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Since we hobbyists rarely have access to accurate field strength readings, it is difficult to appreciate just how inefficient a Part 15 AM installation can be. The installation discussed here uses a radiator nine meters long, forming a grounded dipole antenna fed at two thirds of the total height. The radiation resistance is about twenty times higher than for a three meter radiator fed at its base. The input power to the final transmitter stage is twelve times what is allowed by Section 15.219(a). Even so, the FCC field strength reading corresponds to only 22.2 uW radiated from an electrically short vertical monopole. This is an efficiency of only .00183 %!
I presume that this is a particularly poor system. It should be possible to get more than this amount of radiated power from a very ordinary system with a three meter antenna and 100 mW of input power to the final stage. A really good system should be able to get ten times the radiated power calculated here.