I am building a 400W power amplifier that is designed to work between 10MHz to 15MHz. I have a schematic that uses a set of isolation transformers and a set of step-up transformers. I ordered some powdered ferrite transformers from Magnetics (www.mag-inc.com). The isolation set is a 1:1 ratio torroid that is about twice the size of your thumb. the isolation transformers work well at the designed frequencies. The step-up torroids need a 4:1 ratio and they are about the size of a hockey puck. I used 16awg magnetic copper wire to wind the large torroids. The torroids work well at a range of 2KHz to 2MHz. I have checked every other component involved in the circuit and everything is still working and works at the desired 10Mhz to 15MHz frequency range. Therefore, my only conclusion is that the large torroid transformers are my limiting components that are hurting my power amplifier. I have conducted some research into air core transformers and I think that replacing the torroids with air core transformers would give me the step-up I need at the frequency I need. first, I would like to know if an air core transformer can work at 10MHz to 15MHz frequency range? if it can work, then I would appreciate any help with designing the air-core transformers for my power amplifier.