When people say that one cannot squeeze blood out of a turnip, it means that one cannot get something from a person, especially money, that the person does not have. One cannot squeeze blood from a turnip because it is basically impossible. Many years ago, who would have thought that methane could be obtained from rock? Doing so would be right in line with squeezing blood from a turnip.
Standards & References
TIA-222 G provides the requirements for the structural design and fabrication of new structural antennas, antenna-supporting structures, mounts, structural components and foundations as well as the modification of existing ones.