New MEIA™ Series Establishes Next Generation RF High Power Interconnect Capabilities
By Norm Brodeur, TRU Corporation
Today’s system and component design engineers are increasingly being challenged across a broad range of applications to transmit higher RF power using coaxial cable assemblies.
While everyone’s definition of high power is specific to their application, whether a board level, semiconductor power amplifier or a system level RF generator, our discussions regarding high power focus on coaxial power transmission between 5 – 50kW (average). Frequency considerations are typically 1 GHz or less with typical applications below 200 MHz. As found in most RF applications, there is the ever present compromise between competing performance attributes across mechanical, electrical and environmental needs. As power requirements push above 5 kW, the trade-offs become more acute and the safety/reliability factor becomes a significant focus. Development of the new MEIA™ high power connector interface was undertaken to address the evolving needs of the high power RF market and eliminate many traditional compromises that engineers faced in their RF interconnect solutions.
High power RF transmission traces much of its original innovation back to the 1950s, when military radar and commercial broadcast markets began to emerge significantly. Rigid copper coax structures with various impedance (30, 50 and 75 ohm) outfitted with EIA (Electronic Industries Association) defined RF connectors provided a means of transmission that was well shielded, thermally predictable and could handle the high power application requirements. Limitations of this form factor, however, were considerable. These early assemblies were fully inflexible, required additional space and layout considerations, and added overall weight and mass. Fabrication of bends and termination of the connectors required additional specialized soldering/brazing process steps in the manufacturing cycle. Mounting of the connector flange interfaces to the equipment was time consuming and accomplished manually by the use of multiple bolts and nuts. Thermal conditions drove the use of a removable bullet center conductor to accommodate the movement resulting from heat build-up in the transmission line.
Several innovations emerged over the following decades to address the inflexibility of the cable in these early designs. Coaxial cable using a corrugated outer tube design was introduced as a means of providing additional, yet still limited, flexibility in the cable assembly. Corrugated copper cable, while well shielded with good loss and some additional flexibility, was sometimes subject to cracking during installation or when subjected to multiple bends.
Termination of the connectors often proved mechanically more variable. Over the past 30 years, the introduction of more flexible, multi-shielded braid construction and microporous PTFE dielectrics has enabled flexible cable assemblies to replace corrugated or solid copper tube. Thermally, microporous PTFE dielectrics remain stable when exposed to extreme temperature that allows them to be used in such high power applications. Flexible cable designs, 0.50 inch diameter or greater, provide a thermal profile that could generally ensure proper operation at high power. While cable development progressed, the connector choices remained fairly consistent as EIA 7/8, EIA 1-5/8 or EIA 3-1/8 were specified for power applications > 5kW and above 500 MHz.
The MEIA™ Solution
The MEIA™ series is a complete family of cable assembly, adapter and panel mount receptacle designs as illustrated in Figure 1. While the inner coaxial structure, defining the 50 ohm impedance, is the same as corresponding EIA line sizes (hence equivalent power rating), the outer mechanical design has been optimized for greater ease of installation, improved density and weight reduction making it a logical choice for new system designs or upgraded retro-fits. MEIA™ is offered in two size options: MEIA-1625 which operates to 3 GHz and aligns to the 1-5/8 EIA line size and MEIA-875 which operates to 6 GHz and aligns to the 7/8 EIA line size. See Figure 2 for dimensional comparison between MEIA™ and EIA cable connectors.
MEIA™ cable connectors have the knurl/hex combination coupling nut for efficient mating and the elimination of multiple bolt and nut fasteners to mechanically mate the flange. The knurl allows rapid initial hand mating and the hex facilitates final torque of the connection. High power EIA connectors and cable assemblies have traditionally been large and heavy, causing in some cases excess mechanical loads on the host equipment. The weight and size reduction of the MEIA™ series design lowers the bending moment of the cables on the mounting panel and leads to less mechanical stress on the connector and cables as well as the component/system equipment.
MEIA™ series provides the maximum power handling capabilities in the market today. Higher power applications exceeding 5 kW begin to eliminate practical use of standard 7-16 and LC connector interfaces that do not have the required power handling or design margin in frequencies above 100-200 MHz. MEIA-1625 will handle approximately 3 times more power than a 7-16 interface and 2 times greater than a LC interface, up to an operating frequency of 3 GHz. Even at average power levels below 5 kW, MEIA™ series provides additional margin for peak power situations compared to 7-16 and LC. Figure 3 illustrates the average power handling capability of the MEIA™ interface.
The MEIA™ series provides design engineers with several advantages for use in high power RF applications including:
• Superior power handling capabilities vs. 7-16 or LC interfaces (2-3 times greater)
• Equivalent power handling capabilities to EIA 7/8 or 1-5/8 defined interfaces
• >30% smaller than a comparable EIA connector
• >40% lighter than a comparable EIA connector
• Shorter form factor allows tighter bends and higher density on the equipment
• MEIA™ cable assemblies exhibit a minimum static bend radius of 1.70 inches
• High-efficiency threaded coupling on the MEIA™ interface eliminates the multiple bolt and nut fasteners needed for installation of an EIA
The design of the MEIA™ series connectors is optimized for high power performance, safety and reliability. Internal dielectric components have overlapping/telescoping design elements to lengthen voltage paths and prevent arching that could lead to short circuits and failure. The silver plated, female center conductor is fabricated from beryllium copper for excellent spring memory forces during mating. The center conductor and dielectrics are mechanically captivated to ensure dimensional stability and long service life even after multiple mating cycles. The MEIA™ interface is rated for > 500 mating cycles. MEIA™ panel mount receptacles employ high power back-end launch geometries that facilitate transition to equipment PCB or component level structures.
The area of greatest concern for failure in any cable assembly is the cable-connector junction, where the two elements are terminated together. In high power applications, this is especially true as any weakness in the mechanical cable-connector attachment, errant braid or other debris can result in a voltage arc. The MEIA™ series utilizes a unique TRU cable-connector attachment technology called TRUtie™ that has been designed to provide superior connector retention and torque resistance. The innovative design is extremely repeatable and eliminates the cable junction as the point of mechanical stress failure. Figure 4 illustrates the TRUtie™ cable-connector attachment.
MEIA™ cable assemblies offer a unique combination of high power capability and unmatched flexibility. Two TRU cables are offered for standard MEIA™ cable assemblies: TRU-560 and TRU-500. Each cable has been designed by TRU to optimize power handling capability, shielding and flexibility. Each cable operates to 6 GHz, has > -75 dB shielding effectiveness and exceeds the power rating of the MEIA™ interface. TRU-500 has a FEP outer jacket with a recommended operating temperature of 165°C. TRU-560 has a PVC outer jacket that affords greater overall flexibility with a slightly lower recommended operating temperature of 105°C. The minimum static bend radius of the cables is 1.70 inches. The flexibility of these two TRU cables provides greater system design and configuration options because the cable assemblies can be routed and reconfigured as required for each application. See Figure 5 for TRU cable construction.
The attributes of the MEIA™ series, including superior power handling, improved flexibility, reduced weight and proven reliability, are ideally suited for many commercial applications. Greater power levels above 5 kW are being developed in support of next generation industrial equipment applications that employ high power amplifiers and RF generators. The ISM (Industrial, Scientific & Medical) market continues to drive equipment advances in plasma generation, laser and non-invasive medical therapy that require high performance power solutions.
MEIA™ attributes are also well suited for military applications where weight savings and size equate to significant cost reduction or situations where ruggedized performance is critical.
MEIA™ series connectors and cable assemblies have been qualified and are currently in use in production, high power airborne military radar systems. In order to provide additional weight reduction, the outer housing components are fabricated in aluminum rather than the standard brass composition. The weight and size reduction is critical in airborne platforms where it translates to greater system density, longer flight time and lower costs in fuel consumption. MEIA™ is also available with optional stainless steel outer housings and triple-shielded cable to meet critical battlefield requirements.
There remains a large base of EIA connector designs on system components that are in use. Various combinations of MEIA™ to EIA adapters are available to facilitate the transition of system equipment that already has EIA interfaces on them to realize the advantages of more efficient MEIA™ designs.
There are growing RF power and flexibility needs across an expanding range of applications in both commercial and military markets. The development of the MEIA™ cable assembly and connector series provides component and system engineers with an unprecedented combination of power handling, flexibility and size advantages to meet these new challenges.
Installation is greatly enhanced with a threaded coupling design and flexibility of the cable allows the assemblies to be routed for the greatest range of configurations. The smaller and lighter form-factor facilitates higher density packaging and less mechanical stress on the cable assembly. MEIA™ continues the innovation in RF power capabilities to solve today’s engineering challenges.
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