[ORAWA01]

Thank you for the great advice. I suspected the storage heaters in the room and halls, because they were switched on a few days ago. And indeed, when the portable radio with the whip was bright to near the heater, the noise level was going up.

But what was the real source of the noise was, the switching power supply of the old VHF scanner. I took out the old scanner a few days ago, and left powered on for a few days scanning some VHF WX and Marine channels. When the scanner was switched off, the noise dropped drastically. But there was still some noise on the 5 Mhz. When the switching power supply of the scanner was unplugged from the mains, the noise has disappeared totally.

When the scanner was in scanning mode, the noise level was highest.

So, now I know, when the scanner is on, I must switch off the HF radio, and vice versa. It affected from 3.9 - 5.2 Mhz more critically.

But around 1AM last night, I was trying to listen the band again, and the scanner was unplugged, and the band was quiet. But suddenly loud noise came up on one frequency, and it was 4.940 Mhz. It was like loud air plane engine noise, and 59+ just on that frequency.

I failed to know what it was, and just went to bed. I will try to see if it will come up again on the frequency tonight.