Why Radar Systems Use Waveguides Instead of Coaxial Cables

Radar systems rely on hollow rectangular metal waveguides rather than coaxial cables to transport high-power microwave energy with minimal signal loss. This article explains the physical limitations of coaxial cables at microwave frequencies and details how waveguides overcome issues like dielectric loss, conductor resistance, and voltage breakdown to ensure efficient radar operation.

Coaxial cables consist of an inner conductor surrounded by a solid plastic or foam insulator, known as a dielectric, and an outer metal shield. At the high microwave frequencies used in radar electronics, this dielectric material absorbs a significant amount of electromagnetic energy and converts it into heat. Hollow rectangular metal waveguides eliminate this problem entirely because they are filled with air or pressurized inert gas. Since dry air has almost zero dielectric loss, the microwave energy travels through the interior of the waveguide without being absorbed by insulating materials.

Conductor loss is another major factor at microwave frequencies due to a phenomenon called the skin effect. The skin effect forces high-frequency alternating currents to flow only on the very outer surface of a conductor. In a coaxial cable, the majority of the current is concentrated on the thin inner conductor, which has a very small surface area. This creates high electrical resistance, intense heat, and rapid signal attenuation. A hollow waveguide does not have a center conductor. Instead, the microwave energy propagates by reflecting off the wide interior walls of the metal tube. This large interior surface area drastically reduces electrical resistance, allowing the energy to travel with minimal loss.

Radar transmitters generate massive amounts of peak power, often reaching into the megawatts, which requires transmission lines that can handle extreme voltages without failing. In a coaxial cable, high power creates an intense electric field between the closely spaced inner and outer conductors. If the power is too high, the solid dielectric insulation breaks down, causing electrical arcing that melts the center conductor and destroys the cable. Because waveguides lack a center conductor and have a wider internal geometry, they can handle much higher voltages before an arc occurs. For extreme power applications, waveguides can also be pressurized with gases like sulfur hexafluoride to further prevent electrical breakdown.

Finally, rectangular waveguides provide superior electromagnetic shielding and mode control compared to coaxial cables. At high frequencies, coaxial cables can accidentally support multiple propagation modes, which distorts the radar signal. The specific width-to-height ratio of a rectangular waveguide is mathematically designed to allow only a single, predictable wave pattern—usually the dominant Transverse Electric (TE10) mode—to pass through. Additionally, the solid metal walls of the waveguide completely contain the microwave energy, preventing signal leakage and protecting sensitive radar receivers from external interference.