RF Adapters Gain Bandwidth While Lowering Return Loss

Oct. 5, 2009
From laboratory test setups to the transmitting equipment connected to base-station antennas, coaxial and waveguide adapters have been guiding RF and microwave signals for a long time. These adapters have increased flexibility by ...

From laboratory test setups to the transmitting equipment connected to base-station antennas, coaxial and waveguide adapters have been guiding RF and microwave signals for a long time. These adapters have increased flexibility by establishing connections between different or dissimilar connectors. At the same time, they have steadily continued to make electrical and mechanical progress in order to meet new performance goals set by modern and emerging applications. Suppliers have been able to continuously augment adapter performance by using newer materials, improved manufacturing methods and plating techniques, precision assembly processes, and clever impedance transformer designs.

Traditionally, waveguide-to-coaxial adapters have been a right-angle solution for applications requiring transition. In many situations, however, it is desirable to have connectors inline with the waveguide. By employing the latest RF techniques coupled with precision assembly methods, RLC Electronics has developed in-line adapters also popularly known as end-launch adapters. According to RLC's Director of Marketing, Peter Jeffery, the only advantage here is mechanical and there is also a disadvantage: very narrow bandwidth. Further details on this customer-specific solution were unavailable.

Numerous firms have taken more standard approaches (see Table). Space Machine & Engineering Corp., for instance, has readied a standard series of end-launch waveguide-to-coaxial adapters that incorporate its proprietary matching structure into the waveguide. To obtain broader bandwidth, the company has developed double-ridge waveguide-to-coaxial endlaunch adapters covering sizes WRD750 through WRD200 (Fig. 1). The adapters boast a maximum voltage standing wave ratio (VSWR) of 1.5:1. Doubleridge waveguides for end-launch-style adapters also have been developed by AR RF/Microwave Instrumentation, Cobham Defense Electronic Systems, and QuinStar Technology. Both AR and QuinStar also offer these adapters with rectangular waveguides using a variety of coaxial connectors.

Among the additional proponents of end-launch-style waveguideto- coaxial adapters are Advanced Technical Materials, A-INFO, Flann Microwave, Maury Microwave, Microwave Engineering Corp., and Unique Broadband Systems. Aside from achieving a low profile with short length and low loss and VSWR, Microwave Engineering's design permits its end-launch adapters to operate over multi-octave bandwidths at high power levels.

Developers of end-launch adapters also provide right-angle models. RLC, for example, has been making standard waveguide-to-coaxial adapters in a variety of configurations for applications in the 3.3-to-40-GHz range with options for a broadband or band-specific model. Broadband waveguide-to-coaxial adapters maintain superior electrical specifications over the entire bandwidth. In contrast, band-specific models offer enhanced electrical performance for a specified bandwidth around the center frequency.

The firm's WAD series comprises 50-Ω coaxial-connector types including N, SMA, and K male or female. The adapters' average power-handling capability is 300 W for N, 60 W for SMA, and 25 W for K-type connectors with the waveguide flange as standard. Although insertion loss ranges to just 0.05 dB between 3.3 and 8.2 GHz, it begins to climb as the frequency goes higher. Hence, insertion loss rises to 0.1 dB between 10 and 18 GHz and climbs to 0.15 dB as frequency scales beyond 18 GHz. Likewise, VSWR is 1.2:1 between 3.3 and 8.2 GHz, but deteriorates to 1.35:1 with frequency ascending to 18 GHz and beyond.

With a narrow bandwidth, however, return loss is slightly better. For instance, the VSWR for broadband unit WR112-to-N/SMA, which covers 7.05 to 10.00 GHz, is 1.2:1. By selecting only a 1.47-GHz bandwidth around a center frequency in the range of 7.05 to 10.00 GHz, the VSWR for the WR112-to-N/ SMA improves to 1.1:1. The insertion loss of 0.07 dB remains the same for both models.

Many more companies in this marketplace offer standard waveguide-to-coaxial adapter offerings. Further additions to this list include ARRA, Inc., Cernex, Inc., JQL Electronics, Narda Microwave, Penn Engineering Components, and Vector Telecom.

On another front, RF-coaxialconnector maker San-tron, Inc. has launched Type-N T connectors, dubbed the UG-28A/U and UG-107B/U, utilizing a unique counterbore technique. Essentially, it has replaced the miter-cut joining technique with a counterbore joining to save time and money in the manufacturing process. The Type-N T adapters are rated for operating voltages of 1000 V RMS. At sea level, they have a maximum dielectric withstanding voltage of 2500 V RMS at 60 Hz. The T adapters come in both male and female versions. They operate from -65 to +165C. For better performance, the T adapters are manufactured with polytetrafluoroethylene (PTFE) dielectric material. They feature gold and silver center contacts. According to the supplier, the connectors are available in albaloy, nickel, and silver body finishes.

The manufacturer also has introduced right-angle solder-free adapters that can handle frequencies to 11 GHz from -65 to 165C. The 50-Ω UG-27 C/U adapters are rated for operating voltages to 1000 V RMS with a maximum dielectric withstanding voltage of 2000 V RMS at 60 Hz at sea level. They flaunt a VSWR of 1.15:1 from DC to 6 GHz and 1.35:1 from 6 to 11 GHz, respectively. Featuring silver and gold center contact plating, the adapters come with albaloy, nickel, and silver body plating.

For their relatively small size and good electrical performance, SMA connectors are commonly found in wireless systems, military/aerospace equipment, test and measurement setups, and Global Positioning System (GPS) antennas. Because these connectors use threaded coupling, they require operator time especially in test environments. It takes time to make the threaded connection and then torque the coupling prior to test. To save time and simplify the testing process, Molex has developed two versions of SMA jack to SMA slide on the plug adapter. While a floating-panel-mount version targets test fixtures (73251-2130), the knurled-body version is designed for use on the end of test cables for production testing (73251-2380).

This adapter mates with standard SMA as per MIL-STD-348A. To create constant ground, it uses a berylliumcopper (BeCu) spring on the SMA push-on side. According to the company, this 50-Ω adapter boasts a maximum VSWR of 1.25:1 to 18 GHz. Its body is stainless-steel passivated while the center contact is gold plated.

To ease interconnections in system applications, Response Microwave has launched a new line of coaxial adapters in frequency ranges from DC through 50 GHz with impedances of both 50 and 75 Ω. The 75-Ω BNC , 1.0/2.3, and 1.6/5.6 in-series adapters are specifically tailored for telecommunications and networking infrastructure, explains Peter A. Alfano, the company's Director of Business Development. Alfano points out that the in-series and between-series adapters offer popular interfaces like SMA, SSMA, SMB, 2.4 mm, 2.9 mm, 3.9 mm, SMP, N, 7/16, BNC, TNC, MCX, MMCX, 1.0/2.3, and 1.5/5.6. Plus, there are coupling options like thread-on, push-on, and quick disconnect (Fig. 2). These adapters also come in a variety of configurations, such as in-line, right angle, T, and U-link. They are available in both stainless-steel and brass housings with silver, gold, nickel, or ternary plating options.

With the proliferation of WiFi and broadband infrastructure for telecommunications and high-speed data communications, a tremendous need has arisen for a variety of connectors to test cables in the field. RF Connectors' Vice President of Marketing, Manny Gutsche, points out that the unavailability of any unique interconnection in the field can pose a problem and delay testing. To simplify this task, RF Connectors a division of RF Industrieshas crafted a universal adapter kit labeled RFA-4028-WIFI.

This kit comprises the Unidapt RF cable tester with an assortment of 30 universal adapters, which include male and female MMCX, N, reverse-polarity (RP) TNC, RP SMA, TNC, BNC, and SMA connector interfaces. By screwing any two interface adapters in this kit to a universal center, Gutsche says that scores of different adapters can be made in seconds. All adapters feature machined brass, silver-plated bodies, gold-plated contacts, and Teflon insulation. They also are sold separately. Without the tester, the universal adapter kit is labeled RFA-4024-WIFI (Fig. 3).

Other suppliers offering such universal kits include Bomar Interconnect Products and MegaPhase. To address the needs of technicians and engineers in the broadcast field, Bomar has readied a 42-piece adapter kit, called ADPT4RP, that contains the parts most often needed by technicians in on-site antenna installations. The product consists of two male and two female Type N, BNC, UHF, TNC, TNC reverse-thread (RT), TNC-RP, SMA, and SMA-RP 50-Ω coaxial parts as well as eight universal adapters and two flat wrenches. These parts are fabricated using precision-machined brass with corrosion-resistant gold bodies, Teflon insulators, and gold-plated contacts. For its part, MegaPhase's universal adapter kit includes tools to properly terminate three different-length cable sub-assemblies with various connector combinations.

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