Cyber threats are defined as those that target a system's availability, integrity, or confidentiality. As the threat landscape increases with 5G and IoT connectivity, so does the potential to directly affect or access connected ‘system of systems’ using Radio Frequency (RF) connections. The open architecture and low-cost nature of today’s Software Defined Radio (SDR) technology present opportunities to easily exploit RF access points with cyber threats in a growing connected cyber terrain. SDRs can provide low-cost, simple discrete attacks on a system’s confidentiality, integrity, and availability, without a requirement for physical access. The Cyber Kill Chain, which focuses on external complex and persistent threats, is not fit for purpose for an RF-based threat. While other researchers suggest that traditional cyber attacks may be increasing in complexity and time to render, it is suggested in this paper that RF-enabled cyber-attacks are becoming less complex and easier to implement due to the availability of low-cost SDR technology and associated high-power amplifiers and antennas. In this paper, a new RF Cyber Kill Chain is proposed to aid penetration testers and improve the cybersecurity of connected systems. The RF Cyber Kill Chain is shown through a case study to accurately map three stages of attack: Reconnaissance, Preparation, and Execution. SDR technology is also shown in this paper to have enormous potential when combined with machine learning (ML) to provide early warning of cyber-attacks utilising the RF spectrum.