Detailed explanation of the method of measuring dielectric constant of materials using vector network analyzer

In the last issue, Agitek introduced the application of vector network analyzer in measuring dielectric constant of materials. We learned about the advantages of vector network analyzer in measuring dielectric constant of materials. But how to use network analyzer for measurement? Today, Agitek introduces seven specific test methods: coaxial probe method, transmission line method, free space method, arch method, resonant cavity method, parallel plate capacitance method and inductance measurement method. If you are interested, please collect them.

1. Coaxial probe method

By immersing the probe in a liquid or contacting it with the flat surface of a solid (or powder) material, the screen analyzer measures S11 and converts it to the dielectric constant.

1. Requirements for the material to be tested: liquid, soft integrated solid (flat surface that can fit tightly with the probe without air gap), semi-solid, "semi-infinite" thickness, non-magnetic, isotropic and homogeneous, no gaps

2. Covered frequency range (determined by the probe): 200MHz-50GHz

3. Test host: Keysight full range of network analyzers, E4991A/B impedance analyzer

4. Keysight test software N1500A-004

5. Keysight probe kit N1501A, including three

a) High temperature probe N1501A-101

The frequency is 200MHz to 20GHz, and the glass-to-metal sealing structure can prevent corrosion or friction from chemicals. The probe can withstand the temperature range of -40 to +200℃ and can measure the changes of material properties with frequency and temperature. The large edge enables the probe to measure flat solid materials, liquids and semi-solid materials.

b) Slim probe N1501A-102

With frequencies from 500MHz to 50GHz, it can easily fit into fermenters, chemical reaction chambers or other equipment with small apertures. At the same time, the slim design also makes it suitable for smaller sample volumes. This probe is best used for measuring liquids and soft semi-solids. For castable materials, the price of the probe is economical enough to extend into the material without retrieving. This probe is used as a consumable part, so it is provided in sets of three, and spare parts can be purchased separately.

c) High performance probe N1501A-104

With a frequency range of 500MHz to 50GHz, the slender design combines excellent robustness and high temperature resistance with a wide frequency range. The probe is sealed at both the probe end and the connector end, making it the most rugged and durable probe. The probe can withstand a temperature range of –40 to +200°C, so it can measure changes in material properties with frequency and temperature. The probe is autoclaved and is ideal for applications in the food, pharmaceutical and chemical industries where sterilization is required.

6. Advantages of the coaxial probe method: ECAL can be used to update the calibration in real time, it is convenient and easy to use, basically no need to process the sample, non-destructive, and has a wide frequency range;

7. Disadvantages of the coaxial probe method: not suitable for high εr', low εr" materials, thickness required> 1cm (typical value), solid materials require a sufficiently flat surface, limited accuracy: εr'±5%, cannot measure μr

2. Transmission Line Method

The transmission line method requires the material to be placed inside a section of a closed transmission line. The line is usually a section of rectangular waveguide or coaxial air line. The dielectric constant is calculated from the measurement of the reflected signal (S 11) and the transmitted signal (S 21).

1. Requirements for the material to be tested: solid, and the size can be accurately shaped into a ring or a square, and the surface is smooth enough to ensure that the sample fills the fixture without leaving any gaps.

2. Covered frequency range: 100MHz to 1.1THz, determined by the air line or waveguide frequency

3. Test host: Keysight full range of network dividers (except N9912A)

4. Keysight test software: N1500A-001

5. Keysight fixture: air line (such as 85051-60010 7mm air line 100MHz to 18GHz), waveguide cavity (such as K11644A 18GHz to 26.5GHz)

6. Advantages of the transmission line method: wide frequency range, simple fixture, and ability to test εr and μr

7. Disadvantages of the transmission line method: high requirements for material plasticity, difficult to ensure dimensional processing accuracy, not suitable for measuring low-loss materials, accuracy ±2%, liquids require special fixtures

3. Free Space Method

This method fills the material between two antennas for contactless measurement.

1. Material requirements: large flat solid material, flat, parallel surface, uniform, using special fixtures to measure granular and powdery materials

2. Covered frequency: 100MHz to 1.1T, limited by sample size

3. Test host: Keysight full series network divider (except N9912A)

4. Keysight test software: N1500A-001

5. Fixture: horn antenna, antenna fixing and alignment platform (it is recommended to test in a microwave darkroom to reduce the impact of the external environment)

6. Advantages of free space method: no contact, non-destructive, high frequency, suitable for high temperature testing, can measure magnetic materials, anisotropic materials

7. Disadvantages of free space method: special calibration parts are required, the distance between sample and antenna should be precisely controlled, and it is not suitable for low-loss materials.

4. Arch Method

The NRL Arch Test Method, first developed by the U.S. Naval Research Laboratory, is a practical technique for measuring the variable angle absorption characteristics of a material. A typical setup is to connect a network analyzer to two horn antennas, mounted on an arched armature above or below the material under test. One antenna is used as the transmit antenna, and the other can receive the reflected signal to complete a single-port measurement. The sample should be in the "far field".

1. Material requirements: large flat solid material, flat, parallel surface, uniform, using special fixtures to measure granular and powdery materials

2. Covered frequency: 100MHz to 1.1THz

3. Test host: Keysight PNA series network distribution, E5071C

4. Keysight test software: N1500A-002

5. Fixtures: horn antenna, arch, absorbing materials, etc.

6. Advantages of the arch method: non-contact, non-destructive, high frequency, suitable for testing absorbing materials, and can measure magnetic materials

7. Disadvantages of the arch method: special calibration parts are required, the distance between the sample and the antenna must be precisely controlled, and it is not suitable for low-loss materials.

5. Resonant Cavity Method

The resonant cavity test method uses a network analyzer to measure the resonant frequency and Q of the resonant cavity fixture, which is set up blank at the beginning of the test and then loaded with the sample under test. When the volume of the sample and other parameters of the resonant cavity are known, the dielectric constant can be calculated from these measurements.

1. Material requirements: Samples are cut into flat slices with a thickness of about 0.05mm~5mm, and a typical value of 1mm

2. Covering frequency: 1GHz to 15GHz, one resonant cavity corresponds to one frequency point

3. Test host: Keysight full series network divider (except N9912A)

4. Keysight test software: N1500A-003

5. Fixture: resonant cavity, such as Keysight 85072A, QWED resonant cavity, etc.

6. Advantages of the resonant cavity method: The accuracy is as high as ±1%, and the sample size processing requirements are not high, which is very suitable for low-loss material testing.

7. Disadvantages of the resonant cavity method: It does not support broadband testing, the measurable frequency of a single resonant cavity is narrow, and the analysis is relatively complicated

6. Parallel Plate Capacitance Method

Using an impedance analyzer, the material to be tested is placed between parallel plate electrodes and the dielectric constant is calculated by testing the capacitance value.

1. Material requirements: Smooth and uniform thin sheets or films; different fixtures have requirements for thickness and diameter.

2. Covered frequency: 1MHz to 1GHz

3. Test host: Keysight E4991B, E4990A, E4980A/AL

4. Keysight test software: N1500A-005

5. Fixture: 16451B, 16453A, liquid material uses 16452A

6. Advantages of the resonant cavity method: suitable for flat plate materials, thin film materials, economical price, high accuracy up to ±1%

7. Disadvantages of the resonant cavity method: low test frequency, high requirements on material size, and no support for testing magnetic permeability

7. Inductance measurement method

The magnetic permeability is tested only for magnetic materials and can be derived from the self-inductance of a core inductor containing a closed loop (such as a toroidal core).

1. Material requirements: Ring core structure

2. Covered frequency: 1 kHz to 1 GHz

3. Test host: Keysight E4990A, E4980A/AL

4. Keysight test software: N1500A-006

5. Fixture: 16454A

6. Advantages of the resonant cavity method: high accuracy up to ±1%, simple operation

7. Disadvantages of the resonant cavity method: low test frequency, only testing magnetic permeability

The above is the method shared by Antai Testing for measuring the dielectric constant of materials using vector network analysis, and the advantages and disadvantages of these seven methods are also introduced in detail.