This article introduces a novel power amplifier integrated active antenna (PAIAA) with increased efficiency that integrates a Class-$F {^{-}1 }$Gallium Nitride (GaN) power amplifier (PA) and a stub-loaded wide slot antenna (SLSA) for wireless power transmission (WPT) applications. Unlike conventional PA integrated active antennas in which antennas and PAs are separately matched to 50 ohms, the input impedance of the SLSA is meticulously engineered to match the fundamental impedance and modulate harmonic impedances of the GaN transistor by optimizing the dimensions of the SLSA's stubs. This approach eliminates the output matching network (OMN), the PA's harmonic modulation network (HMN), and the antenna's input matching network (IMN), which typically incur unavoidable insertion loss but are essential for conventional Class-$F {^{-}1 }$PAs and antennas. Consequently, it enhances the power-added efficiency (PAE) within the 3.3–3.8-GHz range and reduces the overall size of the PA integrated active antenna. Thanks to the extended design freedom offered by SLSA, precise impedance values can be achieved across a broader frequency range, resulting in increased efficiency across a larger spectrum. In addition, an innovative insertion loss measurement method for impedance matching network with a non-50-$\Omega $port is introduced to accurately measure the Class-$F {^{-}1 }$PA's PAE incorporated in the proposed PAIAA. The measured PAIAA demonstrates a peak PAE of 70.1%, surpassing the performances of reported integrated active antennas. The measured effective isotropic radiated power (EIRP) of the proposed PAIAA is 44.52 dBm. A conventional PA-antenna design, where an antenna and a Class-$F {^{-}1 }$PA are separately matched to$50~\Omega $and simply cascaded, is also designed and measured as a contrast, whose measured peak PAE and EIRP of the conventional design are only 61.5% and 43.9 dBm, respectively. The proposed PAIAA, with its increased efficiency, can be practically utilized for WPT applications in the Internet of Things (IoT), where efficiency performance is critical.