Mitigating electromagnetic interference (EMI) and RF interference (RFI) is a growing concern in aerospace and defense applications. Consumer communications technology is rapidly moving to over-the-air (OTA) transmission, with the need for more information to be communicated over longer distances, driving wider bandwidth, higher frequency and greater transmit power. Defense systems such as electronic warfare (EW) and radar have similar trends toward higher frequency and greater power, placing more stringent demands on the shielding of cable assemblies and electronic components. As the RF environment, which is already filled with noise sources and interference, becomes even more crowded, systems and components resistant to EMI and RFI will play a greater role in system reliability.
IMPACT OF EMI/RFI ON CABLE PERFORMANCE
Figure 1 Troubleshooting signal interference problems is challenging, especially in an aircraft.
EMI describes noise or interference over a very broad frequency spectrum. For example, the noise source could be a vacuum cleaner motor emitting low frequency interference in the double-digit range. RFI is associated with emissions within the RF spectrum, nominally from 20 kHz to 300 GHz.
For microwave cable assemblies, EMI and RFI are two-way streets. If a cable assembly can produce sizable amounts of interference, it will also be susceptible to receiving the same. The objective of cable shielding is to keep the noise and signals outside of the cable and keep in the signal of interest. When a cable assembly is sensitive to EMI/RFI, the signal of interest - the signal propagating through the cable assembly - can be compromised, even rendered unintelligible by interference.
As frequency increases, between 300 MHz and 300 GHz for microwave applications, shielding a cable assembly becomes more challenging. Higher frequency signals, coupled with increased transmit power, will increase the power radiated through any leakage in the shield. This combination increases the risk of interference with electronic equipment and cable runs adjacent to the radiating component. Troubleshooting a signal interference problem, especially in an aircraft, can be daunting (see Figure 1). Highly effective shielding is critical to system performance.
Figure 2 RF leakage from a cable assembly can degrade an EW system’s capability detecting and countering threats.
Figure 3 The cable, connector and cable termination determine the shielding effectiveness of an RF cable assembly.
With cellular communications, 5G OTA testing requires new cell phone designs to be validated without a cable connection to the device under test (DUT). Instead, all test signals are transmitted OTA in a controlled environment, to approximate real world conditions. In this environment, microwave cable assemblies with poor shielding will be prone to radiating and receiving unwanted signals, which could negatively influence a design validation.
Cable assemblies susceptible to EMI/RFI can have an especially profound effect on airborne EW applications. Modern EW systems gather large amounts of data from the electromagnetic environment surrounding the aircraft, using it to determine threat type, location, proximity and severity. Poorly shielded cable assemblies, or those with compromised shields, can degrade the signals from the system’s antennas and the system’s ability to identify and determine how to respond to a potential threat - particularly since the design goal for threat detection is to detect a threat with a minimum of 2x the threat’s lethal striking distance.
With an EW system, the critical nature of processing time is apparent by considering this scenario (see Figure 2): An aggressor fighter aircraft has just launched a subsonic air-to-air missile against a friendly fighter, as the two rapidly close on one another. At launch, the two aircraft were 4 miles apart, converging at 1200 mph. From the time of launch and at the current closing speed, the friendly aircraft’s EW system has less than 12 seconds to identify the threat and determine a course of action, including displaying the threat and its position to the pilot and launching countermeasures. With poorly shielded cable assemblies, the EW system may not extract the threat signal from the cacophony of electromagnetic noise and interference - not unlike trying to pick out a single voice in a stadium full of cheering spectators.
MITIGATING EMI/RFI
With any technical problem, the first step toward resolution is understanding, which comes through testing and characterization. W. L. Gore & Associates performs shielding effectiveness testing of its cable assemblies using the latest mode-stirred chamber technology and incorporates these findings into its cable assembly and connector designs and termination techniques. Shielding effectiveness is not strictly a cable phenomenon; the connector and cable termination are part of the equation (see Figure 3).
For defense aircraft applications, EMI/RFI challenges can be addressed through the following tactics:
- Use appropriate shielding enclosure designs for the application.
- Where cable installation or aircraft servicing can potentially damage cables, use ruggedized microwave cable assemblies with proven shield designs capable of withstanding installation and servicing.
- Ensure all coaxial connections are clean and tightened to the proper torque.
- In high vibration environments, ensure cables are properly secured using locking connector coupling nuts or coupling nuts drilled for lock wire holes.
- In microwave cable airframe installations, do not stretch a cable that “last little bit” to make a connection; use properly sized P-clamps to secure cables.
- As cable placement is critical, do not position cables close to antennas transmitting high-power.
- In commercial applications, instruct technical personnel on microwave cable assembly care and handling. This is particularly important when using non-ruggedized or general-purpose cable assemblies, as they are considerably more fragile than their ruggedized counterparts.
- Cable shielding problems often result from damage caused by improper handling. Discourage repeatedly bending cables just behind the connector or coiling cables in tight loops for storage, which exposes them to unnecessary wear.
- When bundling cables with plastic zip-ties, use just enough force to contain the bundle; zip-ties can exert damaging crush forces if cinched tightly.
- To maintain performance, proper microwave connector care and maintenance are key to preserving mechanical and electrical integrity.
