Shielding Effectiveness in RF Cables and Cable Assemblies

Shielding Effectiveness refers to the ability of a cable or cable assembly to either reject the electromagnetic (EM) interference from the surrounding environment or prevent the RF signal carried by the cable from leaking into the surrounding environment and causing interference to other nearby electronic systems. In other words, it is defined as the ratio of the RF energy transmitted on one side of the shield to the RF energy transmitted to the opposite side of the shield. The unit of shielding effectiveness is indicated in decibels (dB) and the quantity provides a numerical value of how well the RF cable can provide high signal integrity and strength within various wireless systems. In general, when the shielding effectiveness increases (numerically), the shielding ability is improved in a cable. For example, an RF cable with a shielding effectiveness of 80 dB offers better shielding against EM interference than a cable with a value of 60 dB.

The interference that causes degradation in the signal quality can come from two primary sources which are natural and man-made. Man-made interference can be due to signals interfering from nearby cellular towers or networks, Wi-Fi devices or routers, and other electronic systems. Natural sources generate electromagnetic signals resulting from various phenomena such as lightning and radiation from cosmic effects outside the Earth.

The two sources can cause either temporary or permanent damage to the RF cable if it is not properly shielded to mitigate the effects of EM interference. Therefore, it plays an important role in determining the overall performance of an RF cable. This parameter is often used in product datasheet in various fields such as wireless infrastructure, wireless modules (with interconnects), cable assemblies, and cables.

Common Types of RF Shielding Employed in RF Cables

Several types of shielding exist to protect RF cables from damage or signal distortion. The shield can be a conductor that covers the signal carrying wire or an external material. Different shields deliver different performance characteristics in different environments and therefore, the right shielding method is dependent on the application requirements.

Braided Round Wire Shield or Type 1 Shield: It is one of the common types of shield used in flexible microwave cables. A braided wire represents multiple strands of thin wire which is then wound using machineries, weaving the wire together and shaping it into a flexible and strong round structure, called a braid. This shield is usually constructed using Tin or Silver-plated Copper and is easy to manufacture. The number of wires used or the braid density around the center conductor determines the shielding effectiveness of the cable. A higher braid density will improve the shielding effectiveness. However, when braid density increases, cables will become more rigid or less flexible. Furthermore, the cost of manufacturing high braid density cables is also high. The typical shielding effectiveness is 40 dB (through 18 GHz).

Braided Flat Wire Shield or Type 2 Shield: This type of braid offers a robust structure and better shielding than type 1, typically around 85 dB (through 18 GHz). It also takes less time to be manufactured compared to type 1 and is usually constructed using Silver-plated Copper. The only limitation is that this type has a higher contact resistance along with lower amplitude and phase stability when the cable is bent. Therefore, this cable type is less suitable for applications where components are located in places where cable needs to be bent to a certain extent in order to be connected properly.

Helically-Wrapped, Flat Wire Shield or Type 3 Shield: This type of cable provides relatively better amplitude and phase stability when flexed compared to type 2 cables and lower contact resistance. This cable type offers standard shielding effectiveness of 120 dB (through 18 GHz). The limitation is that the process of manufacturing this shield is demanding as the braid needs to be shaped into a perfect helix and wrapped with high precision. As a result, the labor cost is high for this type of shield.

Helically-Wrapped, Metalized Polymer Foil of Type 4 Shield: In this type, the cable is shielded using Mylar, polyimide or polyester. Polyimide offers high bending strength, chemical, and heat resistance. However, the limitation is that it provides relatively lower shielding effectiveness compared to the above types of shields. Special techniques are required such as improving conductivity of metal deposition of the braid material, which improves the reflection of EM waves from the environment. The typical shielding effectiveness of this cable type is from 20-40 dB. The below figure provides a shielding effectiveness performance vs. frequency for the above types of shielding methods. 

It is therefore important that the proper choice of material combined with the braid size determines the shielding effectiveness. Every material has its own dielectric or magnetic permeability property, which decides the ability of EM field to penetrate a material into the center conductor of the cable. Proper choice of material for the right application will help designers to provide better shielding for the cable to mitigate EM interference. In addition to these, special materials that can further improve the shielding effectiveness may also be deposited on the surface of the braid.

While it is clear that improving shielding effectiveness improves signal integrity, it is also important to consider the fact that doing so increases the cost of manufacturing. Therefore, developing RF cables that can trade-off shielding effectiveness effectively for cost, without compromising on the performance will be an active area of research and will continue to be, as wireless systems shrink in size and achieving high precision becomes critical to ensure high performance for various applications.