Some time before, I noticed the noise comes from my computer. It was a really broadband noise covering all HF bands, with some 100 Hz reminiscences and some peaks along the spectrum, separated about 80 kHz between them. This was the definitive clue: 80 kHz peaks and 100 Hz noise pointed directly to the PC power supply. That power supply was working fine, so what could be the cause for that big noise?
After some time, I decided to open the computer for clean it ( Fans can accumulate a lot of dust ) and I checked the power supply also. Once I removed the supply cover, I just spent about one millisecond to figure out what was the problem with it:
The power supply does not was filtered at all!!! All the filter components were removed from the PCB (better saying, never put on it) so the noise generated could reach the mains and get radiated all along the house.
I remembered I had some faulty PC power supplies, so I opened all of them and I compared the PCBs inside. All of them seems to be made by the same manufacturer, with small modifications between them and same serigraph on all. So I decided to compare the input for all power supplies, just to see how the filter looks like. The noisy unfiltered power supply input stage looks in this way:
And one of the filtered faulty power supplies input stage looks in this another way:
As you can see, there were notable differences, In fact, there was these differences:
A toroidal coil, a 100nF capacitor and a 680Kohm resistor. So, I soldered these components in the working noisy power supply and tested it:
The computer was working fine, so I tested one of the indoor antennas. Results where really incredible. Noise level was zero. I had a really clean HF band, as you can see with this image taken with an indoor loaded dipole located at only one meter from the computer:
Why the power supply does not had the EMI filter components?? The answer is clear: Those cheap Asiatic importers. A capacitor can cost under a cent. But for thousand of power supplies build every day means a nice buck of dollars in the pocket. Add many non-essential components and you will get many bucks of dollars. Of course, these power supplies does not conform with any EMI standard (but they are marked as compliant!!!).
You are lucky if YOUR power supply is noisy, because you can replace or fix it, but... what if the noise comes from your neighbour's power supply?? You will loose your HF bands... This is a real and serious problem for today's ham radio.