Engineering Note: Torque Recommendations For Coaxial Connector Coupling Nuts

Engineering Note:
Tips To Avoid Cable Assembly Breakage

INTRODUCTION:
Most cable assembly breakages occur at or near the cable entry area at the rear of the connector. The larger the connector is relative to the outer diameter of the cable, the more prone the assembly is to damage. For example, a type N or 7/16 DIN connector on RG-316 cable is a popular combination, but requires special care since the connectors are large compared to the cable size. Summarized below are some tips that can extend the life of cable assemblies. They entail assembly procedures and also cautions when actually handling and using the cable assemblies.

If you are purchasing complete cable assemblies, rather than doing them in-house, specify the application that they will be used for (routine lab use, production test or a “set-and-forget” application, high power, environmental conditions, etc).

FLEXIBLE CABLES:
When trimming flexible cables prior to assembly, care must be taken not to nick and/or cut into the braid components or the center conductor. Special care must be paid to cables with very small diameter center conductors. The COAXICOM 402BF and 405BF (BF = BlueFlex) cable assemblies are flexible equivalents to RG402 and RG405 semi-rigid cables with comparable performance and should be treated as delicate flexible cables since the diameter of the braid wires are made small to retain flexibility. The FEP outer jacket increases the overall BlueFlex OD slightly compared to the equivalent semi-rigid cable by a little over .020”. COAXICOM can also supply the flexible equivalent to RG401 (401BF) or to the Semi-Flex cables discussed below.

SEMI-FLEXIBLE CABLES:
Semi-Flexible cables (also called conformable or hand formable, Ref: Coaxicom 402UF, 405UF) are also very similar to the equivalent semi-rigid cables in performance and OD, but have a copper-tin composite outer jacket which provides the same 100% shielding as semi-rigid. Both BlueFlex and Semi-Flexible cables have solid center conductors that must not be nicked during trimming.

PREVENTIVE MAINTENANCE:
At initial cable assembly it is recommend that one or two layers of adhesive backed shrink tubing be used at the connector/cable interface, extending 1” or more beyond the cable entry area. This can be done on flexible, semi-flexible and flexible cables, and is particularly important when using large connectors with small diameter cables or when the cable lengths are long or they will be subject to rough handling. On RG58 and RG316 sized cables with BNC series connectors, strain relief boots are often used over the connector/cable interface.

When handling and using cable assemblies do not bend cables tighter than their specified static and dynamic bend radii. During lab usage or installation into a system, minimize bending close to the connector/cable interface area. Use loops instead of sharp bends on the cable where possible. Avoid abrasion points such as sharp edges on test equipment and on units under test. Further, do not pull on cables to free them after connector disengagement. Grip the cable assembly firmly at the connectors to remove them.

Engineering Note:
SMB, SMCs Connector Discussion

STANDARD 50 Ω SMB, SMC SERIES:

The standard SMB and SMC series are miniature 50 Ω connectors. The SMB features snap-on mating and is specified to 4 GHz. SMC connectors are the same size as the SMB but utilize a threaded coupling and are specified to 10 GHz. The Coaxicom CMS series is similar to the SMB, but has slide-on mating. Note that CMS plugs will mate with SMB jacks.

75 Ω SNAP-ON AND THREADED CONNECTORS:

This is where confusion often sets in. As described in the Coaxicom catalog, these series have interfaces similar to (but 1.5 times larger) than the 50 Ω SMB and 50 Ω SMC series. The 75 Ω Snap-On series is commonly called the SMZ in the industry. They are sometimes also called “Screw-On and Snap-On Conhex series” as a holdover from the original 1960’s designer’s nomenclature.

MINI-75 Ω CONNECTORS (USUALLY CALLED MINIATURE 75 Ω SMB):

Mini-75 Ω connectors are matched impedance SMB connectors, normally used with 75 Ω miniature coaxial cables such as RG-179, 187, and Belden 9221. They mate without interference with 50 Ω SMB’s and are most commonly available as cable plugs and jacks, as well as printed circuit board straight and right angle plugs. Because of their small size, they are occasionally called “High Density 75 Ω SMB Connectors”. Some manufacturers use a colored dielectric to differentiate the mini-75 Ω SMB from standard 50 Ω SMB’s, which use a white dielectric. Coaxicom adds a -75 suffix to the 50 Ω part number.
They are fully mate-able with each other.

The table below itemizes the Coaxicom part numbers for each of the series discussed above. By using the information given here and referring to the website www.coaxicom.com, a user should feel confident that he is ordering the correct part for his application. For additional information, the factory is always available for additional information.

Legend: XX = cable group where applicable; * = connector body finish code

SUMMARY

It is apparent that care must be taken when discussing SMB and SMC connectors in general and mini-75 Ω SMB’s in particular. It is important to understand exactly what the user needs and equally important is that he/she understands what is being supplied. The most likely mistake is supplying the large “75 Ω SMB or SMC like” connectors, when the requirement is actually for the “mini”. It can also go the other way. Customers have been known to order SMB connectors, and then were surprised that our SMB was smaller than expected. It turned out that the requirement was actually for the large 75 Ω “SMB-like” type.

The table below summarizes various characteristics of the SMB, and SMC series connectors.

NOTES:

1/ .195 is the diameter of the bulkhead mounting circle, and  the .180 is the dimension across D flat. These are key dimensions to be checked when cross-referencing.

2/ .200 is center-to-center spacing for the PCB leads, and .250 sq. is the most common footprint size. Again, these are key dimensions to be checked when cross-referencing. Some manufacturer’s footprints are slightly larger, and/or oriented differently relative to the PCB pins.

3/ Inside diameter of the jack (male) interface. The .082/.092 and the .131/.135 is the allowable range.

4/ Coaxicom and other manufacturers specify the VSWR to 2 GHz. Other manufacturers say “useable to
 4 GHz with low reflections”, but have no published VSWR specification.