Ensuring the secure communication is the prime concern for all kinds of data communication infrastructure. Several data encryption techniques are in use for secure communication. There is a scope for further improvement in the security level, as well as for the received signal quality. The selection of a proper combination of encryption and modulation techniques can improve the overall effectiveness of both the security level and the received signal quality. The combination of asymmetric cryptography (here use RSA (Rivest–Shamir–Adleman) algorithm) algorithm with the proposed reflex klystron-based frequency shift keying (FSK) modulation technique provides a high level of secure communication system. The performance of the proposed technique is experimentally verified. The conventional FSK modulation technique operates in the low-frequency band, but the proposed FSK modulation technique is powered by high-power microwave devices, which operate in the X band. The experimental result has been demonstrated and successfully verified for secured communication using the X band.