Thanks Chris and COMMSIGMA. I use both your KiwiSDRs on a pretty regular basis, and the WBCQ signal on 9330 kHz is insanely powerful (especially on the Westminster, MD KiwiSDR).
A notch 3x up in frequency? 9330 kHz x3 is 27990 kHz (27.990 MHz). Hmmm....would this mean a similar sized notch effect just below the 28.000 MHz 11 meter freeband / 10 meter amateur band interface? While there isn't a lot of activity in the Americas in the upper portion of 27 MHz (unless the band is really busy), the UK FM band, CADS and WPAS bands (Irish Church Radio, UK FM CB, etc.) are 27601.25 to 27991.25 and 27605 to 27995. Would the UK FM Channel 19, 27.78125 MHz / 27781.25 kHz be notched out as well?
While most 11m CB activity tops out in the 27.8 MHz to 27.9 MHz region (channel 40 on the high band is 27.855 MHz), the 27.8 MHz to 28.0 MHz region is heavily used by 27 MHz Church transmitters and other 11m DX targets coming out of Europe, Ireland and the UK.