[regenfreak]

I have the Spitfire transmitter using the switching power supplay. I never have any hum using battery powered radio. My earth is connected to the copper pipe of the main water tank.
I dont use the short antenna on its own that comes with it becuase it has poor efficiency in transmitting the RF signal. Instead I made an air core variable inductor with enamel copper wire winding on a 3" postal cardboard tube and giving the maximum inductanceof 1.4mH with about 10 tappings. You can also use ring- ferrite core inductor with tappings. The variable inductor is connected in series with the short antenna. I use a passive field meter and peak its reading at 1MHz or whatever the chosen transmitter frequency by trying different tappings. It makes day and night in difference in performance.