what causes this reception phenomenon?

[wenzeslaus]

a webSDR radio shows this on the waterfall display.

periodic frequency ranges of inconsistent width that are heavily biased at the lower end, about linear throughout most of the middle, fades out at the high end.

says the antenna is some model of DX Commander. must be a vertical antenna.

what causes this?

 

[KB4MSZ]

Rap music?

 

[merlin]

First, consider RF is some really strange stuff.
Your first display is nothing new, the strong signals down around 1 Mhz is AM broadcast.
The roll off getting to 15 Mhz is probably slow AGC gradually turning the gain down.
I can see 20 meters is a bit strong along with some short wave BC.

 

[Boombox]

Either seeing broadcast / ham activity, or it's RFI, or both. Some of it looks like it could be RFI.

 

[krokus]

AGC surging, possibly due to local broadcast overload.

 

[spongella]

It could be CODAR in your first photo. Google that for more info.