source scale changing on export

Hi -

I'm a total newbie with Premier and Media Encoder.  I've tried searching for a similar topic but haven't found one so asking here.

I'm just trying to convert a video from .mov to AVI (uncompressed).  My issue is that after exporting, the height and width of the video have changed.

The source video is 640x360.  After export, the new video is 720x480.

In the Media Encoder CC 2018 Export Settings, if I try to reduce the w:h from 720:480 to 640:360, the values I enter are immediately reverted and not accepted.

I have also tried selecting "change output size to match source" for Source Scaling, but the new file is still created at 720x480.

Is there some way that I can convert this video and maintain the original source scale?

Any help is greatly appreciated.

Thanks,

- Sal