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FM transmitter interference

Started by bossier17, September 04, 2021, 01:22:05 PM

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Quick question - I'm not sure what interference to expect from an FM transmitter, or if what's happening to me is expected or if I have a bad transmitter.  I bought and have it on the one watt setting.   If it's within 5 feet or so of my FPP setup, I get power fluctuations in the mean well power supply where the green "on" light will flicker and the pi becomes non-responsive.

If it's anywhere within line of sight or the same room, the power supply seems stable but the pi will become unreachable through wifi and it will play random sequences without the music.  Even after I turn the FM transmitter off I can't connect to the pi until I unplug it and plug it back in.   

It doesn't seem to affect wifi or power to any other item.  I have a 3d printer with octoprint running off a raspberry pi, and even if I set the transmitter up next to that octoprint is still reachable through wifi and works fine.  It seems to just be the power supply and raspberry pi I have running FPP.   

Is this normal interference?  Thanks!


Quote from: bossier17 on September 04, 2021, 01:22:05 PMI bought and have it on the one watt setting.
Be careful broadcasting with 1 watt of power as that exceeds the FCC allowance.  If they catch you it will be a $10,000 fine per day up to $75,000.  I would not recommend advertising on a forum that you are doing it at least.  You might want to search the FCC website regarding FM transmission.

FM frequency is 88 to 108 MHz and Wifi runs in the 2.4 Ghz range.  FM and Wifi should not interfere with each other.  Transformers can generate undesired radio interference, but Wifi should recover after the interference is gone.   Almost sounds like you either have too much attached to the Meanwell draining power below limits, or possibly the power supply is defective.  If the power is fluctuating... that is what you need to address.  Power fluctuation on a circuit board can cause all sorts of undesired results.  That could be why you have to power cycle to recover.  I have never seen a radio cause a transformer issue, but I have certainly seen transformers cause radio interference.


Back to Amazon it goes.   Does anyone have a recommendation for a FM transmitter.  I've seen people recommend the EDM transmitters but the order page is down.


I tend to recommend people watch this Youtube Video, which is all about audio and FM Transmitters, with explanations of how to do it right :


Many in the hobby have had tremendous success with these little guys:

DSP PLL 87-108MHz FM transmitter -- for < $10 off eBay. Simply solder a piece of wire to use as the antenna, connect it up to your SoundBlaster Play 3!, power it with a decent 5VDC wal-wart (I put an RF choke on the output wire to suppress interference) and you're up and running!


I used the DSP-PLL for 2 yrs, it is 100mw. It would get me over a mile. As an electronic tech of 43 yrs I thought that was a bit much. Not a good way to attenuate it aside from shielding the antenna. I now run a more expensive Vast with RDS turned pwr down so it just runs about 2 blocks.
On a side note, my company is bidding to assist the FCC with rf interference tracking. Will be interesting if we get it, as I will have access to better rf signal strength equipment which would allow me to let people know which ones are legal and which not. Definitely anything over 100mw or 0.1 watt is exceding the rules.

FCC Licensed Tech since 1986

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