Waveguide Tube Testing

Waveguide inspection mainly conducts scientific testing and analysis on the dimension and geometric precision testing, surface roughness and finish testing, straightness and coaxiality testing, VSWR and insertion loss testing, power capacity and breakdown strength testing and air tightness testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Dimension and geometric precision testing: Including the cross-section dimensions of the waveguide inner walls (such as the wide side a and narrow side b of the rectangular waveguides), inner diameters of the circular waveguides, flange mounting dimensions and position degrees, etc. These parameters directly determine the operating frequency bands, cutoff frequencies and impedance matching characteristics of the waveguides, and are the basis of ensuring that they meet the design specifications.

Surface roughness and finish testing: Evaluating the micro unevenness of the waveguide inner walls. The excessively high surface roughness will significantly increase the signal transmission losses, especially more sensitive in the high-frequency bands such as the millimetre waves. Usually, the profilometers or high-precision roughness testers are used for the measurement, ensuring that they meet the Ra or Rz values specified in the standards.

Straightness and coaxiality testing: Detecting the bending degrees of the waveguide axes, as well as the axis alignment precision when the multiple waveguide sections are connected. The straightness deviations will cause the signal mode conversions and reflection losses. Usually, the laser collimators or high-precision optical measurement systems are used for the non-contact measurement.

VSWR and insertion loss testing: The vector network analysers are used for the key electrical property testing. The VSWR reflects the port matching characteristics, and the insertion loss characterizes the power attenuation degrees after the signals pass through the waveguides. The two together determine the transmission efficiencies of the waveguide systems.

Power capacity and breakdown strength testing: Evaluating the ability of the waveguides to work safely under the high-power conditions. Through the high-voltage testing, the breakdown voltages of the media inside the waveguides (usually the air or SF6) are detected, and combined with the thermal analysis, the temperature rises under the continuous wave or pulse power are evaluated, preventing the damage caused by the arcing or overheating.

Air tightness testing: For the pressurized waveguides or the waveguides used in the vacuum environments, the strict sealing testing must be carried out. The common method includes the helium mass spectrometer leak detection method. After the vacuum pumping, the helium leakage rates are detected, ensuring that the waveguides can maintain the stable internal air pressures in the harsh environments, preventing the performance degradation.

Testing Scope

Rectangular waveguides: Widely applied to the microwave relay communication, radar systems and satellite ground stations. The testing needs to cover the series from the standard waveguides (such as WR-90) to the millimetre-wave precision waveguides (such as WR-10), emphatically focusing on their wide and narrow side tolerances, inner corner verticality and flange surface flatness.

Circular waveguides: Often used for the rotary joints, high-power transmission and mode converters. The testing emphases are the roundness tolerances of the inner diameters, uniformity of the inner walls, as well as the dimension precision of the internal steps used for supporting the dielectric rods or diaphragms.

Flexible waveguides and bendable waveguides: Used for the occasions requiring the connection of the non-aligned ports or with the vibrations. In addition to the electrical properties, the testing also needs to pay attention to their performance stability after the repeated bending, fatigue lives of the corrugated tubes, as well as the tensile strengths of the connectors.

Waveguide elements and assemblies: Including the bends, twisted waveguides, directional couplers and filters, etc. The testing scope extends to their specific electrical indicators, such as the effects of the bend radii on the VSWR, out-of-band rejection and passband insertion losses of the filters, etc.

Plating and surface treatment testing: The waveguide inner walls are often plated with the high-conductivity coatings such as the silver and gold to reduce the losses. The testing needs to evaluate the plating thicknesses, adhesion, porosity and surface oxidation conditions, ensuring that they maintain the low-loss characteristics during the long-term use.

New material and special process waveguides: Such as the metal waveguides made by the additive manufacturing (3D printing) and composite material waveguides. The testing needs to carry out the special evaluation on the potential defects caused by their special processes, such as the internal lack-of-fusion blowholes, interlayer bonding strengths and electrical property differences caused by the anisotropy.

Testing Methods

Coordinate measuring machine method: Used for the high-precision measurement of the internal and external geometric dimensions, flange hole positions, flatness and position degrees of the waveguides. Through the precision probe contact scanning, the three-dimensional digital models can be established and compared with the design drawings for the analysis. It is the method of the dimension acceptance.

Optical projector and image measurement method: Suitable for the fast testing of the two-dimensional dimensions such as the waveguide end face contours and flange shapes. Through the high-magnification lenses magnifying the workpiece contours and overlapping the comparison with the standard patterns, the dimension deviations can be intuitively evaluated. With the high efficiency, it is often used for the production process quality control.

Vector network analyser testing method: This is the core electrical property testing method. Through the calibrated VNA, the waveguide-coaxial converters are used to connect the tested parts, directly measuring their S-parameters (S11 and S21) within the operating frequency bands, thereby precisely calculating the insertion losses, return losses, VSWR and phase characteristics.

Time domain reflectometer method: A fault location method based on the TDR principle. A fast rising edge pulse is injected into the waveguide, and through the analysis of the amplitudes and time delays of the reflected pulses, the positions of the impedance discontinuities (such as the dents and deformations) inside the waveguide or the positions of the poor flange connections can be precisely located.

Metallographic microscope and scanning electron microscope analysis method: Belonging to the micro testing methods. Used to observe the grain structures and uniformity of the inner wall platings of the waveguides, as well as to analyse the morphologies and causes of the defects such as the micro cracks and corrosion pits generated by the processing or use, providing the basis for the process improvements.

Air pressure method and helium mass spectrometer leak detection method: Used for the sealing testing. The air pressure method judges the macro leakage by filling with the gas of a certain pressure and monitoring the pressure changes; the helium mass spectrometer leak detection method is more sensitive, finding the micro leak holes by detecting the leakage rates of the tracer gas helium. It is the preferred method for the high-standard air tightness requirements.

Testing Standards

Domestic Standards:

1. GB/T 1408.1, Insulating materials — Methods of test for electric strength — Part 1: Tests at power frequencies

2. GB/T 11313, Radio-frequency connectors — General specification

International Standards:

1. IEC 60153-1, Hollow metallic waveguides — Part 1: General requirements and measuring methods

2. IEC 60153-2, Hollow metallic waveguides — Part 2: Relevant specifications for ordinary rectangular waveguides

3. IEC 60153-3, Hollow metallic waveguides — Part 3: Relevant specifications for circular waveguides

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Vector network analyser: The core equipment of the electrical property testing, which needs to be equipped with the waveguide testing kits of the corresponding frequency bands (including the waveguide-coaxial converters and calibration kits, etc.). The high-end VNAs have the extremely wide dynamic ranges and measurement precision, can carry out the multi-port and multi-channel measurement, and support the frequency domain and time domain analysis functions.

High-precision coordinate measuring machine: The reference equipment of the geometric dimension testing. With the sub-micrometre measurement precision, temperature and humidity compensation systems and precision probing systems, its measurement results are usually used as the final basis of judging whether the waveguide dimensions are qualified, and can be used for the reverse engineering.

Laser interferometer and collimator: Used to measure the straightness, parallelism and flatness of the long waveguides. The laser interferometers evaluate the micro deformations by measuring the optical path differences, with the extremely high precision; the laser collimators provide a spatial reference straight line for the fast calibration of the coaxiality of the multiple waveguide sections.

Surface profilometer/roughness tester: Specially used to measure the surface textures of the waveguide inner walls. Usually adopting the contact probe or white light interference non-contact methods, which can precisely output the roughness parameter curves such as Ra, Rz and Rq, evaluating the processing technology levels.

Spectrum analyser and signal sources: In the system-level testing, used to evaluate the performance of the waveguides when transmitting the specific modulated signals, such as detecting the non-linear effects including the harmonic distortions and intermodulation products caused by the waveguide non-uniformity, which is particularly important for the communication systems.

High-power testing system: Composed of the high-power signal sources, directional couplers, power meters/power sensors and water loads, etc. Used to simulate the actual high-power working conditions, test the average power and peak power capacities of the waveguides, and monitor their temperature rises and possible breakdown phenomena.

Environmental test chambers: Used to evaluate the reliability of the waveguides under the extreme environments. Including the high and low temperature cycling chambers, damp heat test chambers and salt spray test chambers, etc., detecting the geometric dimension stability and electrical property changes of the waveguides after the tests such as the temperature shocks and humidity corrosion.