Editor's Note

RF Connectors: SSMA, SMA, ETC.

By Walter Shawlee 2 | August 1, 2003
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In my last column (July 2003), we looked at the design issues associated with popular N, BNC and TNC radio frequency (RF) coax connectors. This month, we will look at the other two main groups of coax connectors. We also will examine the interconnect problems with connectors that a scalar network analyzer can reveal.

The second main group of coax connectors, including SSMA, SMA and related 3.5-millimeter (0.14-inch) precision fittings, uses the outer mechanical coupling as both the mechanical and electrical ground connection. This connector style often is used internally in microwave and RF assemblies and is capable of low-loss operation to 18 GHz, with semirigid cable. Because the coupling’s tightness is critical to correct operation, this connector body has flats and must be torque-wrench tightened, not finger tightened. In these connectors the range of contact engagement is extremely short, typically under 0.09 inch (0.23 centimeter), and catastrophic internal connection failure can occur easily, while still giving the physical appearance of a good connection.

Contact failure creates a lossy high pass filter due to the center conductor air-capacitor coupling. Connection failure also produces many unpredictable resonances due to the outer ground discontinuity. These contact artifacts are quite vibration-sensitive, so you can imagine the resulting consequences in flight. Any airborne RF assemblies using this style of connector need to be properly torqued (7 to 10 inch-pounds) and, ideally, witness-marked to make obvious any later loosening.

The final group of connectors, which has no distinctive name, has what is in essence only the center portion of the first group of connectors (BNC, TNC, Type N). Simple sliding pressure mates them together. No mechanism exists to ensure positive engagement or to provide supplementary ground connections. These connectors see use in some ARINC tray mounts; Mil-C-38999 circular connectors and related types; some panel-mounted, tray-style avionics as free-standing contacts; and D-subminiature connector housings. Generally, these connectors are better suited for shielded analog signals and data than for low-impedance (50-ohm) RF. And, unless fairly large contact inserts are involved, they are both lossy and easily damaged in the mating process, if misaligned.

Use of this connector style as a system’s primary RF feed should be carefully reviewed at the design stage. The design is difficult to test or inspect in tray mounts and within other connectors. It also can have significant vibration-induced noise.

Front panel, tray-mounted systems often use this slide-on type of connector for frequencies up to and including transponder/DME operation. But it does not perform well at 1 GHz. In addition, poor installation of the coax feed line to the tray connector can result in intermittent operation and frequent trips to the test bench. These isolated panel tray fittings also are normally silver and can tarnish, especially if the avionics boxes are removed for security and the connectors are left exposed for long periods. Contacts within other nonconductive inserts (multipin connectors) are almost always gold-plated and do not have the problem of tarnishing.

These contacts have nothing to really ensure tight contact integrity beyond the nonconductive outer housings and may exhibit motion in and out. Even small-motion artifacts and resistance are significant at 50 ohms and appear as noise. To see how significant this noise can be, just use a simple video graphics array (VGA) monitor switch box on your computer. The noise on the switch contacts or connectors often will make the computer unusable. It causes intermittent losses in the even-more tolerant 75-ohm video feed lines and makes the colors change constantly. In low signal-level receivers this contact noise can dwarf any incoming signal.

Connector Faults Illustrated

Shown, left, are scalar network analyzer pictures illustrating severe internal connector faults. In a correctly coupled system, the flat line (photo 1) shows insertion loss is minimal even up to 1.2 GHz, with hardly any resonances ("zero loss" is the center line on the cathode ray tube [CRT]). Once the center or ground contact is lost, however, the capacitive coupling effect ruins low-frequency operation, and many parasitic resonances appear (photo 2).

Photo 2 also shows performance where the center pin was deliberately disengaged (the outer ground remains). This was set up to be right at the point of disengagement, so the contact gap is in thousandths of an inch, representative of a pushed-back center pin or a tarnished interconnect. The CRT’s midpoint is 600 MHz horizontally, and loss is 10 dB/division, about 25 dB of loss at midband. Photo 3 shows the faulty connector swept to 200 MHz, expanding the display at VHF frequencies. At 100 MHz, losses are already 40 dB, which, for a VHF comm or nav system, can result in severely degraded range and communications. Photo 4 shows SMA connectors and photo 5 shows a D-subminiature type with coax contacts.

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