I found this in South Dade.   It's a well to check the underground water level.  Data from this is transmitted on UHF up to a satellite.   A solar panel on the top powers it.   It looks like it was just installed.  I think it was in this area:

Aerial Photography

This is where the data ends up:

http://water.usgs.gov/fl/nwis/uv?dd_cd=01&format=gif&period=45&site_no=252928080332401