Explanation: An SSB automatic tuner connected to an insulated backstay effectively uses the backstay as an end-fed wire antenna. Such antennas, especially when operating on lower frequencies or mismatched, can develop extremely high RF voltages at their feed point – potentially thousands of volts, even with moderate transmit power. **GTO-15 high-voltage cable** is specifically designed to withstand these very high AC (and thus RF) voltages, typically rated for 15,000 volts. Its robust insulation prevents arcing and power loss at the antenna connection point. **RG8U and RG213** are 50-ohm coaxial cables. While suitable for carrying RF power to 50-ohm loads, their insulation is not designed to handle the extremely high voltages and high impedances present at the feed point of an end-fed wire antenna, which can be thousands of ohms. They would likely arc over and fail. **16-gauge two-conductor wire** is a general-purpose electrical wire with minimal insulation for high RF voltages, typically rated for only a few hundred volts. It would immediately arc and fail in this application.