What are Sinuous Antennas?

A Sinuous Antenna is a combination of a spiral antenna and a log-periodic antnena. This unique structure results in an antenna that provides broadband performance, is dual-polarized, has a high gain and is stable impedance with frequency. Sinuous antennas are used in direction finding systems, human health monitoring, electromagnetic pulse (EMP), close-in sensing systems such as ground penetrating radars (GPR), UWB applications such as high-precision indoor positioning and for various other applications.

Structure of a Sinuous Antenna

A Sinuous Antenna has four arms, each shifted and placed 90˚ away from each other as seen in the figure above. The part of the arm that is close to the center has a short wavelength while the element at the end (away from the center) has a longer wavelength. This unique configuration, enables sinuous antnenas to have wide bandwidths.

Each of the four arms has an angular width of δ with the distance between the elements continuously expanding until the end of last antenna element. This angular width δ can be used to control the impedance of this antenna and has typical values that lie between 30˚ and 70˚. 

An appropriate transmission line, such as a stripline or microstrip balun can be used as a feed line to each of the four arms. A pair of antenna arms that are opposite to each other is excited by feeding power via this line that is equal in magnitude but 180˚ out-of-phase with respect to each other. Any two opposite arms are physically separated by a length equal to . As a result, this power selection will lead to producing two signals that are linearly polarized in their respective directions. Since, these antennas are also spiral in nature, they exhibit circular polarization in the two directions. And depending on the line of reference and choice of axes, one can be regarded as Left hand Circular Polarized (LHCP) and the other as Right hand Circular Polarized (RHCP).

Thus, these antennas exhibit polarization diversity due to this dual-polarization nature. Due to this polarization in both the directions, they can be used to receive signals from transceivers from any orientation and across a wide area or region. In addition, these two polarizations are orthogonal to each other i.e. they are spatially independent beams and not interfere with each other.

With an appropriate choice of wavelength or frequency and bandwidth, this antenna can be used to radiate Ultra-Wideband (UWB) signals. These are special type of signals that have bandwidths equal to greater than 500 MHz. Therefore, UWB radiation along with polarization diversity is exhibited by this single sinuous antenna, which makes it ideally suitable for various UWB application and other scenarios where a high-precision is required.

The geometry of sinuous antennas in general is quite complex and thus it is difficult to manufacturer.