  The study of exclusive $\pi^{\pm}$ electroproduction on the nucleon,
including separation of the various structure functions, is of interest for a
number of reasons. The ratio $R_L=\sigma_L^{\pi^-}/\sigma_L^{\pi^+}$ is
sensitive to isoscalar contamination to the dominant isovector pion exchange
amplitude, which is the basis for the determination of the charged pion form
factor from electroproduction data. A change in the value of
$R_T=\sigma_T^{\pi^-}/\sigma_T^{\pi^+}$ from unity at small $-t$, to 1/4 at
large $-t$, would suggest a transition from coupling to a (virtual) pion to
coupling to individual quarks. Furthermore, the mentioned ratios may show an
earlier approach to pQCD than the individual cross sections. We have performed
the first complete separation of the four unpolarized electromagnetic structure
functions above the dominant resonances in forward, exclusive $\pi^{\pm}$
electroproduction on the deuteron at central $Q^2$ values of 0.6, 1.0, 1.6
GeV$^2$ at $W$=1.95 GeV, and $Q^2=2.45$ GeV$^2$ at $W$=2.22 GeV. Here, we
present the $L$ and $T$ cross sections, with emphasis on $R_L$ and $R_T$, and
compare them with theoretical calculations. Results for the separated ratio
$R_L$ indicate dominance of the pion-pole diagram at low $-t$, while results
for $R_T$ are consistent with a transition between pion knockout and quark
knockout mechanisms.
