What is the Main Factor Affecting the Characteristic Impedance of Coplanar Waveguide?

As we may know, during most of the RF design, it is recommended to adopt coplanar waveguide microwave transmission form with characteristic impedance of 50Ω. But, what is the Main Factor Affecting the Characteristic Impedance of Coplanar Waveguide?

Normally, the main factors affecting the characteristic impedance of coplanar waveguide include the Dielectric Constant (usually 4.2~4.6, here is 4.4), Height between Signal Layer and Reference Ground Layer (H), Microstrip Linewidth (W), Ground Clearance (S) and Copper Foil Thickness (T).

Below table listed the recommended Microstrip Linewidth (W) and Ground Clearance (S) corresponding to 50ohm impedance, with height between different Signal Layers and Reference Ground Layers (H), as well as the Copper Foil Thickness (T) = 0.035mm

Height between Signal Layer and Ground Layer (H)	Microstrip Linewidth (W)	Ground Clearance (S)
0.076mm	0.1188mm	0.15mm
0.1mm	0.1623mm	0.2mm
0.15mm	0.24mm	0.2mm
0.8mm	0.8mm	0.18mm
1.0mm	0.8mm	0.17mm
1.2mm	0.8mm	0.16mm
1.6mm	0.8mm	0.15mm
2mm	0.8mm	0.14mm

Recommended Values for Coplanar Waveguide with 50Ω Impedance

If it is a 2-layer PCB, the signal layer is the Top layer and the Reference Ground Layer is the Bottom layer, as shown in Figure 1 below. If it is a 4-layer PCB, the Reference Ground Layer can be the second layer, the third layer or the fourth layer. If the Reference Ground Layer is the third layer, the second layer directly below the signal layer shall be prohibited from paving, and the width of the prohibited area shall be at least 5 times of the signal linewidth, as shown in Figure 2 below. If the Reference Ground Layer is the 4th layer, the 2nd and 3rd layers directly below the signal layer shall be prohibited from paving, and the width of the prohibited area shall be at least 5 times of the signal linewidth, as shown in Figure 3 below. If it is a 6-layer PCB, the above rules are similar…

Figure 1: LAYOUT Diagram of Two Layers PCB

Figure 2: LAYOUT Diagram of Four Layers PCB (Reference Grounding is the 3rd Layer)

Figure 3: LAYOUT Diagram of Four Layers PCB (Reference Grounding is the 4th Layer)

Here is also an experimental formula to calculate the Microstrip Lines:

Z (Microstrip) = {87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)]

Note: W is the Microstrip linewidth, T is the copper foil/plate thickness (of the microstrip lines), H is the Height between Signal Layer and Reference Ground Layer, and Er is the Dielectric Constant of the PCB board material.

In fact, above INSTRUCTION is applicable to all the Sub-GHz and 2.4GHz RF modules produced by DreamLNK, which include 433/868/915MHz RF modules, 2.4G RF modules, FSK transceiver modules, Bluetooth modules, LoRa modules, UART module, etc.

You may visit our website to know more about our RF modules! If any product meets your demand, please feel free to contact us james@dreamlnk.com