Spiral antenna
[2] Archimedean spiral antennas are the most popular, while logarithmic spiral antennas are independent of frequency:[3] the driving point impedance, radiation pattern and polarization of such antennas remain unchanged over a large bandwidth.
Spiral antennas are reduced in size with its windings making it an extremely small structure.
The traveling wave, formed on spiral arms, allows for broadband performance.
Fast wave is due to mutual coupling phenomenon occurring between arms of spiral.
Leaky wave “leaks” the energy during propagation through the spiral arms to produce radiation.
corresponds to speed of light in the metal of the antenna, mainly determined by the electrical permittivity of the substrate the spiral lies on, and its over-coating (if any).
Often-used Archemedian spirals satisfy a particularly simple equation
The consequence is equal spacing between successive turns, which limits the width of the spiral arms, which is usually kept constant.
Different designs of spiral antenna can be obtained by varying number of turns for each arm, the number of arms, the type of spiral, the spacing between its turns, the variation of the width of its arm(s), and the material(s) that surround it, such as the substrate it lies on.
The antenna usually has two conductive spiral arms, extending from the center outwards.
Additional spirals may be included as well, to form a multi-spiral structure.
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 output of a two-arm or four-arm spiral antenna is a balanced line.
If a single input or output line is desired – for example a grounded coaxial line – then a balun or other transformer is added to alter the signal's electrical mode.
The spiral can be printed or etched over a specifically chosen dielectric medium, whose permittivity can be used to alter the frequency for a given size.
Dielectric mediums like Rogers RT Duroid help in reducing the physical size of antenna.
[7] Spiral antennas transmit circularly polarized radio waves, and will receive linearly polarized waves in any orientation, but will drastically attenuate circularly polarized signals received with the opposite-rotation.
One application of spiral antennas is wide-band 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).
