An article to understand the commonly used microwave transmission lines (I)

btty038

An article to understand the commonly used microwave transmission lines (I) [Copy link]

summary:

Transmission lines that transmit microwave signals and microwave energy in a directional manner are called microwave transmission lines. Commonly used TEM mode transmission lines include coaxial lines, microstrip lines, strip lines and coplanar waveguides. TE mode and TM mode transmission lines include rectangular waveguides, circular waveguides, elliptical waveguides and barren waveguides. This post will briefly introduce the theory and simulation analysis of several typical microwave transmission lines.

The software used in this article is CST2018 and AnsysEM 18.2

Coaxial and Stripline

Among the transmission lines that transmit TEM mode, the most common ones are naturally coaxial lines and microstrip lines.

Coaxial line is a broadband transmission line. The cutoff wavelength of its TEM main mode is infinite, but its first higher-order mode is

mode. When transmitting in single mode, it must meet . Where and are the inner and outer radius dimensions of the coaxial line respectively.

Using CST's impedance calculation tool, we can calculate the parameters of the coaxial line.

As in the theoretical formula, the characteristic impedance of the air-filled coaxial line with inner and outer diameters of 0.8mm and 1.84mm respectively is . The coaxial line with this size basically has a characteristic impedance of 50 ohms.

Click Build 3D , set the solving frequency and boundary conditions, and set the wave port's Number of modes to 2 to facilitate viewing of higher-order modes. Excite Port1 in the time domain solver and check Calculate port mode only for fast calculation.

From the simulation results in the figure below, we can see that the cutoff frequency of the high-order mode is 73.56 GHz . In addition, the difference in the electric field vector distribution between the two modes is also obvious.

Cross-sectional electric field distribution of two modes of coaxial line

The left picture shows TEM mode, the right picture shows TE11 mode

The stripline consists of two floors with a distance b between them and a rectangular cross-section conductor with a width W and a thickness t in the middle. The middle of the two floors is filled with a uniform medium, as shown in the following figure:

Similar to coaxial line, TE and TM modes will appear. Usually the stripline size is selected:

The horizontal width of the floor is 5 to 6 times the width of the strip line to avoid high-order modes.

Similarly, the calculation and modeling simulation of strip lines can also directly call CST's impedance calculation tool.

Next, the design of the wave port is very important. Follow the steps below to open the Calculate port extension coefficient .

However, for lazy modeling and simulation, you can first pick the cross section of the strip line, then open this interface, click Construct port from picked face , and then complete the setting of the wave port excitation. The same is true for the other wave port. You can also set the Numbers of modes of the wave port to 2 to facilitate viewing the first high-order mode.

After the simulation is completed, you can check under 1D Results -> Port Information - > Line Impedance that the port impedance value of the stripline is basically 50 ohms.

In Port Modes under 2D/3D Results , you can see the electric field vector of the excitation mode:

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Microstrip Line

Microstrip lines are widely used in CMOS integrated circuits. Their structure is shown in the figure below:

Compared with stripline, the upper and lower half planes of microstrip line are not so symmetrical. In fact, the strict field solution of microstrip line is composed of TE-TM wave mixture. However, in practical engineering applications, the thickness of the dielectric substrate is taken into account, so its field is quasi-TEM mode . The effective dielectric constant and characteristic impedance of microstrip line can be approximated by the theoretical calculation formula in the book of microwave engineering . These results are approximate fitting of the curve of strict quasi-static solution, so I will not go into details here.

Calculate and construct the microstrip line model in the same way, and adjust the lateral width and length of the substrate appropriately.

In Port Modes under 2D/3D Results , you can see the electric field vector of the excitation mode:

Cross-sectional electric field distribution of two modes of microstrip line

0.762mm thickness substrate

It can be seen that the first high-order mode appears at 10.69 GHz.

In order to broaden the single-mode operating frequency band of the quasi-TEM mode, the thickness of the dielectric substrate needs to be reduced. As shown in the figure below, with a 0.254mm thick substrate, the cutoff frequency of the first high-order mode is increased to 24.54GHz.

Cross-sectional electric field distribution of two modes of microstrip line

0.254mm thick substrate

From 94 Cancer boy

freebsder

Antenna RF is simply metaphysics...

btty038

freebsder posted on 2021-8-17 20:23 Antenna RF is simply metaphysics. . .

Always on the road of learning

btty038

btty038 posted on 2021-8-18 21:31 Always on the road of learning

Never dare to stop. Every day, data processing + theoretical analysis. Alas, it’s still not right. Modify and test again.

freebsder

btty038 posted on 2021-8-18 21:32 Never dare to stop. Every day, data processing + theoretical analysis. Alas, it’s still not right. Modify and test again...

Yes, just like us, although we are not doing antenna microwave simulation, we are also adjusting parameters.

btty038

freebsder posted on 2021-8-19 17:43 Yes, just like us, although we are not doing antenna microwave simulation, we are also adjusting parameters

We need to test these things. If the test fails, we need to find the cause, subdivide the problem, and then eliminate it first. Simulation is a theoretical test, and then there are still many problems in engineering implementation that need to be subdivided and studied.

qwqwqw2088

btty038 posted on 2021-8-19 22:28 We need to test these things. If the test is not correct, we need to find the cause, subdivide the problem, and then eliminate it first. Simulate this...

Can you tell me which software you used for simulation?

btty038

qwqwqw2088 posted on 2021-8-20 08:29 Can you tell me which software you used for simulation?

Are you sure you read the article?

The software used in this article is CST2018 and AnsysEM 18.2

This is mentioned in it.

I use HFSS and ADS