As digital technology increasingly permeates various aspects of daily life, the security of information and network systems has emerged as a critical concern, garnering significant attention from researchers. This paper proposes a novel transmission method that integrates MIMO and OFDM technologies with the STBC encoding scheme to improve PLS in multi-hop UAV communication systems. In the proposed method, a UAV is introduced as a relay node to facilitate packet forwarding at hops characterized by weak connections. To achieve full diversity gain, the STBC encoding scheme is employed with two transmit antennas and a single receive antenna. In this method, it is proposed that the conventional STBC encoding pattern be adapted into 16 different alternative patterns while retaining orthogonality and preserving the advantages of traditional STBC encoding. At each packet transmission time, one of the STBC encoding patterns is selected based on the RSSI value between the transmitter and the legitimate receiver. The RSSI value depends on the transmission channel and the distance between the transmitter and the legitimate receiver. It varies randomly and is only known by both the transmitter and the legitimate receiver. The eavesdroppers are unable to access this value. Consequently, the RSSI value is proposed as a potential secret key for secure communication between the transmitter and the legitimate receiver. The selection of the STBC encoding pattern is designed to ensure randomness and uniform distribution at the transmitter. This makes it difficult for eavesdroppers to determine the transmitted STBC encoding pattern, thereby strengthening the overall security of the system. Based on the analysis and simulation results, the BER performance at the legitimate receiver is maintained, whereas that at the eavesdropper substantially increases. As a result, the PLS of the system is significantly improved.