Arturo Mediano teaches courses on EMC/EMI and Signal Integrity for Besser Associates. He is a frequent contributor to industry magazines and journals on these topics. This article appears in the "Practical Tips" section of InCompliance magazine.

Ferrites for EMI suppression are usually chosen looking for high (resistive) impedance at the frequency of interest; but, sometimes, that ferrite is not working as expected. Perhaps you have saturation effects?

Instructor Arturo Mediano has written an article for In Compliance magazine:

Digital and power electronic systems can reduce the radiated and conducted emissions profile using spread spectrum techniques. Typically, no more than 10-12dB can be obtained with those techniques but the result can be useful to comply with regulations...

Instructor Arturo Mediano has published an article with In Compliance magazine on reducing VHF EMI from a flyback power supply.

Usually, EMC design guidelines in flyback literature are related with the switching effects of the output transistor to minimize emissions for conducted tests in the 9 kHz to 30 MHz range. But failure in radiated emissions in the VHF range is typical when testing the product for EMC compliance.