The local German papers are a good place to look for German immigrant information despite what local researchers will tell you. There are about 60 Bartels entries in "Milwaukee's German Newspapers; an index of death notices and related items" (1844-1950). These include entries for Ferdinand. There are also about 100 entries for Bartel and Bartell which should be checked.
Indexing Milwaukee's German newspapers continuously since 2007 ...
Gary Rebholz, compiler/editor