If there were such a thing as a perfect connector it would consist solely of an outer conductor, a center conductor and a homogeneous dielectric, forming a perfect 50 ohms line. A product that comes close to that perfection is an airline, as used for calibration and testing with network analyzers. In reality, the regular straight connector is much more complicated, full of irregularities and a number of capacitive and inductive sections, caused by the conductors stepping up and/or down, to accommodate dimensional changes.
Also, the captivation of the center conductor with the outer conductor isolates the axial and rotational forces associated with the coupling of connectors from the elements to which the center conductor is attached and maintains the specified dimensions at both ends of the connector or adapter. Captivation techniques are epoxy, shoulder, single barb and double barb. Although captivation adds necessary ruggedness, it also causes electrical disturbance that has to be compensated for. The soldering process also has to be taken into consideration. It is almost impossible to completely control the solder flow or to compensate for excessive solder.
As can be seen, making straight connectors with tight tolerances is not easy, so making parts that go around corners is even more complicated. Basically, angled connectors have to be made from joining straight pieces together, or the straight components have to be formed into the angled configuration, or a casting has to be manufactured, which usually cannot be achieved within tight tolerances. All of these methods were evaluated when Spectrum Elektrotechnik developed its SA2-Line (Smooth Angle Air) swept angled adapters and connectors and MA2-Line (Mitered Angle Air) adapters and connectors.
The swept angled connector (shown in Figure 1) or adapter is often considered to be the best option, although it does have its drawbacks. One reason being that the outer conductor is cast, but as liquid metal shrinks when cooling, the surface will never be smooth.
In addition, the process dimensionally can only be controlled to a certain level. Therefore, steel balls are forced through the outer conductor to make the surface smoother and to get the dimensions to an acceptable level. The center conductor and the dielectric are originally straight and then forced mechanically into the angled body and through the bend. Also, when the dielectric (Teflon, PTFE or similar) surrounding the stiff center conductor (usually beryllium copper) is forced through the angled stainless steel body the ends of the dielectric may be deformed and need to be machined off — a secondary operation.
Additional parts for the front and/or back end of the dielectric may need to be added as well. At the interface the center contact is also normally not completely in line with the outer conductor. To bend a center conductor into a 90° angle, it would need to be bent to an angle greater then 90°, depending on the stiffness of the material. A further aspect that must be taken into consideration is that the electrical performance of the product may change over time. Another technique mentioned above is bend tubing, although this method does not have much advantage over the cast product and is prone to similar problems.
Having taken these points into consideration, Spectrum Elektrotechnik has developed new design and machining techniques to produce a true mitered angled connector that exhibits superior performance. By this method the outer conductor of the connector shown in Figure 2 has been made in one piece, offering both mechanical and especially electrical advantages.
Also, as parts are not joined mechanically, very tight tolerances can be set. In fact, tolerances in the important areas at the outer conductor internally are ±5 µm (±2/10,000 inches). As for the center conductor and the dielectric, special machining and assembly techniques are used, employing machined parts with similarly tight tolerances as those applied to the outer conductor. The improvement of the electrical performance is measurable, with the adapter, a TNC male to TNC female, shown in Figure 3, having a VSWR of 1.15 max. up to 18 GHz, and the typical performance being even better. Similar performance is achieved with the complete MA2-Line N, TNC and SMA connectors and adapters using a Teflon dielectric.
A completely different approach is needed to achieve such good performance for the SA2-Line connector series using air as the dielectric and a bead for captivation. Figure 4 identifies the different components that make up the new Spectrum SA2-Line 2.4 mm connector.
The adapter operates to 50 GHz and exhibits a maximum VSWR of 1.25 up to 50 GHz. The outer conductor is made from two pieces that fit snuggly together and are designed with perfect alignment guaranteed. The smooth angled airline section has been milled for optimum performance and each part is completely finished when leaving the computerized numerical controlled (CNC) milling machine, with no secondary operation needed. Even the half threads are made during the primary operation, guaranteeing that the threads meet and that the male and female interface parts thread easily. The center contact assembly consists of five parts, the three-piece center contact and two beads. The smooth, bent part of the center conductor is the only part in the assembly requiring a secondary operation. After machining it is bent to an angle of exactly 90°, using special tooling. The ends are polished to the tolerance requirement outlined earlier, before the part is sent out for plating.
Figure 5 shows the S-parameter characteristics of the 2.4 mm right angle adapter. In addition to the 2.4 mm connectors, other airline adapters in the series include the 2.9 and 3.5 mm versions with a 1.85 mm adapter currently under development. The adapters are available in all sex configurations — male-tomale, female-to-female and male-tofemale for 2.4, 2.9 and 3.5 mm connector configurations. The operating characteristics of the 2.92 mm adapter are shown in Figure 6. For these airline connectors and adapters the standard bead allows an operating temperature of between –54° to +85°C.
However, as these bead captivated airline connectors are being required for higher and higher power applications and to operate in harsh environments, the company has developed a new generation of beads that can operate with no degradation up to 150°C. This bead is available for some of the connector series used in straight and right angle configuration products and will be available shortly for all the connector series from 7 to 2.4 mm. Also, the high power bead has similar electrical performance as the regular bead, in terms of insertion and return loss. Furthermore, glass beads have been developed for airline connectors and adapters for hermetically sealed applications. At present the operating frequency ranges of these hermetically sealed connectors and adapters, straight or angled, do not cover the entire possible operating frequency range of the connector series due to the higher dielectric constant of the glass currently available.
This article began with a discussion of the perfect connector. There is no such thing, of course, but with its new generation of right angle connectors and adaptors, Spectrum Elektrotechnik has developed a family of high precision, high performance components. This has been achieved by using the latest manufacturing techniques and by working to tight tolerances.