Here’s a timely reminder, not to assume anything when it comes to radio.
I recently bought an ICOM IC-7300. With its SDR receiver it performs very well, better than any other receiver I have used. Not only is it sensitive, it’s noise blanking and noise reduction features are very good and add to that its twin pass band filters and the manual notch capability and if there’s a signal there, I can usually hear it.
What happened about 6 weeks ago however, is my base noise level on 40m went up from S4/S6 to S9/S9+10db. I initially thought the noise was from a local farmer connecting up his (faulty) electrical fence again but after “coping” with this noise for several weeks, hoping the farmer might move his animals and with them the fence, I realised that the noise was not the same pulsing noise that I had last year and rather a white broadband constant noise. Having just changed Internet connectivity, I wondered if that could be part of the problem. Then I decided rather than guessing what it might be, I’d plan a way to track down the noise.
Step 1. Check whether it’s noise coming from your own house.
As Computer systems are known noise generators, I turned off both my computer and that of the wife (both of which are on almost constantly during the day) and nothing changed with regards to the high noise level.
To to do further investigation I’d be dropping mains power so I had to be able to run the IC-7300 on battery power. A new battery connector (Molex) plug was ordered and a new power lead made up. I then ordered a LIPO battery to power the rig (this would be used later for portable and demonstration operation of the IC-7300 - but I’m not taking this rig up a summit - at least not one, where there is any amount of climbing or even walking to do, as its too heavy).
Now I had to wait for an opportunity when the main power switch to the house could be turned off. When this came I first took a sample reading on 7.150MHz in normal operation - the S meter on the IC-7300 was constant on S9. I then switched to battery power - no change (so the PSU I’m running the IC-7300 from is not the problem).
I switched off the main house power and came back o the rig and the noise level indicated on the S-meter had dropped to S6 - Aha! the problem is somewhere in the house! (or at least the S6-S9 portion of the noise was being generated in the house).
I put the power back on to make sure the interference came back and not that the interference had coincidentally stopped - It hadn’t - the 40m noise level was back up at a constant S9 on the meter on the IC-7300.
Step 2. - Track down where the noise is coming from in the house.
The first place to check is the room where the rig is located, so a day later, I once again powered the IC-7300 off the LIPO battery, checked the noise level - again S9 and then simply unplugged all units including power boards from the four wall sockets in the room.
Guess what - the noise level went down to S6 again! So the problem is actually in my “shack” room. From there on it was a matter of plugging the items back into each of the wall sockets until the noise returned. Of course it came up again as I plugged in one of the power boards. In this case I had two power boards “daisy chained” to power various computer related items. I was able to exclude items on the first board by unplugging the cable to the second board from it.
So the noise producer was plugged into the second board. I reconnected the second board at which point the noise level increased up to S9 again and then proceeded to unplug each plug until the noise went down again. The culprit? A switched mode power supply, supplying power to a USB hub. I had suspected a “wall-wart” but we use so many these days it could be any one and the only way to find it was by the process of elimination.
Needles to say this wall wart is now in the rubbish bin and as it happens the powered USB2 hub is now plugged into the USB3 socket on my laptop and is drawing its power from there, so I don’t need to replace the switched mode PSU.
If you follow this guidance and turning off your main house power does not reduce the noise level, you know it is something either outside or in a neighbours house. That is obviously more difficult to find, but perhaps you’ll be lucky like me and find you were sitting right next to the source and you can remove it easily.
Now how to get that S6 outside noise level down somewhat …