Home › Forums › temp › Hum-free, Indoor LPAM transmission – even possible? › Ground Loop or RF coupling.
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If you have a ground loop, normally you will hear a smooth 60 cycle or so hum. If you are coupling RF back into the rig, it usually sound like a buzz. Try this: put your hand near the counterpoise (ground wire), then around the antenna and notice if there is a detectable difference in the sound. Your audio ground (unbalanced) and your DC ground should (must be) tied together. Otherwise the RF will couple back into the modulator.
Second, get rid of the wall wart power supply, period. They possess NO filtering. And finally, use shielded wire on all wire pairs (audio, DC) into the transmitter, grounding only the source end of the shield.
And finally, there is little question in my mind that impedance match between the transmitter and the antenna is not correct. In your make of transmitter, that is why the manafacturer uses the swamping resistor in the output. And BTW, a balanced antenna won’t solve your problem; only make it worse.
Marshall Johnson, Sr.
Rhema Radio – The Word In Worship