arXiv:2610.19040v1 Announce Type: new Abstract: Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS) enable programmable wave control on both sides of a surface, extending wireless coverage beyond single-sided operation. However, practical STAR-RIS demonstrations in literature have narrow bandwidths, because it is challenging to achieve wideband reflection and transmission simultaneously from the same aperture. To overcome this limitation, a wideband STAR-RIS architecture is proposed based on shared-aperture polarization conversion and diagonal switching topology. Here, multiple resonances enable wideband magnitude responses and mirror-symmetric current reversal maintains $180^{\circ}$ phase difference in both reflection and transmission modes. Accordingly, a diagonal four-branch unit cell configuration is developed, where the tailored top patch directs the received wave either through a pair of blind vias connected by a buried strip to the shared top aperture for reflection, or to the bottom ring patch using a through via for transmission. Based on the proposed design, a 10 by 10 STAR-RIS prototype is fabricated and experimentally characterized. The measured 3-dB bandwidths reach 19.8% in reflection mode and 21.9% in transmission mode. Near-field measurements show average signal enhancements of 19.8 and 22.6 dB for reflection and transmission, respectively, over the wide frequency range of 6.8-8.4 GHz. Far-field measurements further demonstrate average received-power enhancements of 11.2 dB in reflection and 11.9 dB in transmission. To the best of the authors' knowledge, this work presents the first experimentally demonstrated wideband STAR-RIS covering the 7.125-8.4 GHz band within the emerging 6G FR3 spectrum.