When and where are Flexible Waveguides Useful?

Feb. 19, 2019

Waveguide interconnect and assemblies are used in a wide variety of microwave and millimeter-wave applications. These applications include military, aerospace, Satcom, radar, microwave/millimeter-wave imaging, industrial heating/cooking, and more. In several of the applications and specific cases, a rigid waveguide assembly, or interconnect routing, must be done in a geometry which would require a waveguide structure that is too expensive, complex, or too rigid to meet successful design criteria.

Such a scenario could occur because the geometry requires very irregular bends, difficult to reliably make or too expensive to make with rigid waveguide. Another case is that it may be desirable to provide some mechanical isolation between assemblies or structures. For these reasons, flexible waveguide interconnect was invented and is used in a wide variety of application. Though helpful in many circumstances, flexible waveguide also has its limitations, and a designer must be mindful of the tradeoffs when finalizing a waveguide routing and a waveguide assembly.

How are Flexible Waveguide Different from Rigid Waveguide?

Unlike rigid waveguide, which are constructed of solid structures and welded/brazed metal, flexible waveguide are made using tightly interlocking sections of folded metal. Some flexible waveguide are augmented with soldered to seal the seams within the interlocking metal sections of a flexible waveguide. What these interlocking sections allow for, is some minor flexure at each joint. Hence, a longer piece of flexible waveguide can flex more than a similarly constructed shorter piece. The interlocking sections are also designed to maintain as strait a waveguide channel within the waveguide as possible.

There are variants of flexible waveguide that allow for flexion at the widest wall, some that allow flexing at the short wall, and some that allow for flexure at both walls. Other variants, called “twist” waveguide, allow for the waveguide to rotate along its length. There are also waveguide that are capable of combinations of these functions.

With rigid waveguide, a designer must either base their design on available waveguide sections, or commission a custom rigid waveguide part. Flexible waveguide, however, can be purchased in standard lengths and bent/flexed for mating. For greater structural performance, some flex waveguide are manufactured with a sturdy outer jacket, and the waveguide can be “pre-formed” to the desired shape.

When/Where are Flexible Waveguide Used?

Flexible waveguide are used in applications where rigid waveguide would be prohibitively complex, expensive, or exceeds a necessary production schedule. Occasionally, redesigns are necessary, and rigid waveguide sections are replaced with readily available flexible waveguide to account for varying design changes. Prototypes are often assembled with flexible waveguide as proof-of-concepts prior to finalized design geometries.

Certain applications actually require flexible sections, as a flexible waveguide will not transfer as much mechanical energy along its structure, than would rigid waveguide. For example, if there is a junction whose relative position changes dramatically under varying environmental conditions– such as temperature variations, humidity, or under load–then a flexible waveguide section could be used to provide the extra “slack” between the shifting joints. Also, some flexible waveguide could also provide some shock and vibration isolation, though such use may also reduce the lifespan of the flexible waveguide.

When is it a bad idea to use Flexible Waveguide?

Flexible waveguide are typically less rigid and less physically robust than rigid waveguide structures. Where some rigid waveguide can be used to also provide mechanical support, a flexible waveguide could be damaged and its electrical performance could be compromised if put under any significant mechanical strain or load. Excessive vibration and shock will also lead to mechanical and electrical failure of a flexible waveguide. Flexible waveguide are also not typically designed for repeated flexure, which could wear down the joints, damage the jacketing, and lead to early failure. The thinner metal, contact resistance between the sections, and non-ideal inner waveguide surface of flexible waveguide also lead to reduced electrical performance compared to some rigid waveguide. Hence, flexible waveguide typically have slightly poorer transmission characteristics and lower power handling than rigid waveguide. Flexible waveguide may also not have the same temperature range of operation as rigid waveguide, as the jacketing material and joint solder may be a critical component of the flexible waveguide structures performance, and these materials likely won’t have the same temperature performance range as rigid metal.

If a flexible waveguide isn’t jacketed or otherwise sealed, humidity and other environmental contaminants can enter the waveguide structure through tiny gaps in the joints. Though purging and desiccants can be used to minimize the amount of humidity inside a flexible waveguide, high humidity or environmentally contaminated environments can compromise the performance of flexible waveguide over time.

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