# Source Methods and Field Methods

Solver methods can be categorized as either source-based methods or field-based methods. Understanding the main differences between these two categories helps to understand and choose an appropriate solution method for each application.

## Discretization

In source methods, only the structure is discretized (meshed) but not the free-space regions between the structures. In field methods, the whole solution domain is discretized, that is the structure as well as the free-space region between structures.

## Boundary Conditions

In field-based methods, the propagating fields, and therefore the fields' associated
mesh, requires a proper termination (truncation). This is not a problem for closed
regions such as waveguides or cavities where the PEC boundary provides a proper
termination. However, for open radiating problems such as shown in Figure 1 and Figure 2, the mesh would be
required to extend to infinity. An artificial absorbing region within the mesh is
used to solve this problem. This termination or absorbing region is denoted a
boundary condition. Source-based methods do not require a termination of the mesh
(boundary condition). A special function (denoted the Green's function) built into
the method automatically accounts for the field behaviour at infinity or any point
in space.^{1}

## Solvable Model Size

Field-based methods are generally more limited in terms of the size, specifically the
electrical size (in wavelengths), of models they can solve. This is because a
growing model size implies a larger **volume** of mesh elements to mesh and
solve. For source-based methods it is only a larger **surface** area of mesh
elements. It is assumed, however, that acceleration techniques for the source-based
method is employed such as the multilevel fast multipole method. In addition it must
be noted that increasing usage of GPU acceleration is increasing the solvable sizes
of field-based models.

^{1}Computational Electromagnetics for RF and Microwave Engineering, Second Edition, David B. Davidson, p. 14