  We present results from observations of the Galactic Center magnetar, PSR
J1745-2900, at 2.3 and 8.4 GHz with the NASA Deep Space Network 70 m antenna,
DSS-43. We study the magnetar's radio profile shape, flux density, radio
spectrum, and single pulse behavior over a ~1 year period between MJDs 57233
and 57621. In particular, the magnetar exhibits a significantly negative
average spectral index of $\langle\alpha\rangle$ = -1.86 $\pm$ 0.02 when the
8.4 GHz profile is single-peaked, which flattens considerably when the profile
is double-peaked. We have carried out an analysis of single pulses at 8.4 GHz
on MJD 57479 and find that giant pulses and pulses with multiple emission
components are emitted during a significant number of rotations. The resulting
single pulse flux density distribution is incompatible with a log-normal
distribution. The typical pulse width of the components is ~1.8 ms, and the
prevailing delay time between successive components is ~7.7 ms. Many of the
single pulse emission components show significant frequency structure over
bandwidths of ~100 MHz, which we believe is the first observation of such
behavior from a radio magnetar. We report a characteristic single pulse
broadening timescale of $\langle\tau_{d}\rangle$ = 6.9 $\pm$ 0.2 ms at 8.4 GHz.
We find that the pulse broadening is highly variable between emission
components and cannot be explained by a thin scattering screen at distances
$\gtrsim$ 1 kpc. We discuss possible intrinsic and extrinsic mechanisms for the
magnetar's emission and compare our results to other magnetars, high magnetic
field pulsars, and fast radio bursts.
