Quasi-Planar Transmission Lines in Millimeter Frequency Applications

Key Takeaways

• In millimeter frequency applications ranging from 30 to 300 GHz, there are various categories of waveguiding media including hollow metal waveguides, planar transmission lines, quasi-planar transmission lines, dielectric integrated guides, H and groove guide structures. 

• Quasi-planar transmission lines are formed by mounting dielectric substrate with printed fins on it in the E-plane of a standard rectangular waveguide. 

• Types of quasi-planar transmission lines used in millimeter frequency applications include slotlines, finlines, suspended striplines, inverted striplines, coplanar waveguides, and conductor-backed coplanar waveguides. 

Millimeter frequency ranges from 30-300GHz

In millimeter frequency applications ranging from 30 to 300 GHz, such as in telecommunications, there are categories of waveguiding media: hollow metal waveguides, planar transmission lines, quasi-planar transmission lines, dielectric integrated guides, and H and groove guide structures. Each of these different types of waveguiding media can be applied in microwave and millimeter-wave systems according to their propagation characteristics and varying features. For example, rectangular-shaped hollow metal waveguides are useful to transmit high power in frequency applications up to 100 GHz. Another example: planar transmission lines are utilized in microwave integrated circuits (MICs) of low and medium power within the 100-140 GHz frequency range. 

One useful type of waveguiding media, quasi-planar transmission lines, exhibits low loss and good integration in the 30 to 120 GHz frequency range and eliminates the need for high dimensional tolerances on inner walls. 

In this article, we will discuss a few of the different types of quasi-planar transmission lines and their applications. 

Quasi-Planar Transmission Lines

Quasi-planar transmission lines are formed by mounting dielectric substrate with printed fins on it in the E-plane of a standard rectangular waveguide. They offer the advantages of planar technology and allow active devices to be easily incorporated. There are several types of quasi-planar transmission lines, including slotlines, finlines, suspended and inverted striplines, and coplanar waveguides. 

Mixed Format Transmission Lines

Slotlines

Slotline transmission lines consist of slots in a conductor material coating placed on a dielectric substrate. The guided mode in a slotline has a region of elliptical polarization and is suitable for application with ferrite material. Compared to other quasi-planar transmission lines, slotlines are the least popular for use in circuits. 

Finlines

Finline transmission lines are comparable to slotline transmission lines; however, finline structures are bound within a rectangular waveguide. Finlines are suitable for millimeter-wave applications, with a range from 30GHz to 110 GHz. 

In finline transmission line construction, a partially metalized dielectric substrate is enclosed within a rectangular metal path. Finlines get their name from the fact that their metallization is mostly in the form of fins; however, their metallization can also be in the form of strips of conductors of different widths placed in symmetric and antisymmetric positions on the substrate. 

Based on the fins, a finline transmission line can be classified as one of the following: 

• Unilateral finline 

• Bilateral finline 

• Antipodal finline 

• Edge-coupled finline

• Insulated finline

In unilateral finlines, the fins are located on only one side of the substrate, with slots in the center or off-center. A finline is considered bilateral when the fins are located on both sides of the substrate. Antipodal finlines are best used when the millimeter-wave application requires a transition between two kinds of planar transmission lines. These types of finlines are employed in power dividers, phase shifters, mixers, and filters. These structures offer advantages such as low loss, good integration, easy mounting of active devices, and mass production capabilities. 

Suspended Striplines and Inverted Striplines

Suspended striplines and inverted striplines are placed in a shielded waveguide. These structures reduce the attenuation of the signals at higher frequencies. They are similar to strip transmission lines, with multiple layers of dielectric material. 

Coplanar Waveguides

The coplanar waveguide is a quasi-planar transmission line made of a center strip on the surface of a substrate, with two ground planes placed nearby and parallel to the strip. In a coplanar waveguide, all the conductors are on the same side of the substrate. The three separate conductors are useful in accommodating three-terminal active devices. When one ground plane is made on the other side of the substrate, then the coplanar waveguide becomes a conductor-backed coplanar waveguide. 

Quasi-planar transmission lines provide advantages such as low loss and good integration, as well as making it easier to mount active devices in millimeter (mmWAve) frequency range circuit applications like antenna design. An understanding of the various types of quasi-planar transmission lines is beneficial for improving the layout and performance of microwave and millimeter-wave frequency circuits. 

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