Waveguide-to-coax adapters are a necessary component in the design and development of the ever-increasing range of mmWave systems. For W-Band testing and prototyping, Fairview Microwave recently released a gold plated, male waveguide-to-coax adapter, the SMW10AC001-VM. The adapter combines a WR10 waveguide with UG-387/U-Mod round flanges and a 1 mm precision coaxial connector, creating a transition with a typical VSWR of 1.4:1. This W-Band transition is designed to serve high performance applications.
WAVEGUIDE APPLICATIONS
An increasing number of W-Band systems are under development and being fielded: automotive radar, point-to-point radio links, satellite services, instrument landing systems, imaging radar for security. Most require antenna and over-the-air characterization in anechoic chambers to analyze and assess performance. As shown in Figure 1, connecting from the antenna or system being tested to test equipment such as vector network analyzers or spectrum analyzers requires waveguide “plumbing” with coaxial interconnects.
Figure 1 Test setup for mmWave antenna characterization, showing waveguide-to-coax adapters.
The advent of highly integrated MMIC modules is enabling many of these applications, which take advantage of W-Band to achieve small IC, module and aperture form factors. Many systems maximize the output power to extend range and low loss waveguide is the ideal interconnect to achieve this in the antenna feed. Waveguide is also the preferred transmission line for high-power applications, as coaxial assemblies can overheat when carrying continuous high-power levels. Coaxial cables, however, are more flexible and reduce the need for waveguide twists and bends for routing. This helps with runs through tight spaces. Also, waveguide runs introduce risks from misalignment, discontinuities and the excitation of higher-order modes, which degrade the signal. Practically, both waveguide and coaxial transmission lines are needed, requiring transitions between the two.
ADAPTER CONSTRUCTION
The function of a waveguide-to-coax adapter is to convert the TE mode of the waveguide to the TEM mode of the coax. This is typically accomplished with a right-angled connection, where the center pin of the coaxial connector is extended into the broad wall of the waveguide. The cross-sectional view facing the iris of the waveguide reveals a probe tip entering normal to the H-plane of the waveguide. The probe is positioned so that the reflected waves from the back wall of the waveguide are in phase with the incoming fields.
An in-line waveguide-to-coax adapter typically consists of a shorting metallic elbow meeting with the coaxial probe. The short circuit forms a time-varying magnetic field that couples the signal into the waveguide. Typically, the in-line transition has a higher power handling capability but presents a higher VSWR than right-angled waveguide-to-coax adapters.
Figure 2 Measured VSWR and |S21| of the waveguide-to-coax adapter.
PERFORMANCE
The SMW10AC001-VM is a right-angled design with a typical insertion loss of 1 dB, with a maximum of 2 dB, and a typical VSWR of 1.4:1, with a maximum of 1.9:1, from 75 to 110 GHz (see Figure 2). The transition is a precision 1 mm male coaxial interface constructed with stainless steel. The WR10 waveguide comprises UG-387/U-Mod flanges constructed in aluminum and gold-plated for durability and repeatable performance.
The losses through waveguide depend on the conductivity of the final plating material on the inside walls. At mmWave frequencies, gold plating is often needed on the inner walls because of the skin effect, i.e., where electrons migrate to the surface of the inside walls. Materials with greater skin depth experience less loss at higher frequencies, driving the choice of aluminum for the waveguide with a gold finish.
The SMW10AC001-VM is an addition to Fairview Microwave’s broad portfolio of precision coaxial and waveguide products and assemblies, including a female version of the W-Band adapter, SMW10AC001-VF. With waveguide-to-coax adapters available from K- to W-Band, Fairview Microwave offers transitions for a wide array of military, commercial and industrial platforms.
Fairview Microwave
Lewisville, Texas
www.fairviewmicrowave.com