In 2012, the European Organization for Nuclear Research (CERN) set the world of science abuzz and the hearts of futurists everywhere alight by announcing that they had detected neutrinos moving faster than light. The excitement built when they managed to repeat the test with the exact same results. When a thorough inspection of the equipment traced the anomalous test results back to a loose connection between a GPS unit and a computer, the disappointment was crushing.
Data centers, cellular networks, and many other industries that depend on radio-frequency (RF) signals are subject to similar errors. Ultimately, much of what we take for granted in the modern world are rather basic calculations derived from values of signal strength and timing. If the SubMiniature version A (SMA) connectors that tie radio frequency modules together are too loose or too tight, the signals, the timing, and the values input into the calculations will be off. These connections must be tightened exactly to specifications with an SMA torque wrench to ensure that the equipment is working within tolerances.
The Importance of Torque for SMA Connections
There are three types of SMA connectors. Two are F-SMA connectors for fiber optic cables. The more widespread type is a screw connector developed in the 1960s for coaxial connections between RF equipment. It has a screw-type coupling mechanism and an electrical impedance of 50 Ohms and is used in the microwave systems that power radar, cellular networks, and Wi-Fi antenna systems. This type looks a great deal like the coaxial connections that tie your cable box into the wall.
Torque is important for these connectors from both a material standpoint and an operational one. An SMA RF connector is rated for up to 500 cycles, meaning it can be unscrewed and re-tightened up to 500 times and pass signals in the range specified for the equipment. However, a single tightening cycle that is not done to spec can be problematic. If it’s too tight, the result can be cross-threading; if it’s too loose and impedes signals, then the SMA connector’s impedance, frequency, and other electrical values can no longer be relied upon. In some applications, like HAM radios for the home hobbyist, this may not matter much. In fields like radar, telecommunications, and radio astronomy, however, it matters a great deal.
The Types of SMA Connectors
There are three grades of SMA connectors, each one meant for different uses. Torque specifications for SMA connectors vary depending on these classes. The least expensive type is made of softer materials and requires a lower level of torque. SMA connectors intended for more precise applications are made of sterner materials, cost more, and require higher torque values to make a firm connection without interference. The three classes of SMA connectors can be seen in the chart below:
| SMA Connector Classes | |||
| Class | Material | Torque lbf.in | Torque N.cm |
| Economic | Brass | 4 lbf.in | 45 N.cm |
| Standard | Stainless steel | 9 lbf.in | 100 N.cm |
| Hermetic | Stainless steel plated for application | 17 lbf.in | 195 N.cm |
Brass SMA connectors are the most common type. If you have a high-grade Wi-Fi router in your home, the antenna attaches to one of these economical brass connectors. When you first set up the router, you likely tightened these by hand and noticed no ill effect. Industrial and telecommunication applications are more stringent, and a torque wrench should be used to avoid damaging these soft connections in a professional setting.
The same is true of standard grade and hermetic grade SMA connectors. They are made of corrosion resistant materials to avoid signal loss and interference due to corrosion of the material. Signal interference due to improper fastening is a hazard, and its consequences will likely be severe. Hermetic SMA connectors derive their name from being designed to hermetically seal the electrical connection from interference and alteration due to changes in the surrounding environment. In addition to these seals, they are plated with a material specific to the application they are being used for, like gold, chromium, or titanium. These higher grade SMA connectors are used in some of the most sensitive and delicate operations humans engage in, and the consequences of fastening these connectors out of specification can range from telecommunication networks going down to the loss of years of scientific research. Therefore, when working with SMA connectors professionally, a precise SMA torque wrench is mandatory.
SMA Torque Wrenches for Accurate RF Signals
SMA connectors tie together cables that prevent the full seating of a tool on the fastener. As such, SMA torques wrenches must be open-end inch-pound torque wrenches. Break-over wrenches are the best SMA torque wrenches for industries to equip their technicians with. Specifically, break-over wrenches with a 5/16 inch open head. While an adjustable break-over torque wrench can be used to tighten SMA connectors to specifications—and in situations where multiple classes of SMA connectors in a setting are common, this may seem economical—it does create the potential for improper connections due to user error. The potential consequences in applications where accuracy cannot be compromised suggest that the possibility of the user missetting a torque wrench should be ruled out.
The best choice One option is to use preset break-over wrenches that are already calibrated to the torque specifications of the types of SMA connectors that are to be worked with. If you’re working with a range of different grades of SMA connections, then a preset torque wrench with the correct settings can be ordered for each type in a different color to help ensure that technicians use the correct SMA torque wrenches for each grade of connector. The association of color and connector type will help to prevent costly fastening errors.
Alternatively, you could use a crowsfoot spanner head. These attach to either preset screwdrivers for manufacturing applications or adjustable screwdrivers for field service. These spanners work well for the standard connector sizes.
SMA torque wrenches aren’t devices that are usually brought up during discussions of high technology. Nevertheless, the signal strength and timing that allows us to know our position globally and estimate the distance between ourselves and the stars in the sky depends on turning a simple threaded fastener correctly. That’s why your company needs to use an accurate and precise SMA torque wrench.
Mountz, Inc. has seen the advance of technology firsthand from our home of Silicon Valley. We’ve supported innovation in telecommunications, manufacturing, and assembly for over forty years with a range of quality precision torque tools. To see our full range of products, shop our store. Contact us anytime to ask a question. To inquire about price, request a quote. If you’d like to see our equipment in person, schedule an appointment.
