Explore FCC ham radio database

1. Ingest and process (label columns) an FCC pipe-delimited file.
2. Analytics of call sign patterns.
3. Analytics of VHF/UHF repeaters (different CSV, not from FCC).

Are there forbidden radio callsigns?

See R Markdown file.

Which, if any, 3-letter amateur radio (ham) callsign suffixes do not exist anywhere in FCC's database (presumably because they are banned or actively shunned)?