Miniaturized Omnidirectional Horizontally Polarized Antenna

A novel miniaturized impedance matched antenna with omnidirectional horizontally polarized radiation pattern is presented. The antenna structure resembles a circular loop formed by a circular array of shunt miniaturized n-fold resonant dipole antennas, which will be referred to as miniature composite wire-loop antenna (MCWLA). The proposed antenna does not require an external matching network and can easily be matched to balanced ports with any desired impedance. MCWLAs can be made to be a very small fraction of the wavelength and yet to provide a relatively high radiation efficiency. The antenna input impedance can be adjusted by the number of array n-folded dipole elements, the number of folds and the diameter of the composite loop. The input impedance can be lowered by increasing the number of elements and/or by decreasing the loop diameter. Conversely, the input impedance, for a fixed number of elements and diameter, can be increased by increasing the fold number. The antenna efficiency increases with increasing the loop diameter. To demonstrate the proposed concept, a MCWLA is designed with diameter ${\lambda}/8$ and is designed and fabricated using the standard PCB approach on a thin substrate ( $\lambda /500$ ). To achieve omnidirectional radiation pattern in H-plane, a minimum of three elements fed in-phase around the inscribing loop are used. It is shown that impedance match to a $50\hbox{-}\Omega$ line can be achieved using fourfold miniaturized dipole elements. The simulation shows strong uniform current distribution on the A transformer balun that is used to connect the balanced port of the antenna to a small coaxial line at the center of the loop. The measured gain, including the loss through the balun, is to be 0 dBi and the gain variation in the H-plane is shown to be less than 0.5 dB for the prototype MCWLA.