Low profile, single feed, tri-filar, linear- or circularly- polarized helix antenna

We have designed and built a small, low-profile tri-filar helix antenna, which can have either linear or CP polarization, with a single feed and no internal feed network. This antenna is comprised of three bent quarter wave monopoles (an element), positioned at 0, 120, and 240 degrees, respectively. One of the bent monopoles is fed, and the other two are parasitically coupled. Perturbations on the parasitic elements determine whether the antenna is linear or CP, and control the phases. To induce linear polarization, no perturbations are used and both parasitic elements are coupled at +120 degrees. To induce CP polarization, the resonant frequency of one or both of the parasitic elements is slightly shifted using perturbations. The elements can be supported using a dielectric substrate, or can be free standing. A ground plane is necessary only directly under the elements. This antenna can replace a dielectrically-loaded single feed linear or CP patch, and does not require the dielectric loading. Hence this design is a less expensive choice for narrow band antenna applications.