Selecting Shielded Cables: Why Transfer Impedance Matters
Posted by Joe Wigginton on 7th Aug 2024
Ensuring the reliability of electronic systems is mission critical in the aerospace and defense industries. Shielded cables are designed to protect signals from external electromagnetic interference (EMI) and to prevent the emission of interference from the cable itself ensuring that communication, navigation, radar, and other vital systems continue to function without disruption.
A key factor in choosing the right shielded cables is understanding transfer impedance. In this guide, you’ll hear from our product specialists on what transfer impedance is, why it matters so much, and find resources on how to select the optimal EMI protection for your application.
What is Transfer Impedance?
Surface transfer impedance is the intrinsic electromagnetic property used to characterize the effectiveness of shields. It measures the ability of a shield to block EMI. This was shown by Shelkunoff in the 1930s and defined as:
Zt = (1 / Io ) dV/dz
Io = Current flowing on Shield
dV/dz = Voltage per unit length on inside of shield
For practical applications it can be simplified as Zt = V / (l * Io) where l is cable length (Spira Manufacturing) .
Factors Influencing Transfer Impedance
- Materials: Nickel-plated Copper, Tin-plated copper, and Stainless Steel offer excellent conductivity and low transfer impedance. Aluminum is less effective and is not recommended for aerospace and defense applications.
- Construction: Unlike common metal braids, the woven structure of EMI protection products remains stable, ensuring consistent EMI shielding regardless of the installation diameter within the recommended application range.
- Layering: Double layer shielding offers lower transfer impedance and better EMI protection. A double layer also provides added benefits such as abrasion protection when an interior layer of PTFE tape is included or water resistance with an outer layer of Nomex® is added.
- Connectors and Terminations: Proper grounding and high-quality connectors are essential to maintaining low transfer impedance. Poor connections can negate the benefits of a well-shielded cable.
Why Transfer Impedance Is The Most Important Measurement For EMI Shielding
Transfer impedance is not the only measurement used when discussing EMI Shielding products. Multiple measurements including shielding effectiveness, insertion loss, common impedance coupling, surface transfer admittance, and charge transfer elastance can provide helpful information in context. But, if you were to choose only one metric to rely on, transfer impedance is the most important and reliable measurement for characterizing the electromagnetic shielding properties of cables and connectors across a wide range of applications and frequencies. It provides a direct, intrinsic, and practical assessment of a shield's effectiveness, making it an indispensable tool for engineers in ensuring the reliability of electronic systems.
How to Select the Right Shielded Cable for EMI Applications
Application Requirements: Match cable specifications to the specific needs of your application. For instance, avionics systems require cables with excellent EMI suppression to avoid interference with navigation instruments. If your application has risk of moisture exposure, look for a product with a waterproof exterior layer like Nomex.
Performance vs. Cost: While budget constraints are always a consideration, investing in high-quality shielded cables with low transfer impedance can prevent costly system failures and downtime. The long-term benefits often outweigh the initial investment.
Trusted Manufacturers: Source products from reputable manufacturers and distributors who have a longstanding history of working in the aerospace and defense sector and are familiar with the stringent requirements.
Top Recommendations for Optimal EMI Protection in Aerospace & Defense Applications
| Product Name | Standard | Triaxial Method | Inductance (Lt) | Transfer Impedance | Additional Features | Applications |
| Roundit EMI FMJ | IEC 62153-4-3 | Triaxial method on straight installation | 0.6 nH | Max size 8: 5.5 mΩ, Other sizes: 5 mΩ | 95% coverage for enhanced EMI protection | Cable assemblies, Wire harnesses |
| Roundit EMI C27 XWS | IEC 62153-4-3 | Triaxial method | 2 nH | Max size 5: 6 mΩ, Other sizes: 5 mΩ | Optimized weight reduction, corrosion protection | Cable assemblies, Wire harnesses |
| Roundit EMI C4 XWS | IEC 62153-4-3 | Triaxial method | 2 nH | Max size 5: 6 mΩ, Other sizes: 5 mΩ | Optimized weight reduction | Cable assemblies, Wire harnesses |
| Roundit 2000 NX EMI (A/B) | EC 60512-23-3 | Triaxial method | Not specified | 3.45 mΩ/m (Size 11) | Abrasion protection, available with/without PTFE liner | Cable assemblies, Wire harness |
Titan Electronics Is Here to Help
Browse our EMI shielding products for aerospace and defense applications or contact us today for help finding a product with the transfer impedance measurement you need for your specific application.
About The Author
Joe Wigginton is a seasoned Sales Manager with a rich background in the aerospace and defense industries. With over 20 years of experience in the cable and cable protection sector, Joe initially served as a manufacturers' representative for 5 years before taking the entrepreneurial route. For the past 15 years, he has successfully owned and operated a business dedicated to supporting customers with top-quality products and solutions.
Beyond his professional achievements, Joe has traveled extensively worldwide to forge meaningful relationships and source the best products to meet our customers' needs. His global perspective and commitment to excellence make him a valuable asset to our team and clients alike.