Waveguide can provide outstanding electrical performance in terms of low insertion loss and low VSWR at microwave and millimeter-wave frequencies, but it may often be necessary to transition between waveguide and coaxial components. Doing so requires the right Waveguide-to-coaxial adapter and knowledge of how to integrate it into a design without introducing un-wanted chaos. The right adapter can provide convenience as well as performance.
Waveguide-to-coaxial adapters allow power to be propagated in either direction, with each side of the adapter providing the full frequency range of its waveguide size. Although the coaxial connector side of the adapter theoretically works broadband, the frequency band is limited by the waveguide structure on the other side of the adapter.
A general procedure when connecting an adapter to a waveguide should involve making certain that the rectangular waveguide ports are oriented the same. The ports should be carefully aligned with respect to the waveguide opening in order to minimize reflections. Then the flanges should be bolted or clamped securely together to evenly distribute pressure over the contacting surfaces of the waveguide portions. A tight fit provides a good impedance match between the waveguide sections, whereas a loose fit can result in mismatches, leakage, and distortion.
When considering a waveguide-to-coaxial adapter for an application, the waveguide side of the adapter is selected to fit a given waveguide and flange size, while the coaxial side of the adapter needs to match the connector type (SMA, Type-N, etc.) of the mating component (antenna, coaxial cable, etc.) to be linked in a system. Again, although the coaxial interface is a broadband connection, the waveguide and flange size on the other end of the adapter will determine the frequency range of the adapter. (Example: a WR-75 rectangular waveguide interface has a frequency range of 10 to 15 GHz.) The overall performance of the adapter depends on the transition between the waveguide and coaxial component. How well this transition is accomplished translates to the performance parameters, such as insertion loss and VSWR. These performance parameters should be used to compare the quality of different adapters. (I.e., Low VSWR and insertion loss adapters have a very good transition design.)
Ideally, a waveguide-to-coaxial adapter should not degrade the performance of the transmission line in which it is inserted while making the link between the two different transmission-line topologies. Over its operating frequency range, a good waveguide-to-coaxial adapter will typically introduce additional insertion loss of less than 0.5 dB below 18 GHz and often typically around 0.2 dB or less for frequencies below 10 GHz.
An adapter’s voltage standing wave ratio (VSWR) specification is an indication of the impedance match or mismatch that a waveguide-to-coaxial adapter will introduce into a circuit or system. Ideally, the adapter’s VSWR should be as low as possible, so that minimal reflections occur at that point in the circuit or system with the addition of the adapter. As an example, rectangular-waveguide-to-Type N coaxial adapters machined by MDL (www.mdllab.com) exhibit maximum VSWR of 1.25:1 with pressurized versions offering a maximum VSWR of 1.10:1. These Type N standard adapters cover a total frequency range of WR-650 (1.12 to 1.70 GHz) through WR-75 (10 to 15 GHz), maintaining low VSWR.
At higher frequencies, standard rectangular waveguide to SMA coaxial adapters exhibit a maximum VSWR of 1.25:1 from 2.6 to 40.0 GHz, with some showing typical values of 1.14:1 over that frequency range. For frequencies to 40 GHz using 2.9-mm coaxial connectors, some double-ridge-waveguide-to-coaxial adapters are capable of maximum VSWRs of only 1.30:1. Again, the adapter’s goal is to provide a mechanical link between a waveguide portion of a system and a coaxial component or portion, while remaining electrically “invisible.” Adapters with a low VSWR can electrically disappear once installed.
In many applications, waveguide-to-coaxial adapter will be used as part of a test system, to introduce and detect signals for analysis to a coaxial test port from a waveguide transmission line or component. Such applications will generally be at relatively low power levels, of +10 dBm or less. But often, waveguide-to-coaxial adapters are used as part of a radar system, typically with high-power pulsed signals, and the power rating of an adapter may prove to be a specification of interest. Many commercial standard waveguide-to-coaxial adapters and double-ridge-waveguide-to-coaxial adapters, depending on the coaxial connector type and are rated for continuous-wave (CW) power levels as high as 1 kW and peak power levels to 5 kW when using rugged Type N connectors on the coaxial side.
Traditional waveguide-to-coaxial adapters mount a waveguide flange at one end and a coaxial connector on the topside of the assembly, typically at a right angle to the waveguide flange, for ease of access. But various other adapter configurations are available, including end launch adapters, which essentially mount the waveguide and coaxial connectors in a straight line, so that the adapter can be installed as part of an in-line addition, such as to run a coaxial cable from a waveguide fixture. These types of adapters are available with similar waveguide sizes and connector types as traditional waveguide-to-coaxial adapters, and offer comparable electrical performance in terms of loss and VSWR, with the added convenience (where needed) of in-line installation.
Although this blog has detailed various types of waveguide-to-coaxial adapters, it should be noted that it may at times be necessary to mount together two different waveguide components within a system, in which case, a waveguide-to-waveguide adapter (Transformer) will be needed. Such adapters are specified by two different waveguide sizes, larger and smaller sizes, and they exhibit an optimum frequency range, typically the frequency range midband between the two waveguide bands. Waveguide-to-waveguide adapters (Transformers) are also specified by the usual electrical parameters, such as insertion loss, maximum RF/microwave power, and VSWR.
Waveguide is a viable transmission-line configuration that is still very much in use in high-frequency applications from RF through millimeter-wave frequencies. The adapters described herein are just a sampling of the different products available, for waveguide-to-coaxial or waveguide-to-waveguide transformations. Practical solutions can be found throughout the MDL website at www.mdllab.com.