Key Takeaways
- The double shielding of a typical quadrax cable provides a high degree of electromagnetic isolation needed for reliable signal transmission.
- Quadrax cables are used for high speed avionics and defense data buses where protection from EMI is critical.
- Quadrax cables are typically designed for balanced-data applications unlike inherently unbalanced coaxial cables.
There are dozens of communications protocols and cable types to support data transmission, and because of this, system designers have a wide range of options to meet their system performance requirements. As a system matures, the performance requirements will typically help “box in” the protocols and cable types needed for the application. In the case of high reliability, high-bandwidth data transmission, quadrax cables may emerge as a viable option for some designs. This article discusses the potential applications, performance capabilities, comparisons to similar cable types, and installation/maintenance challenges of quadrax cables.
Cable Construction
In accordance with AS6070, the quadrax cables construction starts with four conductors arranged as two differential pairs with dual concentric shields (an inner braid and an outer braid); to put it another way, quadrax cable can be thought of as two balanced pairs with two layers of shielding each with at least 85% optical coverage. Where this stands out from most other cable constructions is the double layer of shield that provides the high degree of electromagnetic isolation needed for reliable signal transmission. While quadrax cables are typically built with two concentric shields, there are some solutions that achieve similar performance with one shield; these claims should, of course, be verified before use in aviation.
The double shielding of a typical quadrax cable does carry a weight penalty meaning that, at least for aerospace applications, the cable use will often be limited to those critical systems needing the isolation.
Those seeking quadrax cables viable for aerospace applications should consider those qualified to SAE AS6070/1, /3, or /8 cable standards. The different slash sheets offer different impedance values for the user.
Cross-sectional view of a typical quadrax cable.
Impedance
Quadrax cables are typically designed for balanced-data applications and, at least within the AS6070 cable family, come in 100 Ω and 110 Ω versions. In comparison, coaxial cables are usually 50 Ω (common in RF/microwave) or 75 Ω (video/broadcast) and are inherently unbalanced.
EMI/Noise Immunity
Quadrax cable dual shielding solution provides an effective means to reduce the penetration of EMI to the signal wires. Furthermore, because quadrax cables are balanced lines, they also reject common-mode noise better than unbalanced cable types like coax.
When considering the shielding options for cables, the following is a good rule of thumb for shield performance (based on comparison at 200MHz using the absorption clamp method) sorted from worst to best.
- (Single shield) Aluminum shield with drain wire
- (Single shield) Served Sheild
- (Single shield) 60% optical coverage braided shield
- (Single shield) 95% optical coverage braided shield
- (Double shield) Double Served Shield
- (Double shield) Aluminum foil shield with 90% optical coverage braided shield
- (Double shield) 90% optical coverage braided shield + second 90% optical coverage braided shield
It is important to note that with each additional layer of shielding, the cable diameter and weight increase and the cable flexibility decreases. While the typical recommended cable bend radius is 10x the outer diameter of the cable, the performance impact should be assessed before seeking to install a double shielded cable below this guidance.
Typical Applications
Quadrax cables are used for high‑speed avionics and defense data buses where protection from EMI is critical. Commonly found in military connectors (MIL-DTL-38999, ARINC-600/404) with Quadrax contacts, it carries two independent differential channels in one cable. Some of the common applications for quadrax are radar display buses, high-speed video, and network communications (e.g. IEEE 1394b, Gigabit Ethernet, FiberChannel). The quadrax cable’s balanced dual-pair design also makes it suitable for ruggedized fiber-channel links in aircraft.
Summary Table
Construction |
Four conductors (two twisted differential pairs) in dual concentric shields. Usually star-quad geometry. |
One central conductor + dielectric + one outer shield (single or multiple layers). |
Shielding |
Inner and outer shields; full 360° coverage per pair. |
Single or double coaxial shield; covers center conductor. |
Impedance |
Typically, 100, 110, or 150 Ω differential per pair. |
Typically, 50 Ω (RF) or 75 Ω (video). |
Bandwidth |
Very high – characterized up to ~3 GHz; supports Gigabit data. |
Very high – RF/microwave; multi-GHz. |
EMI/Noise |
Excellent immunity: balanced pairs + dual shields confine interference. |
Adequate via shield, but unbalanced nature makes it more susceptible to noise. |
Typical Uses |
Avionics data buses, high-speed video. |
RF/microwave links (radar, antennas, comms), video transmission, sensors. |
Quadrax Cable Installation and Termination Guide
As with any EWIS component, there are a set of recommended connectors for the proper termination of quadrax cables. It is possible to find aerospace-grade connectors with Quadrax contacts (e.g. MIL-DTL-38999 Series I/III or ARINC-600 connectors equipped for quadrax), but, because of the high data rate and performance requirements, additional factors need to be part of the evaluation. It is important to note that MIL-DTL-38999 connectors are not impedance controlled, but there are vendor specific connectors that use a MIL-DTL-38999 shell with a custom insert design for minimizing interference and achieving impedance matching. To ensure high system reliability, the terminating component performance should be assessed and the terminating technique should be tested/verified.
From a shielding perspective, termination of the quadrax cable’s has two shields can be a challenge. For this reason, the shield termination must be done in steps with the inner braid first handled and pulled over the ferrule/bushing, trimmed to remove excess shielding, then followed by the outer braid being captured under the crimp bushing. This two-step process ensures each shield is electrically bonded. If solder sleeves are necessary, the termination should occur as close to the connector as possible.
Common Issues & Troubleshooting
Termination of any shielded cable can be a challenge. The dual shield construction of quadrax cables is a process that requires practice and patience.
While there are benefits to use of the quadrax cable, there are some common issues that emerge with their use.
- Misaligned or Bent Contacts: Quadrax contacts can become bent when the contacts are misaligned during mating. While keyways on the connector should help to prevent scooping damage, low loss connectors may use longer contacts that are more prone to bent contact damage.
- Poor Termination Quality: Inadequate crimping or poor soldering can cause high resistance termination leading to signal attenuation. The risk of poor termination performance can be reduced with crimp verification and performance checks as part of both visual and electrical harness acceptance testing.
- Untrimmed Shield Strands: If braid strands are not properly terminated, they can short to the inner conductors. After trimming braid, it is important to clear the area of any loose wire pieces; in other words, FOD control is an important part of quality quadrax termination. The inner braid should not contact inner conductors.
- Impedance/Skew Problems: Excessive skew or signal loss can arise if there is uneven wire lengths or excessive untwisting of the conductor pairs. To control this usually means well documented build procedures and careful attention to cut and strip back lengths. Again, this is something that should follow the manufacturers guidance, and if not is available, be verified through testing.
- Connector Leakage: If leakage current or poor EMI performance is detected with testing, start with a visual check to determine if the shield is fully under the crimp sleeve and that the sleeve is crimped correctly. The cable’s ground braid should make solid contact with the connector shell (using specified braid ferrules or bonding techniques) to maintain shield continuity.
Conclusion
Each system has a unique set of requirements, and those needing high speed reliable signals transmission for aerospace applications should consider the use of quadrax cables. Finding the right cable for the application to match performance needs, impedance, and termination technology will take time, but the design of the quadrax cable will help to minimize signal loss and EMI impacts during operations. As with any technology, there are limitations and challenges for fielding the solutions, but they can be overcome with training and proper use and installation. Contact Lectromec to find out how we can help with your EWIS design principles, testing, and/or EWIS certification support.
Michael Traskos
President, Lectromec
michael.traskos@lectromec.comMichael has been involved in wire degradation and failure assessments for more than two decades. He has worked on dozens of projects assessing the reliability and qualification of EWIS components. Michael is an FAA DER with a delegated authority covering EWIS certification and the former chairman of the SAE AE-8A EWIS installation committee.