Images of Mrk 421 for the first observing epoch at 15 GHz (top image), the second observing epoch at 24 GHz (central image), and the third observing epoch at 43 GHz (bottom image). Levels are drawn at (−1,1,2,4...) × the lowest contour (that is, at 1.0 mJy/beam for 15 and 24 GHz images and at 0.65 mJy/beam for the 43 GHz image) increasing by factors of 2. The restoring beam, shown in the bottom left corner, has a value of 1.05 mas × 0.66 mas, 0.67 mas × 0.40 mas, and 0.33 mas × 0.19 mas for 15, 24, and 43 GHz, respectively. The overlaid color maps show the linearly polarized intensity, and bars represent the absolute orientation of the EVPAs.