Editor's Note

Close Look at RF Connectors

By Walter Shawlee 2 | July 1, 2003
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Avionics systems designers face a wide range of radio frequency (RF) connector choices. Each has important operational implications. Since high-frequency applications aggravate failure mechanisms unique to these connectors, understanding the devices’ strengths and weaknesses becomes important. Although there seems to be an almost bewildering range of commonly used connectors, there really are only three basic types:

  • Connectors with an electrical ground contact separate from the mechanical coupling,

  • Connectors in which the mechanical coupling is the ground connection, and

  • Captive coax contacts within multipin connectors or trays, which have no secondary mechanical coupling.

Connectors with ground contacts are the most commonly used and include type N, BNC and TNC. While they look different from each other, these fittings are the same fundamental connector and will even intermate (a useful trick on the test bench). A male type N will fit into a female BNC and TNC with no difficulty, and work correctly. The last member of this group–but not intermateable–is the type C, which is a scaled-up BNC fitting. These are all designed to be finger-tightened, and so have knurled barrels. Precision, low-loss versions of the threaded N and C types also are available with flat exterior surfaces for wrench tightening.

This group of connector designs has an inner ground cylinder and a side- or face-wiping contact that makes the shield return connection and maintains the connection’s characteristic impedance. The outer mating assembly is designed mainly to mechanically couple the parts and seal the inner connection from outside contamination. The bayonet-style mechanical coupling of the BNC (or C) in the airframe often is preferred, as it has a positive "click" when mated, averting human error. But the inner contact is not well protected, and the external parallel ground can become intermittent and subject to vibration-induced noise. Generally, a BNC connector should not be used above 500 MHz, but it can work up to 1.8 GHz, as long as vibration is low.

Both type N and TNC connectors have threaded couplings, and although "mated" is a subjective assessment, the inner contact’s protection is excellent, and there are many points of contact for the outer ground. These connector styles can operate at 12.4-GHz with minimal difficulty. The range of center pin engagement is not large in any of this group’s connectors–with only about 0.12 inch (3.05 mm) of travel–and some of that is lost with the contact’s front taper.

All the cable connectors in this group have one problem: the center conductor is not firmly locked with regard to the front surface. It is held in its correct position simply through the careful measuring and trimming of the center conductor insulation when the connector is assembled. This can lead to loss of connection at the center conductor, which produces losses and resonances.

The standard BNC UG88/U mil-spec connector (male) of this type is especially poor. It provides no center contact support and is tarnish-prone, mechanically weak for pulling stresses, and hard to assemble correctly. A redesign of the center contact to include a locking ring finally solved this set of problems. Later crimp BNC versions, such as the Kings KC-59-123, KC59-346 and Mil-Std-M39012/16-0014 are excellent performers and difficult to damage or mismate. Lack of a locking ring for the center pin causes the center conductor to withdraw into the cable when pressure is applied to the contact or when the cable is coiled or externally flexed. This can lead to frustrating field problems in which intermittent or weak radio operation is encountered, but everything checks perfectly.

Plating also is an issue with connectors. Silver has been the plating of choice for decades and is helpful to control losses as frequencies increase. But silver contacts will quickly tarnish in conditions of considerable humidity and atmospheric pollution. Some precision-type N or TNC connectors are available in stainless steel construction and/or gold plating, which extends service life. Kings also has a TR-5 tarnish-resistant finish that is widely used in BNC, TNC and N connectors.

Use of a center pin with silver plating is a bit problematic for BNC connectors, as the connector is not gas-tight or even well sealed when coupled in this design. This often permits contamination to oxidize the center pin, which leads to the "lossy high pass filter" fault mode and badly degraded operation. Most BNC, TNC, N and C connectors are available with gold center pins, which deliver cheap insurance against long-term field problems.

A final caveat, now quite significant, is that many very low-quality RF connectors are now flooding the market, an offshoot of the computer networking, satellite, cable and RF modem industries. They often come with serious contact and plating problems and should be carefully evaluated before use.

Look for more comment on connectors in the August System Design column. Walter Shawlee 2 can be reached by e-mail at [email protected].

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