Millimeter-wave Interconnect Considerations
By: Peter McNeil
There has been a general trend of radio and sensing applications shifting to higher frequencies, either to avoid interference due to the growing use of RF and microwave spectrum or to take advantage of the growing accessibility of millimeter-wave hardware. This is true for consumer, industrial, military, aerospace, and satellite communications applications alike. Regardless of the cause, the result is an increased need for millimeter-wave interconnect, including coaxial cable assemblies, connectors, and adapters, as well as waveguide interconnect. Also, the relatively small size of millimeter-wave hardware compared to RF and even microwave hardware mean that there are often times a need for even more compact interconnect solutions, such as probes and planar transmission lines (microstrip, stripline, coplanar waveguides, slotlines, etc).
Unlike at lower frequencies, there are several phenomena that come into play at millimeter-waves that are often ignored or are only a minor consideration. Some of these phenomena are skin effect, undesired transmission modes, and higher RF losses from dielectrics/conductors. Compounding this are the issues that transmission lines and interconnects are generally geometrically proportional to the frequencies at which they operate. For instance, larger coaxial connectors have a maximum cut-off frequency that is a function of the spacing and dimensions of the inner and outer conductor. This cut-off frequency doesn’t necessarily mean that coaxial connectors are not carrying the higher frequency signal energy, it just means they are no longer operating according to specification (due to the other transmission modes produced by higher frequency signal components).
Other considerations are power handling and breakdown voltage. As millimeter-wave interconnects are subject to higher RF losses and smaller conductors and geometries are often used, the power handling and breakdown voltage for these interconnects is usually lower than interconnects designed for lower frequencies. This can be a compounding issue, as the higher RF losses mean that less power can be injected into a system and more power is lost over the length of an interconnect, which results in a proportionally lower effective maximum transmission range.
This is why waveguides are often used as the choice interconnect for many millimeter-wave applications. Waveguide interconnects generally have higher power handling and lower loss at the same frequencies as coaxial or some planar transmission lines. However, as surface finish becomes a significant component of RF losses at millimeter-wave frequencies it is important to properly machine or coat conductive surfaces. Hence, for millimeter-wave interconnects used in critical or sensitive applications, the conductors are often coated with precious metals, especially gold due to its corrosion resistance and superior conductivity. High RF losses in millimeter-wave interconnects also come from dielectrics, which makes dielectric choice an extremely important aspect of millimeter-wave design.
Another reason that millimeter-wave applications are often designed with waveguide interconnect or specialized cable assemblies is to address the challenge of an operator making the physical connection. With RF and microwave coaxial, for instance, an operator with some skill and experience can make good RF connections without special tooling. However, at millimeter-wave frequencies the size and tolerance of the connectors and assemblies often dictate the need for precision machine assembly. Even connecting millimeter-wave coaxial connectors together may be challenging for some, which is why precision indexed cable assemblies are becoming increasingly common. Another reason for the growing use of these specialized cable assemblies is to be able to accommodate many more RF interconnections in the same or smaller connector footprint.
Learn more about Pasternack’s extensive line of RF, microwave, and millimeter-wave coaxial and waveguide interconnect here:
Pasternack Coaxial Cable Assemblies
Pasternack Coaxial Adapters
Pasternack Coaxial Cables
Pasternack Coaxial Connectors
Pasternack Coaxial Connector Assemblies
Pasternack RF Probes
Pasternack Coaxial End-launch Connectors
Courtesy of Pasternack