna can be obtained by varying number of turns it contains, the spacing between its turns and the width of its arm. A dielectric medium is used with a specific permittivity and dimensions over which the spiral is printed. Dielectric mediums like Rogers RT Duroid help in reducing the physical size of antenna. Thin substrates with higher permittivity can achieve the same result as thick substrates with lower permittivity. The only problem with such materials is their less availability and high costs.^[5]

Applications

A spiral antenna transmits EM waves having a circular polarization. It will receive linearly polarized EM waves in any orientation, but will attenuate signals received with the opposite circular polarization. A spiral antenna will reject circularly polarized waves of one type, while receiving perfectly well waves having the other polarization.

One application of spiral antennas is wideband communications. Another application of spiral antennas is monitoring of the frequency spectrum. One antenna can receive over a wide bandwidth, for example a ratio 5:1 between the maximum and minimum frequency. Usually a pair of spiral antennas are used in this application, having identical parameters except the polarization, which is opposite (one is right-hand, the other left-hand oriented). Spiral antennas are useful for microwave direction-finding.^[6]

Elements

The antenna includes two conductive spirals or arms, extending from the center outwards. The antenna may be a flat disc, with conductors resembling a pair of loosely nested clock springs, or the spirals may extend in a three-dimensional shape like a screw thread. The direction of rotation of the spiral defines the direction of antenna polarization. Additional spirals may be included as well, to form a multi-spiral structure. Usually the spiral is cavity-backed, that is there is a cavity of air or non-conductive material or vacuum, surrounded by conductive walls; the cavity changes the antenna pattern to a unidirectional shape. The output of the antenna is a balanced line. If one input or output line is desired, for example a coaxial line, then a balun or other device is added to so transform the signals.

References

1. Richard C. Johnson, Henry Jasik, Antenna Engineering Handbook, Second Edition, ISBN 0-07-032291-0, 1961, Chapter 14-2
2. Orr, William I. (1976). Beam Antenna Handbook, 5th Edition. Radio Publications, Inc. pp. 185–186.
3. 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
4. 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
5. Asad, M., Gilani, J., Khalid, A. and Iqbal, M.S., “Optimizing Q factor of square spiral antenna ”, PACCS 2010, Page(s): 227 - 230
6. Stephen E. Lipsky "Microwave Passive Direction Finding " SciTech Publishing, 2004 ISBN 1-891121-23-5 page 40

"Practical antenna" references

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