A twistable flexible waveguide accommodates a specified amount of rotation between its ends; a non-twistable flexible waveguide accommodates bending and limited extension or compression, but has no torsion allowance. Choose twistable when the route requires axial rotation. When it needs only bending, compare bend radius, loss, power and pressure ratings before choosing either type. Matching the waveguide size does not make the assemblies interchangeable. [1]
At 8.2 to 12.4 GHz, the military Class 1 twistable specification lists 95 degrees per foot of axial twist and a 3.38-inch minimum E-plane bend radius. Class 2 non-twistable lists a tighter 1.38-inch radius, while both list 0.1 dB/ft insertion loss. Class 3 extra-flexible twistable combines a 1.38-inch E-plane radius with 240 deg/ft of axial twist, but lists 0.13 dB/ft loss. These are requirements from the cited specification editions, not universal ratings for commercial products. [2] [3] [4]
Table of Contents
What the specifications actually compare
| Property | Class 1 twistable | Class 2 non-twistable | Class 3 extra-flexible twistable |
|---|---|---|---|
| Axial twist | 95 deg/ft | Not rated | 240 deg/ft |
| Minimum centreline bend radius, E / H plane | 3.38 / 7.51 in | 1.38 / 3.00 in | 1.38 / 2.25 in |
| Repeated twist at 400 cycles | 47.5 deg/ft | Not rated | 120 deg/ft |
| Repeated twist at the longer test duration | 14.25 deg/ft at 105 cycles | Not rated | 36 deg/ft at 106 cycles |
| Flexure allowance, E / H plane, at 105 cycles | 27.5 / 12.3 deg/ft | 114.50 / 57.25 deg/ft | 34.25 / 17.12 deg/ft |
| Insertion loss limit | 0.1 dB/ft | 0.1 dB/ft | 0.13 dB/ft |
| Pressurization | 30 psi gauge | 60 psi gauge | 60 psi gauge |
The flexure figures are angular allowances per unit length at a stated cycle count, not lifetime predictions. Footnote 4 on each sheet assigns the listed beryllium-copper variants a flexure cycle count of 106 instead of 105. Repeated twist has its own cycle counts, shown separately above. Compare movement amplitude, bending plane, material and cycles together. [2] [3] [4]
For these X-band rows, Class 2 permits a larger flexure angle at the stated cycle count than Class 1 or Class 3. That does not establish how long any of them will last on a different route. Likewise, 60 psi gauge is approximately 414 kPa gauge, while 30 psi is approximately 207 kPa, using 1 psi = 6.89476 kPa. The pressure requirement applies to the assembly. [2] [3] [4]
The word twist also appears in rigid-waveguide specifications with a different meaning. MIL-DTL-85H limits unwanted longitudinal twist to 1 degree per foot: a manufacturing tolerance, not permission to rotate a rigid section during installation. [5]
How construction affects movement
The historical MIL-HDBK-216 describes an interlocked construction made by spirally winding and interlocking a formed metal strip. Its turns slide relative to one another as the guide bends, extends, compresses or twists. The handbook describes conventional seamless corrugated tubing as able to bend and change length, but unable to withstand twisting. These are construction descriptions, not proof that every commercial part follows the same design. [6]
“This type of waveguide flexes by virtue of a sliding motion that takes place between convolutions as it is stretched, compressed, bent, or twisted.” [6]
Seamless does not invariably mean non-twistable. US Patent 4,710,736 describes seamless walls with corrugations inclined at about 45 degrees to the transverse planes, allowing both bending and twisting. The patent supports that design mechanism; it does not establish the construction or performance of an unrelated product. Specify the required movement and use the selected assembly’s ratings rather than inferring them from its appearance. [7]
Loss, power and contact behaviour
Insertion loss depends on the band
| Frequency range | Class 1 | Class 2 | Class 3 |
|---|---|---|---|
| 1.12–1.70 GHz | 0.005 | 0.01 | 0.025 |
| 8.2–12.4 GHz | 0.1 | 0.1 | 0.13 |
| 12.4–18.0 GHz | 0.25 | 0.25 | 0.30 |
| 18.0–26.5 GHz | 0.35 | 0.5 | 0.35 |
Neither label guarantees lower loss. Class 1 and Class 2 have equal limits in two bands above, but differ in the other two. For a 300 mm assembly, 0.1 dB/ft gives a length-scaled limit of about 0.098 dB: 300 / 304.8 × 0.1. This is a calculation from the tabulated limit, not a prediction of a particular unit’s measured loss. [2] [3] [4]
Corrugation geometry matters as well. MIL-HDBK-216 describes increased current-path length and higher attenuation in flexible constructions than in rigid guide. NASA’s study of cut-off corrugated surfaces examines how corrugation density and shape affect power loss; it does not provide a universal twistable-versus-non-twistable loss ratio. [6] [8] On an assembled route, flange alignment and mating surfaces also deserve attention when reducing insertion loss.
Power and PIM need their own requirements
The three class sheets do not tabulate RF power. MIL-DTL-287J specifies a power-handling test when required: apply the specified RF power at the specified frequency, simulated altitude and temperature. Maintain it for one hour after thermal equilibrium, defined as no more than 5 °C variation within a 15-minute period, then measure insertion loss and VSWR. A catalogue power figure needs its operating conditions before it can be used as an acceptance limit. [1]
A sliding-contact construction introduces contact behaviour that should be assessed in a low-passive-intermodulation (PIM) system. Research on flexible metallic contacts identifies compression, gaps, tilt and DC resistance as relevant test variables. It does not rank the three waveguide classes or establish that all twistable guides have sliding joints. [9]
Both types also have flange connections. Waveguide-flange studies link PIM to surface topography, screw torque, oxide or contaminant layers and mechanical load. A non-twistable label alone therefore does not establish low-PIM performance; request an assembly-level requirement and test under the intended mechanical conditions. [10] [11] [12]
Choosing for the installed route
| Requirement | Candidate | Decisive check |
|---|---|---|
| Bending without axial rotation | Non-twistable, or a suitable twistable assembly | E- and H-plane bend radii, loss, match, power and pressure |
| Bending plus axial rotation | Twistable; consider Class 3 where a tighter bend is needed | Permitted combined bend and twist for the actual route |
| Repeated movement | Either type for bending only; twistable if torsion is required | Repeated-motion amplitude, radius, cycles and mounting |
| Continuous rotation | Rotary joint | Rotation duty and the joint’s RF limits |
| Pressurized service | Any assembly with the required pressure capability | Complete pressure boundary, seals and leak-test conditions |
| High power or low PIM | A design qualified for the stated requirement | Power conditions and PIM measurements, not the class name |
For a fixed directional change, rigid waveguide bends may remove the need for a flexible section. A pressure window can separate pressure regions, but it does not raise the pressure rating of an adjacent flexible assembly. Check each component against the pressure it will actually experience.
A worked bend-and-twist screening example
Assume a 300 mm section operating at 8.2–12.4 GHz, one 90-degree E-plane bend and 15 degrees of additional axial orientation mismatch after the route’s change of direction is accounted for. This is a hypothetical layout calculation using the military Class 1 values, not a tested installation.
At a centreline radius of 3.38 in, or 85.852 mm, the quarter-circle bend occupies π × 85.852 / 2 ≈ 134.9 mm. Subtracting that from 300 mm leaves about 165.1 mm. If all that remainder were available to distribute twist uniformly, multiplying 95 deg/ft by 165.1 / 304.8 gives about 51.5 degrees. This simplified split ignores flange-end restrictions, clamps and combined loading. [2]
The calculation does not approve the 15-degree installation. The axial-twist test in MIL-DTL-287J uses a rigid shaft to prevent bending. Its separate bend and twist limits do not establish a combined-load envelope. Ask the manufacturer to confirm the installed shape, usable flexible length and simultaneous movement. Class 2 offers no torsion allowance for this layout; a fixed rigid waveguide twist or revised flange orientation could remove the torsion requirement. [1]
A static fit can fail the repeated-motion check
Dolph’s twistable waveguide datasheet lists WR90 at 315 deg/m static twist and 76 deg/m repeated twist. Its minimum E-plane bend radii are 66 mm static and 264 mm repeated. These are that document’s product values, separate from the military figures above.
Consider two separate screening checks for a hypothetical 300 mm section. First, 30 degrees of uniformly distributed twist requires 30 / 0.3 = 100 deg/m: below the static figure, but above the repeated figure. Second, a 100 mm E-plane radius exceeds the static minimum of 66 mm but falls below the repeated minimum of 264 mm. A layout can therefore satisfy the static checks and fail both repeated-motion checks. Neither calculation establishes combined bend-and-twist performance, and the datasheet does not state a cycle count for its “repeated” values.
What the published Dolph data shows
A comparison of the published documents on 25 October 2026 found differences that should be resolved before specifying a part. The twistable catalogue and non-twistable catalogue use DH-series model identifiers; the twistable PDF identifies rows by R, WR and WG designations. A shared nominal waveguide size does not establish identical revision, construction or acceptance conditions.
| Family and size | Parameter | Web catalogue | Linked PDF |
|---|---|---|---|
| Twistable WR187 | Insertion loss | 0.24 dB/m | 0.16 dB/m |
| Twistable WR62 | Insertion loss | 0.8 dB/m | 0.65 dB/m |
| Twistable WR22 | Insertion loss | 2.7 dB/m | 2.5 dB/m |
| Twistable WR90 | Twist | 308 deg/m maximum torsion | 315 deg/m static |
| Non-twistable WR90 | Insertion loss / average power | 0.4 dB/m / 3 kW | 0.45 dB/m / 2.5 kW |
The twistable PDF and non-twistable PDF supply the PDF values above. The non-twistable WR90 PDF also lists 8.20–12.40 GHz, VSWR 1.1 and bend radii of 66 mm E-plane and 120 mm H-plane. The web catalogue lists its band as 8.2–12.5 GHz. Do not combine the widest band, lowest loss and highest power from different documents into one assumed product specification.
For procurement, request one revision-controlled drawing or datasheet for the exact part number, length, flanges and duty. Resolve conflicting values in that document. A military class sheet becomes a contractual requirement when the applicable specification and revision are invoked; merely naming a class does not demonstrate that a commercial assembly meets it.
Check the length as well as the type
The twistable WR90 PDF lists 0.4 dB/m insertion loss. Scaling that figure gives 0.12 dB at 300 mm and 0.72 dB at 1800 mm. Its return-loss table lists 30.2 dB at 300 mm, 22.1 dB at 1500 mm and 19.7 dB at 1800 mm. These are published entries, not measurements made for this article or a universal law for all flexible waveguides.
As a separate hypothetical requirement, a 1500 mm section with a 0.5 dB loss budget would fail a screening calculation based on 0.4 dB/m: 1.5 × 0.4 = 0.6 dB. An 1800 mm section would also fail a requirement of at least 20 dB return loss if its acceptance value were the published 19.7 dB. Extra length can help distribute movement, but loss, match and mechanical limits must all pass.
Double-ridged products require their own comparison. Dolph’s double-ridged flexible waveguide page lists WRD580, WRD650, WRD750 and WRD180 models and a summary length of up to 600 mm. Do not transfer WR90 radii or twist allowances to those assemblies.
What to put on the drawing
- RF interface: operating band, waveguide size, flange designation and orientation at each end. MIL-DTL-3922 addresses waveguide flanges. Match the aperture, bolt pattern, alignment features and seal arrangement, not just the nominal size. [13] The EIA and IEC flange designations and flange-interface details help identify the required connection.
- Installed geometry: relaxed assembly length, available flexible length, E- and H-plane radii, axial rotation and permitted extension or compression. The flexible waveguide selection guide provides the size, frequency and bend-radius context.
- Movement duty: static installation or repeated service motion, angular excursion, cycles, movement rate and whether bending and twisting occur together.
- Electrical acceptance: insertion loss and maximum VSWR across the complete operating band at the ordered length; average and peak power with their conditions; PIM limits where needed.
- Environment: pressure boundary, leakage requirement, temperature range, jacket and corrosion exposure. Use the exact part-number row rather than a family-wide temperature assumption.
- Mounting: support locations, loads and clearance through the motion. Select standoffs and hangers and bracket positions without forcing the flange ends to carry an unsupported run.
The class sheets tabulate VSWR by assembly length. Their footnote requires the next higher tabulated value for intermediate lengths; for example, a 15-inch assembly in the lowest Class 2 group takes 1.06:1 rather than the 12-inch value of 1.05:1. This is a specification rule, not linear interpolation. [3]
For interfaces above 110 GHz, NIST’s review of IEEE 1785 explains aperture tolerance grades and interface uncertainty. That higher-frequency guidance is not a qualification basis for the X-band examples here. [14] For custom assemblies, provide the route drawing and acceptance conditions together so the mechanical and RF requirements describe the same part.
Specify the relevant service tests
MIL-DTL-287J includes environmental and mechanical requirements for both types, while its axial-twist and repeated-twist clauses apply only to twistable assemblies. Shared test methods do not mean identical ratings or that a commercial product has passed them. The default cold-bending procedure includes seven days at 100 ± 2 °C, two hours at room temperature, then twenty hours at −55 ± 5 °C before three bends at no more than 5 degrees per second. The procedure includes “unless otherwise specified” provisions and a subsequent pressurization test. [1]
The axial-twist test applies five clockwise and counterclockwise cycles at about one cycle per ten seconds, with bending restrained. The flexure procedure specifies at least 30 cycles per minute. For moving service, identify the applicable procedure, excursion and cycle count on the order rather than treating a static bend radius as a life rating. [1]
Frequently asked questions
Can one type replace the other?
Only after checking every relevant requirement. A non-twistable part cannot replace a twistable part where torsion is required. A twistable part may replace a non-twistable part if its bend radii, RF limits, pressure rating and movement duty fit. At X band, the Class 1 minimum E-plane radius is 3.38 in versus Class 2’s 1.38 in, so it cannot follow every Class 2 route. Class 3 matches that E-plane radius but has a higher listed loss. [2] [3] [4]
How much rotation do I need?
Determine the required axial orientation change from the route drawing first. Then compare it with the applicable twist allowance and usable length under the manufacturer’s stated conditions. Rate × length estimates capacity, not demand: 95 deg/ft × (300 / 304.8) ft gives about 93.5 degrees for the idealized Class 1 calculation. It does not approve that angle on a bent or clamped installation. [1] [2]
Can a twistable section rotate continuously?
No. A bounded twist allowance is not continuous rotation capability. Use a waveguide rotary joint specified for that duty. Contactless rotary waveguide joints are established designs, but their own frequency, power and matching limits still apply. [15]
Which type carries more power?
The compared Dolph web tables list 3 kW average power for non-twistable WR90 and 1 kW for twistable WR90. The non-twistable PDF instead lists 2.5 kW. Those published figures do not establish a universal class advantage. Confirm the exact assembly, frequency, cooling, pressure and duty; keep average and peak power requirements separate.
How much can loss change after flexing?
MIL-DTL-287J clause 3.5.2 limits maximum insertion loss in the prescribed stability test to no more than 25 percent above the specified value. Clause 4.7.3 records the maximum observed in the specified deformed positions, not just after returning the guide to rest. The reference is the specified limit, not the unit’s initial measured loss: a specified limit of 0.1 dB gives a stability ceiling of 0.125 dB. Applicable bending, flexure and twist tests also require VSWR checks. [1]
An enquiry is easier to answer when it carries the numbers. Give the operating frequency, the waveguide size or connector, the power requirement, the quantity and the delivery date, and add the model number if you have one. For a custom assembly, send the drawing and the mating-interface details, and the engineering team can work from the same figures used above.
References
- MIL-DTL-287J, Waveguide Assemblies, Flexible, Twistable and Non-Twistable, General Specification For, US Department of Defense, 10 April 2012. Clauses 1.2, 3.3.2, 3.4.1–3.4.4, 3.5.1–3.5.16, 3.6, 4.7.2–4.7.17, 6.1, 6.5.1–6.5.9.
- MIL-DTL-287/3F, Waveguide Assemblies, Flexible, Twistable, Class 1, US Department of Defense, 14 April 2009. Table I and footnotes.
- MIL-DTL-287/4G, Waveguide Assemblies, Flexible, Nontwistable, Class 2, US Department of Defense, 14 April 2009. Table I and footnotes.
- MIL-DTL-287/5G, Waveguide Assemblies, Extra-Flexible, Twistable, Class 3, US Department of Defense, 14 April 2009. Table I and footnotes.
- MIL-DTL-85H (Notice 1), Waveguides, Rigid, Rectangular, General Specification For, US Department of Defense, 22 June 2000, validation notice 15 April 2005. Clauses 3.5, 3.9, 4.5.7 and Table VIII.
- MIL-HDBK-216, RF Transmission Lines and Fittings, US military handbook, Section 6 text dated 12 June 1975, compilation with Notices 1–10, 11 March 1982. Sections 6.1.7, 6.2.7, 6.4.2, 6.4.2.1–6.4.2.1.4, 6.4.2.2, Tables 6.XI–6.XIII, Section 8. Historical construction guidance; quoted with its date.
- US Patent 4,710,736, A. G. Stidwell, Flexible waveguides with 45° corrugations to allow bending and twisting of waveguides, published 1 December 1987.
- NASA, A parametric study of cut-off corrugated surface properties, NASA-CR-2317, Ohio State University, publication date 1 October 1973. Corrugation density, profile and depth in relation to surface power loss.
- S. Xia, Y. He, Y. Wang, H. Wang, G. Mankaruse, D. Chan, H. Zhou, W. Lee, N. McDonnell and C. Hwang, Practical Fixture Design for Passive Intermodulation Tests for Flexible Metallic Contacts, 2022 IEEE International Symposium on Electromagnetic Compatibility & Signal/Power Integrity (EMC+SIPI).
- C. Bai and W. Cui, Analysis of passive intermodulation between rough rectangular waveguide flanges using fractal method, International Journal of RF and Microwave Computer-Aided Engineering, 29(7), e21716, 2019.
- C. Vicente, D. Wolk, H. L. Hartnagel, B. Gimeno, V. E. Boria and D. Raboso, Experimental Analysis of Passive Intermodulation at Waveguide Flange Bolted Connections, IEEE Transactions on Microwave Theory and Techniques, 55(5), 2007, pp. 1018–1028, doi:10.1109/TMTT.2007.895400. Intermodulation response as a function of the torque applied to the flange screws.
- C. Vicente and H. L. Hartnagel, Passive-intermodulation analysis between rough rectangular waveguide flanges, IEEE Transactions on Microwave Theory and Techniques, 53(8), 2005, pp. 2515–2525, doi:10.1109/TMTT.2005.852771. Contact resistance from the oxide and contaminant layer, against applied mechanical load, surface finish and metal properties.
- MIL-DTL-3922E, Detail Specification: Flanges, Waveguide, General Purpose, General Specification For, US Department of Defense, 18 April 2016. Scope and general requirements for the waveguide flanges that couple two sections together.
- R. A. Ginley and N. M. Ridler, A Review of the IEEE 1785 Standards for Rectangular Waveguides above 110 GHz, 89th Microwave Measurement Conference, Honolulu, HI, US, 4–9 June 2017, NIST publication record. Aperture tolerance grades against worst-case reflection coefficient, and interface uncertainty data.
- P. H. Smith and G. H. Mongold, A High-Power Rotary Waveguide Joint, IEEE Transactions on Microwave Theory and Techniques, 12(1), 1964, pp. 55–58, doi:10.1109/TMTT.1964.1125751. Contactless rotary joint with impedance and phase variation under rotation eliminated.
