There's no such thing as "powdered ferrite". Ferrites are ceramics, made
by firing special clays at high temperatures with proprietary processes.
The process is similar in many ways to firing pottery, and making a
ferrite rod from "powdered ferrite" would be as hopeless as making a
ceramic vase from ground-up coffe mugs.

Roy Lewallen, W7EL

Radio Man wrote:
Where in the U.S. can i purchase powdered ferrite in
order to make very large ferrite rod antennas?