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So I am working on a low cost ham radio beacon using my Baofeng UV-5R radio because it is easy to use and portable. I built a simple PTT interface for a Raspberry Pi and wrote some python to transmit the information I want. However, there is one problem. I want the option to have to radio plugged in to its charger to be able to transmit for long periods of time without dying but when I plug the radio into the wall and use Raspberry Pi to transmit audio it produces a very loud hum over the radio signal and it is completely unusable. This does not happen when I plug a battery operated audio source (such as a phone) into the radio, only when I use a wall powered source such as the Pi or a computer. it also does no occur when the radio is using battery power and the Pi is plugged into the wall. It does happen when using a simple male to male aux cable and VOX PTT. I can't seem to find the problem however I suspect it to be an issue with common grounds or something of the like but I have no idea how to solve it if that is the case. Please help!

Thanks,

Mr. Schmuck

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  • $\begingroup$ Fun fact: That hum will cause the noise floor on 80 meters to jump about 10 db. I have to unplug my Baofeng chargers while operating HF. When I suggest to others on our 80 meter net that they do the same, they all reported similar results. $\endgroup$
    – Joshua
    Jan 9, 2018 at 21:44
  • $\begingroup$ @Joshua did you find a solution? $\endgroup$
    – MrSchmuck
    Jan 9, 2018 at 22:31
  • $\begingroup$ MrSchmuck, the solution I found was to unplug them while operating. That fell within the KISS principle so I stopped there. $\endgroup$
    – Joshua
    Jan 9, 2018 at 23:10
  • $\begingroup$ @Joshua you know what? That's a good idea. Connect the charger when transmitting and disconnect when not using. Ill use relays for that. Thanks! $\endgroup$
    – MrSchmuck
    Jan 9, 2018 at 23:35

3 Answers 3

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Probably the charger you're using. I ran into a similar issue with a QRP receiver I was running off a wall wart. Try with a different wall wart, or maybe a few windings through a toroid.

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  • $\begingroup$ I'll give it a try $\endgroup$
    – MrSchmuck
    Dec 29, 2017 at 13:22
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The symptoms you related sound like a classic ground loop problem. One solution is to galvanically isolate the PI circuit from the radio. This technique allows the signals to pass but not stray currents. Here is a simple circuit containing only three parts that can be built for under $10 with new parts.

schematic

simulate this circuit – Schematic created using CircuitLab

The audio transformer isolates the audio signal while the optoisolator isolates the push to talk (PTT) signal. There should be no other direct connections between the PI and the radio (or its power supply).

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    $\begingroup$ @mrschmuck The relay is a good solution for the PTT. The 600:600 ohm audio transformer can be sourced through Mouser, Digikey, Ebay, etc. $\endgroup$
    – Glenn W9IQ
    Dec 29, 2017 at 15:11
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    $\begingroup$ So I installed an isolation transformer and no cigar. Still experienced the hum. Not sure how to fix it. I tried many different transformers it came with and just the same... $\endgroup$
    – MrSchmuck
    Jan 1, 2018 at 4:19
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    $\begingroup$ You have no other common grounds or connections between the PI or its power supply to the Baofeng, or its power supply? Have you listened to the output of the PI through an audio amplifier to see if the PI is generating the hum on the audio line? $\endgroup$
    – Glenn W9IQ
    Jan 1, 2018 at 4:26
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    $\begingroup$ @GlennW9IQ the Pi's audio output is fine on my stereo. I broke my oscilloscope so I can't test different points in the circuit which is a bit limiting but I believe the him is introduced in the radio somehow as the hum is present when the radio is transmitting silence and is plugged into the Pi. It doesn't have the hum when the audio source is my battery powered phone as previously stated. Thanks for helping me with this, it's very frustrating. $\endgroup$
    – MrSchmuck
    Jan 1, 2018 at 6:53
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    $\begingroup$ @mrschmuck Your audio cables should be as short as possible. You may wish to try adding a 0.1 uF capacitor across the audio line to bypass any induced RF. Test your Pi audio into your stereo with the transmitter transmitting close by . $\endgroup$
    – Glenn W9IQ
    Jan 1, 2018 at 14:57
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You are overthinking this. You are using a CHARGER, not an AC power supply. Most chargers are meant for charging batteries, not running the radio. They are not well filtered (if at all) and probably not rated at enough current actually power the radio. Get a REAL AC power supply.

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    $\begingroup$ I guess you mean a DC power supply? $\endgroup$
    – ON5MF Jurgen
    Sep 30, 2019 at 11:32
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    $\begingroup$ To be clear, it is a supply that lets you use AC house current to power the radio, yes, it would have a DC output. $\endgroup$
    – Jim
    Sep 30, 2019 at 11:57
  • $\begingroup$ Hello Jim, and welcome to ham.stackexchange.com! We've seen the term charger used loosely by different folks. For example, my 12 volt car battery charger outputs rectified (but not filtered) DC. However, my mobile devices and laptop chargers output very well filtered DC, indeed. That being the case, I suggest that you edit and clarify your answer so that it includes the text in your comment. :-) $\endgroup$
    – Mike Waters
    Sep 30, 2019 at 18:42
  • $\begingroup$ Also, please consider taking the tour and help pages, to help you get the most from this site. $\endgroup$
    – Mike Waters
    Sep 30, 2019 at 18:42

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