**Abstract**

This chapter presents multiscale auxiliary sources mainly used to solve complex electromagnetic problems, especially those that insert localized elements into circuits. Several equivalence relations (field-circuit) are established to simplify and make more accurate electromagnetic calculations by changing some characteristics of the localized elements known by their field representation as "voltage-current" representation and vice versa. Various examples are illustrated to show the effects of auxiliary sources in planar circuits containing localized elements (dipoles, diodes, transistors) in the millimeter and terahertz bands. An example of a graphene or Gold dipole is demonstrated in this approach. Another typical example of a diode integrated in a radiating structure is also simulated.

**Keywords:** auxiliary sources, multiscale circuits, surface impedances, located element, microwave diode, microwave transistor, equivalent circuits, active microwave components, MM and terahertz waves
