I had posted following message: "I've received IMD from 139MHz taxi dispatch freq to another freq up 42.6MHz. So maybe 21.3MHz is one of IF for BR330T. Hope this helps! And http://myweb.accessus.net/~090/bc-780xlt.html#interfere shows 3 IFs of BC780XLT can give you some clue! " . Out of curiosity, I find the exact IFs of BC72 and BC92 at page 58 of http://www.racingelectronics.com/bc72manual.pdf . It shows 1st IF 380.7MHz, 2nd IF 21.3MHz, 3rd IF 450kHz. So I conclude that BC246, SC230, BC92, BC72 share such IFs. No wonder I always receive 3rd IMD from 506.775MHz taxi dispatch at 464.175MHz. Hope this helpful!