Calvin's program was broadcast on 950 WGBS-AM in New York City. How it happened has been documented by a few reputed sources. Ryan Ellet wrote a great paper that explained much of this in detail:
"Evidence strongly indicates that the Pittsburgh Courier Hour, which traces it's roots to 1927, was the first black radio series. On September 8, 1927, the prominent African-American Newspaper sponsored a 15-minute talk over radio station WGBS, owned by the Gimbel brothers department stores. The talk, given by Ruth R. Dennis...was Some Notable Colored Women..."The book Harlem Renaissance Lives by Henry Louis Gates more assertively puts the start date to October 2nd 1927. But the program was definitely not yet named for the Courier. How long it had been going on is still open to debate. A 1925 radio schedule tantalizingly lists "Radio Columnists" as a 15-minute program on a Wednesday in April. the story of Floyd Calvin is more certain. More here.
The program described by Ellet was a talk given by Ruth R. Dennis who wrote religious features for the Pittsburgh Courier. It was through Ms. Dennis that the PD at WGBS-AM, Terese Rose Nagel. Nagel saw the success of Ms. Dennis' program and offered a similar opportunity to Calvin. So he wrote an article "Some Notable Colored Men" in response. It was broadcast on November 27th. He took the opportunity to editorialize about negro journalism.
And why not? The Pittsburgh Courier had at that time been publishing for two decades. calvin had been worked there since 1924. When the program was cancelled by WGBS, he was successful enough to re-launch the program at WCGU-AM in 1928. A least that's according to the book Harlem Renaissance Lives.
There's not a lot of information on 1400 WCGU-AM. It predates 1935, the earliest year of the Broadcasting yearbook. But it does appear in the 1929 Dept of Commerce broadcasting list. [source]. So I know it was on Coney Island, and owned by the U.S. Broadcasting Company. The book Airwaves of New York reveals it to be a sharetime with three other stations: WBBC, WSGH, and WFRL. The station had first aired in 1926, so it was a competitor (in a sense) to WGBS. it's call letters are an acronym for it's founder: Charles G. Unger. It was sold in 1933 and became WARD. So we know that Calvin left before that date.
Regardless, Calvin's career continued beyond radio. In 1935 he started "Calvin's News Service" a wire service marketed to black weeklies. He died at the age of 1937 at the age of 35.