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 Post subject: AC Hum Modulation Buster Questions
PostPosted: Aug Sun 15, 2021 5:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5005
Location: Texas
I solved the problem of AC hum modulation on my Part 15 transmitter by locating it outside, far enough away from house wiring to silence the hum. This solution isn't always practical, especially when developing and testing these critters.

I'm wondering if there might be a circuit to cancel hum very locally that uses antiphase technology similar to the way noise cancelling headphones work. For example, my VLF whistler receiver is not tuned so around the house all it picks up is hum and hash from anything with a switching supply or microprocessor. Is there a way to retransmit its received signals in antiphase? Even if there was a sweet spot just big enough for a part 15 transmitter, that would be sweet indeed.

Would a loop that has inphase and antiphase windings work?

Would Faraday cage solve this kind of problem? If not, why?


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 Post subject: Re: AC Hum Modulation Buster Questions
PostPosted: Aug Mon 16, 2021 10:11 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1605
Macrohenry wrote:
I solved the problem of AC hum modulation on my Part 15 transmitter by locating it outside, far enough away from house wiring to silence the hum. This solution isn't always practical, especially when developing and testing these critters.

I'm wondering if there might be a circuit to cancel hum very locally that uses antiphase technology similar to the way noise cancelling headphones work. For example, my VLF whistler receiver is not tuned so around the house all it picks up is hum and hash from anything with a switching supply or microprocessor. Is there a way to retransmit its received signals in antiphase? Even if there was a sweet spot just big enough for a part 15 transmitter, that would be sweet indeed.

Would a loop that has inphase and antiphase windings work?

Would Faraday cage solve this kind of problem? If not, why?


I think the hum modulation is still there, but there is less coupling or signal being received by the house wiring for re-radiation when your transmitter is outside. So the strongest signal arriving at you radio is directly from your transmitter antenna, where there is no hum modulation on the carrier.

It is a tricky problem with whips and wire antennas inside a dwelling where multiple line power conductors are all around the transmitting antenna.

The only way I have managed to get around this annoying problem myself is with a loop transmitting antennas and a ferrite rod at the receiver. The rod can go in a small plastic box, tuned to the reception frequency and about 4 or 5 turns of wire for a coupling coil to feed the radio's antenna input.

I have never tried to phase cancel the received hum, in principle it could work.

There will be a position where the orientation of the transmitter's whip/wire antenna, and the one on the receiver, which should be aligned to match that, results in minimal hum, you could try moving them both around to a different axis or angle.


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 Post subject: Re: AC Hum Modulation Buster Questions
PostPosted: Aug Tue 31, 2021 5:13 am 
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Joined: Apr Thu 14, 2016 8:25 pm
Posts: 515
Location: pensacola fl
hi all. There are two things you can try. One is to detect the R.F. output and use that audio to feedback to the input. You must make sure the detector gives you out of phase output to input so as to be negative feedback. This does mean you have to have enough audio gain to allow for the canceling you are adding. This also helps flatten the frequency response and distortion as well. you can detect the R.F. at a convenient place at the transmitter output. There is the old trick broadcast transmitters did back in the early days. They took a sample of the filament supply and fed it to a circuit with two pots. one was for phase shift and the other was for amplitude the output of this was connected to the audio input and adjusted so that it canceled hum on the air.


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