  We demonstrate continuous-frequency electric field measurements of
high-intensity microwaves via optical spectroscopy in a small atomic vapor
cell. The spectroscopic response of a room-temperature rubidium atomic vapor in
a glass cell is investigated and employed for absolute measurements of
K$_a$-band microwave electric fields from $\sim$200 V/m to $>$1 kV/m over a
continuous frequency range of $\pm $1 GHz (15% band coverage). It is
established that in strong microwave fields frequency-specific spectral
features allow for electric field measurements over a large continuous
frequency range.
