A Single-Layer, Differentially Fed Triband Filtenna With Uniform Polarization and Similar Broadside Radiation Characteristics
Dajiang Li, Ming‐Chun Tang, Kun‐Zhi Hu, Yang Wang, Richard W. Ziolkowski
Abstract
An innovative single-layer, differentially fed triband 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 the excitation of the TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\mathbf {1,0}}$ </tex-math></inline-formula> , TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\mathbf {1,2}}$ </tex-math></inline-formula> , TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\mathbf {1,4}}$ </tex-math></inline-formula> , and antiphase TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\mathbf {2,0}}$ </tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.31\lambda _{\mathbf {L}} \times 0.28\lambda _{\mathbf {L}} \times 0.031\,\,\lambda _{\mathbf {L}}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{\mathbf {L}}$ </tex-math></inline-formula> 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.