In this paper, a new approach to analyzing graphene-based antennas in the terahertz frequency range using the characteristic mode analysis method is presented. By the proposed method, graphene antenna parameters are obtained with simpler analysis and better physical visibility compared to numerical methods. In this regard, two structures of a graphene-based rectangular microstrip patch antenna for resonance in the terahertz frequency range are considered. The first structure consists of a metal ground plane and a substrate on which a layer of graphene is placed instead of a 10 μm × 7 μm metal patch. The second structure includes a metal ground plane with dimensions 19 μm × 16 μm, a substrate, a graphene layer on the substrate and a 10 μm × 7 μm metal patch placed on a graphene layer. Characteristic mode analysis method has been applied to the mentioned structure with different substrates, and the effect of graphene chemical potential on the modal behavior of the structure has been analyzed. For the first structure, there are two modes in the frequency range of 1–3 THz, but they do not have good resonance and they can be considered as non-resonant mode. Due to the conductivity of graphene in low frequency range of terahertz, which causes graphene to be a dielectric in this area. For the second structure, the 7.6 THz resonant frequency corresponds to the first mode of the structure and the 10 THz resonant frequency corresponds to the second mode of the structure, respectively, in the vertically and horizontally feeding stimulated.
