Electrically Small, Planar, Horizontally Polarized Dual-Band Omnidirectional Antenna and Its Application in a MIMO System

A method of designing a miniature planar dual-band horizontally polarized (HP) omnidirectional antenna is presented. It combines an electrically small (ES) lower band omnidirectional loop antenna with an upper band Alford loop antenna on a single substrate. Its frequency ratio is higher than two. A technique that effectively increases the bandwidth of the Alford loop is developed. To reduce the gain variation in the azimuthal plane, the ES loop is fed by four symmetrical radial strips extended from a circular patch. For demonstration, the ES loop and Alford loop were designed for the 2.4 and 5 GHz Wi-Fi bands for Wi-Fi applications. The antenna prototype has a diameter of 0.296λ
₀
and a height of 0.012λ
₀
only, where λ
₀
is the wavelength in air at the lowest operating frequency. It covers the Wi-Fi bands of 2.4-2.5 and 5.1-5.9 GHz, with a high total efficiency of ~90%. Based on the single-element design, a four-element multiple-input-multiple-output (MIMO) antenna is developed for Wi-Fi applications. The MIMO antenna has a size of 0.93×0.93×0.012λ
₀
³
, with a small gain variation of less than 3.2 dB in the azimuthal plane. The isolations between any two antenna elements are higher than 16 and 24.5 dB for the 2.4 and 5 GHz bands, respectively.