Monopole antenna with a TEM feed

Two differences with respect to the previous case :

The monopole is no longer a wire, but a its surface is meshed with triangles.
The monopole is fed with the output of a TEM waveguide.

Create a disk with a coaxial guide.

Create lines :

(0.005, 0, 0) => (0.005, 0, 0.25) => (0, 0, 0.25)

(0.0115, 0, 0) => (1, 0, 0)

The first line is the monopole antenna and the second line is the disk. Note the small gap between both line. It is the gap between the core and the outer wall of the coaxial feed. It is supposed to be full of air. The outer radius was chosen to achieved a 50 ohms impedance.

We have to extrude these lines to build the surfaces. This is done with the Utilities => Copy tool. The following settings ask GiD to rotate the lines 4 times ("Multiple copies") around the z axis (vector defined by "first point" (0 0 0) and "second point" (0 0 1)) with an angle of 90°. Surfaces will be created ("Do extrude"). Click on the select button, select the two lines and click on finish. The final geometry is obtained :

The expert's tip :

There are different ways to create such a geometry. For example, you can create the disk and create an hole. You can also directly a cylinder for the monopole antenna. However, the presented way creates 4 curves at the coax feed boundary. This will force GiD to create a fine mesh around this curves, which is an advantage for this small feed.

Set the coaxial waveguide source :

Check the source location (I_guide1 for the inner conductor and O_guide1 for the outer conductor) :

Check the normals orientation.

Assign the electromagnetism properties on the surfaces (not only on the disk, but also on the monopole)

Frequency : 300 MHz

The computed Far field pattern :

and the computed input impedance (called "Z equivalente" in the case of a coaxial waveguide) :

The expert's tip :

This example is not really useful in the case of a thin monopole antenna. On the contrary, it increase the computation time. However, it introduce the use of a waveguide. Moreover, it could have been extended to thicker monopoles or even cones. In such cases, the thin wire approximation is no longer valid and the radiating element must be meshed with surfaces. Note that the voltage source is still available for such geometries : simply a a small piece of wire between the disk and the radiating element.