Home › Forums › temp › Constructing Properly Loaded Antennas for the Lower End of the AM Band – Below 800 Khz. › Reply about the AM25
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Power line coupling does occur, but I suspect that your signal “got out” due to the radiation from the ground lead. The hum could be due to the effect on the transmitter where the output sees a RF ground but the circuit ground is floating or grounded through the audio source producing a ground loop. This is just speculation on my part.
I don’t know the output impedance of the AM25. The circuit has a step up transformer at the drain of the output FET so the designers apparently decided to raise the impedance at this point. This is followed by a lowpass filter which can further change the impedance. I haven’t simulated or measured this circuit impedance.
Transmission lines, even though shielded and grounded to the transmitter, will radiate a signal and that is why part 15 rules limit the total length of the ground, antenna, and transmission line. It is possible to use a balanced transmission line with appropriate baluns which does not radiate, but the rules don’t make any exceptions for this That is why most part 15’er mount their transmitters at the base of the antenna. Doing so makes the transmission and ground lengths minimal so the antenna can approach the 3 meter limit on the total length.