Corralling Coaxial Couplers and Adapters

Corralling Coaxial Couplers and Adapters

Adapters and couplers are passive components that help channel signals through high-frequency systems.

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Microwave/RF adapters and couplers are designed to transfer high-frequency signals between components as invisibly as possible. Adapters may be needed because some components are terminated with different types of coaxial connectors. In some cases, it may be necessary to transfer signals from components with waveguide terminations to components with coaxial fittings.

In all cases, the goal of an adapter is to transfer signals from one component to another with as little electrical “presence” as possible—that is, with low insertion loss, low return loss or voltage standing wave ratio (VSWR), and even low passive intermodulation (PIM) distortion. Couplers provide a somewhat different function: tapping into a single line and sampling a small-level version of the signal for testing or other purposes. In both cases, adapters and couplers can be specified by mechanical as well as by electrical parameters, with reliability one of the characteristics that should never be overlooked.

Coaxial adapters are specified according to the coaxial connectors they fit and, in the case of waveguide-to-coaxial adapters, the waveguide and connector sizes they fit. Because many coaxial connectors are defined by gender, such as male or female, some adaptors provide gender-changing connections between connectors of a similar product type. Examples include SMA adapters (DC to 18 GHz) or 3.5-mm-connector adapters (DC to 26.5 GHz) with female connector on one end and male connector on the other end.

Because waveguide impedances are typically much higher than the typical 50-Ω characteristic impedance of a high-frequency coaxial connector, transitions from waveguide to coaxial connectors can occur in different ways. They can happen via what are known as resistively matched transitions (where the waveguide dimensions are shaped to make the impedances of the waveguide and coaxial interface equal at their interface) and mode-matched transitions (where smooth transitions are made from the high impedance of a waveguide termination to the 50-Ω impedance of the connector interface).

Adapters with waveguide flanges may be specified in terms of EIA rectangular waveguide (WR) size, such as WR90 (8.20 to 12.40 GHz) or WR62 (12.40 to 18.00 GHz)—or their equivalent flange sizes, such as UG135/U and UG419/U, respectively—and the coaxial connector (e.g., an SMA connector) at the other end. For any waveguide-to-coaxial adapter, care should be taken to protect the face of the waveguide flange from damage or debris, which could prevent close surface-to-surface mounting between the two waveguide flanges.

In terms of performance, different adapters are compared by their frequency ranges, insertion loss, return loss or VSWR, and maximum power-handling capability. The frequency range of an adapter is very much a function of the physical dimensions of its terminations, such as its coaxial connectors and/or connectors and waveguide flanges. The quality and the performance of the coaxial connectors in an adapter depend highly on the materials used for the connectors (e.g., stainless steel, aluminum, or gold-plated brass).

For rectangular waveguide flanges, the size numbers denote the usable frequency ranges. Small numbers represent higher frequencies, like WR12 for 60 to 90 GHz, WR10 for 75 to 110 GHz, and WR8 for 90 to 140 GHz. Coaxial connectors with smaller dimensions provide higher-frequency operating ranges, with some common high-frequency connectors defined by dimensions of 7 mm (DC to 18 GHz), 3.5 mm (DC to 34 GHz), 2.92 mm (DC to 40 GHz), 2.4 mm (DC to 50 GHz), and 1.85 mm (DC to 65 GHz).

The power-handling capability of an adapter, which is related to the operating frequency, is usually limited by the power-handling capability of the coaxial connectors (and the power-handling capability of the connectors’ center conductors), since the waveguide ports can typically handle higher power levels than the coaxial ports. The insertion loss and return loss (VSWR) of an adapter can be affected by the quality of the mechanical interfaces.

Pasternack's adapters
1. These adapters operate from DC to 18 GHz and connect components with ZMA connectors to those with SMA connectors. (Photo courtesy of Pasternack Industries)

For adapters with a coaxial connector, a decrease in the outer-conductor diameter results in an increase in the connector’s usable upper-frequency limit. A connector’s performance can also be impacted by the addition of different dielectric material materials in the air spaces around the connector’s conductors and the quality of the interface for the connector’s mated pair. Variations in the mechanical dimensions of the conductors, for example, can result in variations in electrical performance from one set of adapters to the next.

For critical applications, such as in measurement systems with vector network analyzers (VNAs) requiring phase and amplitude stability and repeatability, coaxial adapters are often manufactured to MIL-STD-348 test-grade requirements to ensure stable phase-matched performance. In addition, applications such as in test systems may call for adapters/connectors to be specified in terms of the number of expected connects/disconnects and their relationship to the operating lifetime of the adapters.

Coaxial adapters may sometimes be used to mate a lesser-known connector interface to a more commonly known connector. One example is the recently released line of ZMA-to-SMA coaxial adapters from Pasternack Industries, which are suited for commercial, industrial, and military applications from DC to 18 GHz (Fig. 1). To simplify integration in systems, they are supplied in three- and four-lug keyed attachment designs. The ZMA side of the adapter features a bayonet-style coupling nut that is similar to a BNC connector, allowing connections by hand without need of a torque wrench.

The adapters are available in a number of different configurations, including SMA female to ZMA male, SMA female, to ZMA female, and SMA male to ZMA male three- and four-lug styles. The RoHS-compliant adapters are constructed with passivated stainless-steel bodies and gold-played beryllium-copper (BeCu) contacts to minimize insertion loss and return loss.

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Focusing On PIM

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A recent trend in characterizing coaxial and waveguide adapters stresses the importance of PIM performance, a parameter not included in product comparisons just a few years ago. Because of the impact that high PIM levels can have on wireless communications systems, such as degradation of bit error rate (BER), the PIM levels of newer coaxial and waveguide adapters have come under closer scrutiny in recent years. Many suppliers now offer components with low PIM levels of -165 dab or better for coaxial-to-coaxial adapters.

Although there is some debate within the industry on the preferred composition of connector materials for optimum PIM performance, including silver-plated brass, some manufacturers recommend the use of white bronze to achieve minimal PIM levels. White bronze is actually a blend of copper, tin, and zinc, combined to form a smooth, stainless-steel-like finish. In all cases, the finish as well as the composition of the materials for low-PIM adapters should be carefully engineered to achieve the lowest PIM levels within high-frequency coaxial connectors.

As an example of a low-PIM coaxial adapter, model ANN-NM-M03 from MECA mates Type-N male to Type-N male connectors from DC to 12.4 GHz with better than -165 dBc typical PIM performance. The adapter, with 1.60 in. length and 0.82 in. diameter, is formed with nickel adapters and silver-plated brass connector pins and cables.

Couplers for RF/microwave applications come in many shapes and sizes, including with waveguide and coaxial terminations. Couplers can be designed in various configurations, including as quadrature hybrid couplers, where input signals are split into two equal-amplitude (3-dB split) output signals offset by 90 deg., and as stripline directional couplers, where a small amount of an input signal is available at a coupled port for analysis or testing, with the remainder of the input signal passed to an output port for normal system use.

Krytar's couplers
2. Model 104020030 is a 30-dB directional coupler with low loss from 4 to 20 GHz. (Photo courtesy of Krytar, Inc.)

Directional couplers usually have four ports: input, output, coupled, and isolated. The coupled port contains a portion of the power applied to the input port, with most of the input port power appearing at the output port, while the isolated port contains an amount of power typically symmetrical to the amount of power at the coupled port. This isolated port is usually terminated in a matched load.

As an example, model 104020030 is a coaxial directional coupler from Krytar, Inc. with a 30-dB coupled port (Fig. 2). This stripline coupler is designed for use from 4 to 20 GHz, with maximum VSWR of 1.35:1 and less than 0.60 dB insertion loss across the full frequency range. It can handle 20 W average input power and 3 kW peak (short pulses) input power at temperatures from -54 to +85°C. The coupling remains within ±1 dB across the frequency range in a package that is only 1.40 × 0.40 × 0.66 in. and weighs 1 oz. It is supplied with SMA female connectors (SMA male connectors as an option).

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