  We report on the multi-frequency multi-epoch radio observations of the
magnetar, XTE J1810-197, which exhibited a radio outburst from December 2018
after its 10-year quiescent period. We performed quasi-simultaneous
observations with VERA (22 GHz), Hitachi (6.9 GHz and 8.4 GHz), Kashima (2.3
GHz), and Iitate (0.3 GHz) radio telescopes located in Japan to trace the
variability of the magnetar radio pulsations during the observing period from
13 December 2018 to 12 June 2019. The pulse width goes narrower as the
observing frequency goes higher, analogous to the general profile narrowing
behavior of ordinary pulsars. When assuming a simple power law in the range of
2.3 GHz and 8.7 GHz, the radio spectrum of the magnetar goes steeper with the
average spectral index $ \langle \alpha \rangle \approx -0.85$ for the first
four months. The wide-band radio spectra inferred from our observations and the
literature suggest that XTE J1810-197 would have a double-peaked spectrum with
a valley point in 22 - 150 GHz, where the first spectral peak infers a
gigahertz-peaked spectrum (GPS) feature with a peak at a few GHz. The GPS and
the high-frequency peak have been identified in the spectra of other radio-loud
magnetars, thus they may be intrinsic features that can give a new insight to
understand various emission mechanisms and surrounding environments of radio
magnetars. Our study emphasizes the importance of simultaneous long-term
broad-band observations toward radio-loud magnetars to capture the puzzling
spectral features and establish a link to other types of neutron stars.
