#### 5.2.3.1. Symmetry

You can dramatically speed up your run time in simulations with natural symmetry by using symmetry planes. There are two types of symmetry planes: electric and magnetic. An electric symmetry plane is appropriate when the tangential electric field is symmetric across the plane. Physically, the electric symmetry plane is equivalent to a PMC boundary, so the normal component of the electric field will vanish on the boundary. A magnetic symmetry plane is appropriate when the tangential magnetic field is symmetric across the plane. The magnetic symmetry plane is equivalent to a PEC boundary, so H-normal vanishes on the boundary.

Symmetry planes must bisect any ports. If there is a port that is not adjacent to the symmetry plane, the simulation will return an error and abort. While the electric and magnetic symmetry planes are physically identical to standard PMC and PEC boundary conditions, respectively, they are not interchangeable in the simulation. If you designate a surface as a symmetry plane, you are indicating to RF3p that the geometry has been bisected and only half of the system has been drawn. This size change is taken into consideration in power calculations, far field calculations, and elsewhere within RF3p.

The following image shows a horn antenna in a ground plane.

The top and sides of the domain will be PML (described as follows) and the port is defined on the surface at the bottom of the horn. The user is interested in only fundamental-mode excitation, so that the electric field in the port will be directed in y, as shown in this top-down view of the port field.

The structure contains a natural symmetry plane. We take advantage of this by dividing the structure in half at the y-z plane, bisecting the port.

Since you want to enforce symmetry in the electric field at the bisecting plane, and the electric field is parallel to the bisecting plane, you assign an electric symmetry boundary condition to the bisecting plane. The electric field at the port now looks like this.

As a result, the size of the simulation domain has been cut in half, significantly reducing the computational burden, without compromising accuracy. It is a good practice to use symmetry planes whenever possible.