Battery-less sensors for IoT require energy harvesting from the ambient environment. RF-DC conversion with a rectifying antenna (so-called rectenna) is generally used to obtain energy from ambient electromagnetic waves. However, RF-DC conversion efficiency of the diode significantly decreases for ambient electromagnetic waves due to their weak RF power. We previously reported bulk acoustic wave (BAW) piezoelectric transformers based on c-axis zig-zag ScAlN film/substrate structures (HBAR). However, very narrow multiple peaks in HBAR transformers are not suitable for rectenna applications. In this study, we report relativity wideband piezoelectric transformer based on a self-standing film structure (FBAR). The theoretical voltage gain was simulated by Mason's equivalent circuit model. We also derived the simple formula describing the voltage gain of the FBAR transformers without using the circuit model. This transformer was demonstrated with the wet etching process. S parameters of the FBAR transformers are measured by a network analyzer. As a result, the voltage gain approaching + 10 dB in 500 MHz range was obtained. The relative bandwidth of FBAR transformer is 30 times wider than that of HBAR transformer. This polarization inverted FBAR type transformer is promising for a relativity wideband transformer in the rectenna.