RFPlanar Transmission Line Comparison: Microstrip, Stripline, CPW, and More
This article compares various planar transmission lines, including microstrip line, stripline, suspended stripline, slotline, coplanar waveguide (CPW), and finline. We’ll explore their key differences based on characteristics such as impedance, radiation loss, dispersion, component mounting, and quality factor.
Comparison Table: Microstrip Line vs. Stripline vs. Other Planar Lines
The following table summarizes the key differences between these transmission lines:
| Name | Characteristic Impedance | Radiation Loss | Dispersion | Component Mounting | Quality Factor |
|---|---|---|---|---|---|
| Microstrip Line | 20 to 120 Ohm | High (low εr) / Low (high εr) | Low | Easy (series) / Difficult (shunt) | 250 |
| Stripline | 35 to 250 Ohm | Low | None | Very difficult (totally covered structure) | 400 |
| Suspended Stripline | 40 to 150 Ohm | Low | None | Compatible (shunt & series) | 500 |
| Slotline | 60 to 200 Ohm | Moderate | High | Easy (shunt) / Difficult (series) | 100 |
| Coplanar Waveguide | 20 to 250 Ohm | Moderate | Low | Easy (shunt & series) | 150 |
| Finline | 10 to 400 Ohm | None | Low | Compatible (shunt & series) | 500 |
Key Differences Explained
Let’s dive deeper into what makes each of these transmission lines unique:
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Microstrip Line: This is a common type, consisting of a conductor separated from a ground plane by a dielectric substrate. Its impedance is influenced by the substrate’s dielectric constant. It’s relatively easy to implement but can suffer from higher radiation losses, especially with lower dielectric constant substrates.
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Stripline: Stripline features a conductor sandwiched between two ground planes. This shielded configuration significantly reduces radiation loss and improves the quality factor compared to microstrip. However, accessing the conductor for component mounting is significantly more difficult.
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Suspended Stripline: A variation of stripline where the conductor is suspended in air (or a low-dielectric material) between the ground planes. This further improves the quality factor and is more adaptable for both series and shunt component mounting.
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Slotline: Slotline consists of a slot etched in a metallized substrate. Both conductors are on the same side of the substrate. It exhibits moderate radiation loss and higher dispersion, making it less suitable for wideband applications. Shunt component mounting is easier than series.
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Coplanar Waveguide (CPW): CPW has a central conductor and two ground planes on the same side of the substrate, separated by slots. It provides convenient access for both series and shunt component mounting. Its radiation loss is moderate, and dispersion is lower than slotline.
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Finline: Finline uses a metal fin inserted into a waveguide structure. It features low loss and dispersion and is compatible with both series and shunt component mounting, but the impedance range can be quite high.
Important Considerations
Choosing the right transmission line depends heavily on the specific application. Factors like frequency, power handling requirements, acceptable loss, and ease of component integration all play a crucial role in the decision-making process.
