New Connectors Address Design Limitations of the SMP Family

by David Murray, Technical Fellow

Originally published in Microwave Product Digest

The aerospace and defense industries count on robust, reliable RF systems for mission-critical applications such as satellites and communications systems, electronic warfare, intelligence, surveillance, reconnaissance systems, missile guidance, and more. These advanced technologies must be unfailing and offer consistently high performance—often in demanding, variable, and confined environments on the ground, air, and at sea.

The performance and reliability of any RF system are heavily determined by each of its hardware components and, especially, how they fit together. While some RF installations demand relatively standard products, others require custom solutions to meet unique requirements. For example, several innovative new RF interconnect designs, including locking miniature connectors, locking miniature blind mate connectors, and multiport contacts, were created to address specific needs not fulfilled by standard designs such as the SMP connector.

SMP Family: Limitations

The RF industry has used sub-miniature push-on connectors, or SMPs, for many years. This connector style was sold initially by Gilbert (now part of Corning) in the late 1970s under the name GPO^TM and was designed to scale down SMA’s (Sub-Miniature version A) connectors. Other manufacturers began to incorporate similar designs to create compatible connector versions under the generic SMP terminology, and it quickly became an industry-standard design.

A significant benefit of the SMP design is that they are simple to install by pushing the connectors against each other and pulling them apart to disconnect. SMPs can also mate without threading, eliminating the need for small wrenches and other tools. In addition, the SMP accommodates a slight radial misalignment during mating, which would be unacceptable with threaded connectors.

However, while SMPs remain a valuable connector option for numerous designs, they pose problems as applications operate in higher frequency bands. One of the primary issues is shielding and electromagnetic interference (EMI). EMI is an unwelcome occurrence occurring when an external source causes a disturbance in the signal of interest. The SMP connector utilizes wide slots to ease alignment and mating but creates a path for signal leakage. At the same time, the SMP’s design reduces its capacity to function without affecting other equipment operating in the same environment. This is referred to as electromagnetic compatibility or EMC. The SMP’s lack of proper electrical bonding and shielding exposes the conductor’s signal to external influence, and the signal leakage issues often result in failed EMC tests.

Another significant design flaw is that the SMP connector is vulnerable to ingress from salt, fuel, water, and other contaminants. The lack of an environmental seal due to its mechanical openings makes SMPs prone to corrosion and failure. It can also cause increased VSWR and loss. Devices using SMPs may also fail qualification because of this insufficient environmental seal.

Finally, the SMP connector’s easy connect/ disconnect design makes them prone to de-mating in a high-vibration environment. Coupling nuts and third-turn fasteners can be added to address the lack of attachment strength. However, this creates a larger connector—which defeats the purpose of a miniature connector, particularly as electronic boxes get smaller and smaller.

Viable SMP Alternatives

Locking Miniature Push-On and Locking Miniature Blind Mate Connectors

Smaller-sized, O-ring sealed connector solutions offer improved shielding, environmental protection, and mechanical retention. This includes a new generation of locking miniature push-on and locking miniature blind mate connectors from Times Microwave Systems. The TLMP™ (Times Locking Miniature Push-On) and TLMB™  (Times Locking Miniature Blind Mate) connectors were created for applications where EMI may be an issue, such as an aircraft or ship, that require an environmentally sealed and shielded connector. The TLMP is also ideal for high-vibration environments such as a weapons launch, carrier landing, or other harsh similar platform issues.

TLMP/TLMB connectors maintain the small form factor of SMPs but add improved shielding, environmental, and power capabilities, covering a range of frequencies from DC to 60 GHz. The mating part entirely covers the connector’s slots to prevent signal leakage. This enables the connectors to overcome performance issues from the SMP connector’s susceptibility to EMI and EMC interference as well as liquid and salt ingress. Their sealed, rugged design can withstand harsh conditions and severe environments better. At the same time, the high-power/high-voltage design (overlapping insulators), cutting off a direct path to the ground from the center conductor to the outer shield enables higher-power or higher-voltage functionality.

TLMP/TLMB connectors also feature a latching mechanism to improve mating retention, making it a better choice over options that use threaded bodies. In addition, a visual verification feature with green (locked) and red (unlocked) color coding assures connectors are fully mated and locked. Finally, the TLMB features an additional (to TLMP) outer sleeve to provide fully protected tines for true blind mate applications.

Design offerings for TLMP/TLMB include edge and orthogonally launched to PCB/microstrip, MMIC, probe launch, and straight and right-angle connectors for various coaxial cables. They are also available as 50-ohm terminations for plug or jack.

Similarly, new TLC and TLP connectors are available from Times Microwave Systems that are slightly larger for higher power handling. They are suitable for high CW power applications with frequencies above 18 GHz.

Multiport Interconnect Systems

Advanced aerospace and defense applications must increasingly accommodate extremely restricted space constraints and rising operating frequencies. The high-frequency RF interconnections within those systems are essential components. While many critical systems still operate in the 18 GHz range, newer ones are emerging that are required to work at 40 GHz.

Multiport contacts, including the new M8M contact from Times Microwave Systems, allow equipment manufacturers to have one connection that accommodates all of these frequencies. The M8M is in addition to Times Microwave’s time-tested high-performance M8 family of multiport interconnects, including the original M8 (18.5 GHz), the M8E (23 GHz), and lightweight, low insertion force V8 (18.5 GHz) contact. Throughout its more than 30-year history, the Times Microwave M8 family of multiport interconnect solutions has been the go-to system for a wide range of platforms worldwide, with tens of thousands of parts currently in service on U.S. and allied military aircraft from SIGINT / ELINT platforms to fast fighters—with hundreds of thousands of flight hours logged.

The new M8M contact brings 40 GHz capability to the trusted family of multiport interconnects. It is compatible with all M8 shells to maximize existing infrastructure while adding new assemblies to power high-frequency applications. Its unique construction makes the M8M contact excellent for high-vibration, harsh environments typically found in aerospace and defense applications. Furthermore, it meets reduced size, weight, and power (SWaP) requirements.

Multiport interconnect systems can also address the lack of adequate electrical bonding, and shielding exposes the conductor’s signal to external influence, creating EMI and EMC issues in higher-frequency applications. They replace individual assemblies requiring multitudes of connectors with an alternate solution featuring a single connection port.

Furthermore, fitting the complex RF systems that power many critical functionalities in aerospace and defense applications into tight spaces can also allow unwanted coupling between RF transmission lines, such as coaxial cables. Minimizing the distance between connectors and cables is necessary for the interconnect system to survive high vibration and other extreme environmental conditions. Multiport systems solve this problem with a smaller, modular connector assembly that integrates multiple coaxial contacts into a single housing for a higher interconnection density than individual coaxial connections. This reduces installation time, simplifies system maintenance and testing, and increases reliability.

A vital part of this system is the M8 multiport shell—a machined piece that allows the installation of multiple cables in one housing/interface. Times Microwave Systems works closely with large prime contractors and has developed plating materials for the M8 multiport connection system with excellent salt fog resistance (2,000 hours) and Sulfur Dioxide resistance (668 Hours). This plating type is used on the latest, cutting-edge airframes as it is intended for use in harsh airborne and maritime environments requiring shell-to-shell-conductivity. Times Microwave System also has fiber optic, and octo contacts to fit into the same shells if multifamily interconnects are in the application’s requirements portfolio.

Conclusion

SMP and other standard connectors have been around for a long time and still work for what they were designed to do. However, modern, more challenging RF applications require new interconnect solutions for high reliability, high vibration, and high density. Innovations in this area include locking miniature push-on, locking miniature blind mate, and multiport connectors from world-class suppliers, including Times Microwave Systems.

When selecting RF cables and connectors, it is ideal to work with an experienced partner like Times Microwave Systems, with engineers that can identify the application’s needs and design an optimized solution that is ultimately easier to use—creating better electrical, mechanical, and environmental performance. The supplier should work collaboratively as an extension of your design team. These systems are highly complex and do not use a standard solution, so it’s also vital that the RF supplier’s technical team asks questions and listens to understand the application’s unique needs.