  We present a self-calibrating, SI-traceable broadband Rydberg-atom-based
radio-frequency (RF) electric field probe (the Rydberg Field Probe or RFP) and
measurement instrument (Rydberg Field Measurement System or RFMS). The RFMS
comprises an atomic RF field probe (RFP), connected by a ruggedized fiber-optic
patch cord to a portable mainframe control unit with a software interface for
RF measurement and analysis including real-time field readout and RF waveform
visualization. The instrument employs electromagnetically induced transparency
(EIT) readout of spectral signatures from RF-sensitive Rydberg states of an
atomic vapor for continuous, pulsed, and modulated RF field measurement. The
RFP exploits resonant and off-resonant Rydberg-field interactions to realize
broadband RF measurements at frequencies ranging from ~10 MHz to sub-THz over a
wide dynamic range. The RFMS incorporates an RF-field-free atomic reference and
a laser-frequency tracker to ensure reliability and accuracy of the RF
measurement. We characterize the RFP and measure polar field and polarization
patterns of the RFP at 12.6 GHz RF in the far-field of a standard gain horn
antenna. Measurements at 2.5 GHz are also performed. Measured patterns are in
good agreement with simulations. A detailed calibration procedure and
uncertainty analysis are presented that account for deviations from an
isotropic response over a $4\pi$ solid angle, arising from dielectric
structures external to the atomic measurement volume. Contributions to the
measurement uncertainty from the fundamental atomic measurement method and
associated analysis as well as material, geometry, and hardware design choices
are accounted for. A calibration (C) factor is used to establish
absolute-standard SI-traceable calibration of the RFP. Pulsed and modulated RF
field measurement, and time-domain RF-pulse waveform imaging are also
demonstrated.
