GLENAIR
Performance and Loss in an RF Transmission Line When passing an RF signal through a transmission line, some of the power is invariably lost in a phenomenon known as attenuation. Attenuation is frequency dependent, with high frequency transmissions almost always leading to higher levels of power (signal) loss. Signal loss through coaxial
cable can occur for various reasons, such as radiation exiting the cable via apertures or imperfections in the outer shield, resistive losses in the cable conductors, and signal absorption in the cable dielectric—particularly in long-length cable runs. Better attenuation performance can be achieved through the use of more conductive shielding materials, such as silver plated versus non-plated copper and by lowering the dielectric constant. Using larger-diameter coaxial cable increases the surface area of metal and thus reduces the resistive losses per unit length.
Cable Construction Coax cables are typically supplied in 50-Ohm and 75-Ohms impedances, created by altering the ratio of the inner conductor to the outer conductor. 50-Ohm cable is a “best of both worlds” design, as 30-Ohms delivers best power handling while 70-Ohm delivers lowest signal attenuation (loss). 50-Ohm cable is thus preferred for higher power and frequency video and digital audio signals. For less power intensive interconnection to a radio or RF receiver, 75-Ohm Coax cable is ideal.
The center conductor in the Coax cable carries the signal, with the electromagnetic energy traveling on the outermost portion of the center conductor in a phenomenon called “skin-effect.” Some larger diameter cables may use a hollow tube, as the dielectric constant of dry air in low-frequency applications delivers acceptable low-loss performance. Smaller form-factor cables may use a variety of core and shield plating materials as well as various thicknesses of plating to optimize the electrical signal. The insulator or dielectric surrounding the inner conductor may be solid plastic, a foam plastic, or air with spacers supporting the inner wire. The dielectric helps maintain the concentricity of the center conductor and contributes significantly to the electrical performance of the cable. Dielectric materials used by Glenair include PTFE and extended ETFE for low attenuation at microwave frequencies. We also utilize LPCF to minimize phase shift caused by temperature change with less than 250 ppm/°C phase change from -40 to +60 °C. Many conventional coaxial cables use braided copper wire forming the outer conductive shield. While this improves flexibility compared to solid outer conductors it may result in gaps in the shield layer which can lead to higher attenuation and cross-talk. For this reason, high-performance applications with low-loss budgets typically rely on more highly engineered designs. Glenair constructed cable may incorporate as many as four metallic layers for greater than 90 dB of shielding effectiveness including designs with flat SPC (Silver-Plated Copper braid) flat tape inner shield, aluminum/polyimide foil interlayer, and round SPC braid outer shield. Cables with a solid outer conductor facilitate shape-memory forming where required, are electrically stable, and ideal for use in areas with limited space and high vibration resistance requirements. Semi-rigid cable with solid outer conductors are also used on higher-frequency applications to take advantage of the low-impedance solid tube performance. Designs supplied by Glenair are available with PTFE dielectric and in special low loss / higher temperature versions. The outer jacket, when used, serves as a protective barrier to the coaxial cable. FEP is a typical outer jacket material, however, there are other jacket materials available to meet unique application requirements such as resistance to ultraviolet light, harsh chemical environments, radiation, oxidation, high heat, abrasion, cutting, and so on. For internal chassis applications the insulating jacket is typically omitted.
QwikConnect • January 2023
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