  We present results from eight months of Green Bank Telescope 8.7-GHz
observations and nearly 18 months Swift X-ray telescope observations of the
radio magnetar SGR J1745-2900, which is located 2.4" from Sgr A*. We tracked
the magnetar's radio flux density, polarization properties, pulse profile
evolution, rotation, and single-pulse behavior. We identified two main periods
of activity in SGR J1745-2900. The first is characterized by approximately 5.5
months of relatively stable evolution in radio flux density, rotation, and
profile shape, while in the second these properties varied substantially.
Specifically, a third profile component emerged and the radio flux increased on
average, but also became more variable. Bright single pulses are visible and
are well described by a log-normal energy distribution at low to moderate
energies, but with an excess at high energies. The 2-10 keV flux has decayed
steadily since the initial X-ray outburst, in contrast with the radio flux. Our
timing analysis includes Green Bank Telescope, Swift, and NuSTAR data. When we
include the X-ray data in our analyses, we find that SGR J1745-2900 exhibited a
level of timing noise unprecedented in a radio magnetar, though an analysis of
only the radio data indicates timing noise at a level similar to that observed
in other radio magnetars. We conclude that, while SGR J1745-2900 is similar to
other radio magnetars in many regards, it differs by having experienced a
period of relative stability in the radio band that now appears to have ended,
while the X-ray properties have evolved independently.
