  We implement a technique to characterize the electromagnetic properties at
frequencies 100 to 165 GHz (3 cm$^{-1}$ to 4.95 cm$^{-1}$) of oriented smectite
samples using an open cavity resonator connected to a sub-millimeter wave VNA
(Vector Network Analyzer). We measured dielectric constants perpendicular to
the bedding plane on oriented Na$^{+}$ and Ca$^{++}$-ion stabilized smectite
samples deposited on a glass slide at ambient laboratory conditions (room
temperature and room light). The clay layer is much thinner ($\sim$ 30 $\mu$m)
than the glass substrate ($\sim$ 2.18 mm). The real part of dielectric
constant, $\epsilon_{re}$, is essentially constant over this frequency range
but is larger in Na$^{+}$- than in Ca$^{++}$-ion infused clay. The total
electrical conductivity (associated with the imaginary part of dielectric
constant, $\epsilon_{im}$) of both samples increases monotonically at lower
frequencies ($<$ 110 GHz), but shows rapid increase for Na$^{+}$ ions in the
regime $>$ 110 GHz. The dispersion of the samples display a dependence on the
ionic strength in the clay interlayers, i.e., $\zeta$-potential in the Stern
layers.
