  The silicon vacancy in silicon carbide is a strong emergent candidate for
applications in quantum information processing and sensing. We perform room
temperature optically-detected magnetic resonance and spin echo measurements on
an ensemble of vacancies and find the properties depend strongly on magnetic
field. The spin echo decay time varies from less than 10 $\mu$s at low fields
to 80 $\mu$s at 68 mT, and a strong field-dependent spin echo modulation is
also observed. The modulation is attributed to the interaction with nuclear
spins and is well-described by a theoretical model.
