  Graphene is a valuable 2D platform for plasmonics as illustrated in recent
THz and mid-infrared optics experiments. These high-energy plasmons however,
couple to the dielectric surface modes giving rise to hybrid plasmon-polariton
excitations. Ultra-long-wavelengthes address the low energy end of the plasmon
spectrum, in the GHz-THz electronic domain, where intrinsic graphene Dirac
plasmons are essentially decoupled from their environment. However experiments
are elusive due to the damping by ohmic losses at low frequencies. We
demonstrate here a plasma resonance capacitor (PRC) using hexagonal
boron-nitride (hBN) encapsulated graphene at cryogenic temperatures in the near
ballistic regime. We report on a $100\;\mathrm{\mu m}$ quarter-wave plasmon
mode, at $40\;\mathrm{GHz}$, with a quality factor $Q\simeq2$. The accuracy of
the resonant technique yields a precise determination of the electronic
compressibility and kinetic inductance, allowing to assess residual deviations
from intrinsic Dirac plasmonics. Our capacitor GHz experiment constitutes a
first step toward the demonstration of plasma resonance transistors for
microwave detection in the sub-THz domain for wireless communications and
sensing. It also paves the way to the realization of doping modulated
superlattices where plasmon propagation is controlled by Klein tunneling.
