Ever 20 to 30 khz makes me suspicious of the new high freqency transformers on the market these day . They operate at a fundamental freq of 20 khz and radiate every harmonic from there up begining at 20 khz and goes right passed 30 MHz. Took me a while to figure it out , but when I did... I trashed these 20 khz xformers with a hammer. Good Luck