Probe, Loop, and Aperture Coupling in Waveguides

Key Takeaways

• Waveguide coupling is a process in which the part of electromagnetic energy associated with one waveguide is shared with another waveguide. 

• There are three waveguide coupling methods: probe coupling, loop coupling, and aperture coupling.

• Directional couplers and power dividers are examples of aperture coupling in waveguides.

The electromagnetic energy associated with one waveguide is shared with another waveguide through waveguide coupling

The transmission, propagation, and reflection of guided waves happens in waveguides with or without excitation. Waveguide coupling is a process in which the part of electromagnetic energy associated with one waveguide is shared with another waveguide. Waveguides can be coupled to generators, excitation sources, or other waveguides to exchange energy in and out. When waveguides are coupled to power sources, various propagation modes are excited and there can be evanescent modes to store energy. 

Types of Coupling 

There are three different methods of coupling: probe, loop, or aperture coupling in waveguides. Probe or loop coupling (or a combination of both) is used to couple two waveguides. Aperture coupling utilizes one or more apertures in the common wall of two waveguides for energy transfer. Let’s take a closer look at these three coupling methods.

Probe Coupling in Waveguides

In probe coupling, a probe inside a coaxial line is used to distribute energy into a waveguide. As current starts flowing in the probe, an electric field is set up and it gets detached from the probe to the waveguide. The probe (also called a probe antenna) radiates energy equally into the waveguide where it is inserted. Probe antenna insertion is usually made perpendicular to the length of the waveguide, at a distance equal to the quarter wavelength from the shorted end of the waveguide.

Probe coupling is intended for coupling to the electric field and is made at the center of the waveguide, as the electric field concentration is at its maximum at this position. 

Loop Coupling in Waveguides

Loop coupling enables coupling to the magnetic field in the waveguide. In loop coupling, a conductor is inserted into the waveguide and bends into a loop. The center of the loop is at an equal distance from the top and bottom walls of the waveguide. When current flows through the loop, it generates a magnetic field component that couples with the waveguide fields. For high efficiency, the loop should be inserted at the point where the magnetic field is at its maximum strength. 

Aperture Coupling in Waveguides

When coupling is required between two waveguides of different cross-sectional areas, aperture coupling is preferable. Apertures are small windows or slots in the walls of a waveguide. A small fraction of electromagnetic energy is lost through apertures when they are inserted in a waveguide. They unbalance the electromagnetic field distribution inside the waveguide and result in the energy transfer between two waveguides.

According to a theory developed by Hans Bethe, an aperture can be considered equivalent to a combination of radiating magnetic and electric dipoles. The dipole moments of the magnetic and electric dipoles are proportional to the tangential magnetic field and normal electric fields of the incident wave, respectively.

Aperture coupling in waveguides can be of two types: electric field coupling and magnetic field coupling. The orientation of the aperture with respect to the fields determines the type of aperture coupling. Directional couplers and power dividers are examples of aperture coupling in waveguides, where electromagnetic power is transferred from one waveguide to another through the apertures.

Aperture coupling in waveguides is of great importance whenever the energy transfer between two waveguiding structures is required. Cadence software offers simulation tools that help in modifying probe, loop, or aperture coupling to achieve maximum energy transfer in waveguide couplers.

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