Coaxial cables are electrical cables with three or more layers: a conductor material within an insulating layer surrounded by a tubular shield. Waveguides are structures which guide and direct different types of waves. A waveguide component of a particular dimension is used to propagate electromagnetic waves of a particular frequency. Some waveguides are also used in integrated circuits. Waveguide-to-coaxial adapters are composed of a waveguide component that fits the waveguide tubing and ends with a flange, and a coaxial probe assembly with a coaxial adapter and connection hardware. The coaxial cable adapter is typically tapped through one wall of the waveguide adapter housing. This example shows an HFWorks design of an adapter for a rectangular waveguide to coaxial transition operating at 2.4 GHz.
Figure 1 - Geometry of the coaxial antenna
It is known that waveguides function like high pass filters as they are characterized by a cut-off frequency determined by the dispersion relation imposed on the wavenumber k . We can see this in the simulated results as it rejects some frequencies below a certain limit.
The design of the adapter comes as a result of conception and optimization of a set of dimension variables represented in this figure:
Figure 2 - The simple transition
Through this simulation, we should be able to analyze the properties of the modeled coaxial cable to waveguide transition: the power loss, the return loss along with near field plots in E and H planes. We shall be able to visualize the animated transition in 3D between the coaxial and the waves propagating in the waveguide's hollow.
The waveguide has a cavity with air surrounding the coaxial conductor's stem at the bottom of which we have the input port. We assign the a second port to the open face of the waveguide. We consider the conductors and the waveguide's conductive walls as PEC.
The Smith Chart shows the return loss of the connection between the coaxial support and the waveguide from 2 to 4 GHz. We can get the curve smoother by applying a smaller frequency step.
Figure 3 - Transition's return loss at both ports
Figure 4 - Wave propagation in the antenna at 2.33 GHz (Front and right views)
These captures show the near distribution of the electric field in waveguide to coax adapter. The second view corresponds to a surface clipping: a feature provided in HFWorks to explore the E and H fields distributions in the inner parts of the structure.