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The transmitter I built has a RF output transformer for impedance matching and I left the secondary (antenna side) floating from circuit ground to avoid audio ground loops. In use, one side connects to the antenna and the other to the earth ground.
With no secondary connection there is no detectable signal on my monitor radio. While fooling around with this I disconnected the antenna lead but left the ground connected. I get a usable signal throughout my house with NO ANTENNA! It is down about 20 dB below the signal with the antenna connected but it is usable. It appears my ground is radiating and there is a capacitive coupled return path from the primary to the secondary of the transformer.
RF can be strange sometimes..