A Single-Layer, Differentially Fed Triband Filtenna With Uniform Polarization and Similar Broadside Radiation Characteristics

Abstract

An innovative single-layer, differentially-fed tri-band filtenna is presented. It is formed with a main patch, which consists of two interdigitated-coupled driven patches that are approximately mirror-symmetric, and four parasitic patches. The strong coupling between the two driven patches facilitates excitation of the TM1,0, TM1,2, TM1,4, and antiphase TM2,0 modes of the entire main patch and the consequent very small electrical size of the filtenna. Two pairs of open-ended branches are loaded on the main patch to improve the selectivity of the middle band. A pair of split-ring slots are etched on the main patch and four parasitic shorted patches are loaded near it to introduce additional in-band resonances and out-of-band radiation nulls simultaneously, which improve the bandwidth and selectivity of the lower and higher bands, respectively. A prototype was fabricated, assembled, and tested. Its electrical size is 0.31λL×0.28λL×0.031λL, λL being the free-space wavelength corresponding to the lower band’s center frequency. The experimental results, in agreement with their simulated values, demonstrate operation in three bands: 2.4–2.487, 3.72–3.93, and 5.57–5.88 GHz, with uniform polarization and similar broadside radiation performance. They also show excellent low cross-polarization levels, high common-mode noise immunity, and good filter properties over all three bands.