Graphene has become an outstanding material for revolutionary wireless applications in the microwave and terahertz/infrared frequency ranges. Due to its unique physical characteristics, a family of completely new devices can be developed by exploiting graphene's behaviour as High Impedance Surface (HIS) or highly-Reactive Impedance Surface (RIS). In this paper we theoretically investigate first a 10 GHz-resonant gold dipole antenna suspended over a graphene flake acting as a HIS reflector: the radiator can be placed at a very close distance from the graphene ground by keeping at the same time very good radiation performance with respect to classical metal reflectors. Second, we consider a gold dipole antenna operating at 10 THz directly attached to a graphene layer. Excellent radiation performance can be achieved thanks to the highly-inductive surface impedance of graphene at such high frequencies. An in-depth description of the possibility of varying graphene's conductivity by means of an applied gate voltage is provided, thus demonstrating how radiating properties can be tuned by simply applying a proper voltage to the reflector. Hence, the proposed antennas can be considered in reason as the starting point for innovative graphene-based mm- and µm-systems with unique tunability properties.