An antenna-in-package solution is proposed based on embedded organic substrate package. RF front-end module is integrated with an antenna considering the electrical and thermal performance, dimensions, manufacturing as well as cost. A millimeter-wave low-profile wideband antenna is designed, analyzed, and verified experimentally. The radiator of the proposed antenna consists of three groups of patches with different dimensions and it is fed by a stripline through an aperture etched on a ground plane. Input impedance analysis is utilized to accurately explore resonance characteristics of the aperture-coupled radiator. As a comparison, characteristic mode (CM) analysis is applied to calculate CMs of the source-free radiator. Different results obtained by two methods are analyzed. Over the operating band, three resonance modes, respectively, dominated by quasi-TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> mode of different patches are excited in turn, which contributes to wideband performance. Measured results demonstrate that the proposed antenna with only 0.039λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is wavelength of 28 GHz in free space) substrate thickness achieves an impedance bandwidth ( |S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> | ≤ -10 dB) of 37.5% (22.8-33.3 GHz). The peak gain is up to 10.7 dBi and 3 dB gain bandwidth is 23.7% (25.7-32.6 GHz), while 6 dBi gain bandwidth is 33.6% (23.5-33 GHz).