Hello Juan,
I attached an image of the device in the Spike software exhibiting the same shape as your software. I used 1 dB per div while your plot was 0.2 dB per div. The shape you are seeing is the shape of the IF band, the result of any filters and the RF path. The RF gain/attenuation is going to affect this shape as well as the frequency. Different start/stop frequencies will change the RF path of the input.
I hope this clarifies and validates what you are seeing.
Regards,
A.J.