My node from G7RPG has the hum, but that is usually eliminated if I listen carefully while moving it a few inches this way or that on the desk.
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On 5/2/2020 9:18 AM, Steve Matzura wrote:
I find this assessment a real ear-opener. I've always been able to detect who has a Shari node and who has something else, because almost to a fault, those stations using a Shari have the buzz. I don't specifically know any other kind of Allstar node that uses a Raspberry Pi other than a homebrew one, like the one I'm building with a URI cable, a fob, and my own radio, so after reading this, I'm thinking my node will be subject to this problem, too, telling me it's thankfully not due to the Shari transceiver. I'm also thinking mine might have the problem even more than a Shari node might because it's using ten times the power--five watts--but the antenna, a tri-band mag mount, is at twice the distance--outside on a terrace. Definitely good information to have when I get transceiving and discover I have the buzz, too.
Regarding the choke on the Pi power supply, the Pi 3 that runs my hotspot has one, but unles it's built into the in-line switch, the power supply on my 4 B? which will run my Allstar node does not. Curiouser and curiouser.
On 5/1/2020 5:11 PM, N8AR wrote:
Hi Quentin,
I have just completed an investigation into this issue as I experience it here also.? I am writing a report which I will share with the groups in a few days.
Basically the problem is caused by your Pi power supply or other similar wall warts in the immediate area of the SHARI antenna. What is happening is your SHARI is coupling RF energy into the power lead to the Pi from the power supply.? This energy at your chose transmit frequency is pulse modulated at a 60 Hz rate by the AC power rectifier diodes which only conduct over part of the 60 Hz cycle resulting in sidebands every 60 Hz (i.e. 60,120,190 ....Hz) with a sin(x/x envelope shape about your transmit frequency.? The radio you are using to listen to your node (call it an HT) receives the RF signal from your node plus this pulse modulated signal.? What you hear depends on the strength of each signal at your HT antenna? and how the FM detector in your HT works (especially the capture effect), Sometimes the signal from SHARI wins, sometimes the modulated signal wins and most the time it is a combination.? You move your HT a few inches and it sounds fine, move it a few more and the buzz is incredible.
If you have a phone recharging battery, use it to power your Pi and the buzz should go away if there are no other wallwarts nearby.? (other nearby wallwarts can cause the same problem but at a lower level)
I have "debuzzed" my Pi supply by using two FT114-61 ferrites right at the power supply with 4 turns of the power supply lead through each ferrite.? This keeps most of the SHARI RF energy out of the wall wart and thus the sidebands are vastly lower. ?I have some Fair-rite type 61 cores which should be identical to the FT114-61 cores on order from Digikey (much cheaper than the Amidon cores) and will test them soon.
The problem can also occur on your audio that others hear as the RF from your HT is subject to the same problem.? The closer you get to your node and thus your power supply, the worse the problem.
Another solution to this problem is to use an external antenna on SHARI so that your radiated RF is not near your Pi power supply or any other wallwart.
I created a photo album in the photofolder showing this modulation phenomena.
