HF Broad band interference

[majoco]

Hi Guys, I've been troubled by this broad band of raised noise level between the mid 8MHz to the top end of 11MHz. I've attached three screen dumps, one shows the 8MHz end - a rise in background noise from -110dBm to about 120dBm, the second one at 11.96MHz show the steep drop from -110dBm to -120dBm level, the third the spectrum showing the whole band. Some days, not many, it's not there but it's not necessarily any particular day of the week, but when it's on it's on for 24/7. Unfortunately it's not strong enough to be picked up on a portable and carried around the streets - I may go out to the country with my Kenwood R2000 and a length of wire but I've done experiments like that before and all I get is electric fences ticking away! I know that noise levels as low as this would be a joy to some of you guys in built-up neighbourhoods but it's generally fairly quiet round here.

 

[merlin]

There is a good list of sources, Monitors for one and switching power supplies another.
You can optimize your signal to noise ratio adjusting RF and IF gains. Often lower is better.
Anything you can do in the front end in the way of filtering helps. Reduce your bandwidth, see if that helps.
You won't get rid of noise but you can find a sweet spot of minimum noise/maximum signal, the best that can be done.

 

[majoco]

I don't think it's anything local. I've run the radio off a 12v gell cell into a laptop and connected it directly to the outside antenna with the main breaker to the house turned off and it's exactly the same. The spectrum is not AGC controlled and the front end bandwidth selection only rejects signals outside the selected frequency band but it makes no difference. There is a KiwiSDR about 20 miles away but his receiver seems to be deaf compared to mine. I can run the radio in question in my car from the battery and use a laptop again - I may make a tuned HF loop and go for a drive.....
Thanks for your suggestions but I think I've covered most of the bases already.