RF Transformers
Toroidal cores are ideal for prototyping RF inductors and transformers at frequencies from a few hundred kHz to a couple hundred MHz. They provide a high degree of self shielding and a predictable Q. Reference (3), available from the manufacturer for the asking, provides an excellent overview of RF toroids and winding information for a variety of cores.
The subject of RF transformers is a specialized area worthy of a separate book. Reference (5) is an excellent guide to this subject. A few brief guidelines: (1) The first inclination when winding an RF transformer may be to make sure that the primary and secondary self impedances are many times those which will be connected to the transformer. This isnˇŻt necessarily the best choice, especially at higher RF frequencies. It often results in a large number of turns on a small core. When this happens, the turns are too close to each other and interwinding capacitance effectively shorts them out.
(2) A higher permeability core, particularly a ferrite, with fewer turns will provide better transformer action. Reference (3) and the Inductor Fabrication guidelines provide guidelines for selecting appropriate core materials for the design frequency range. Use the vector network analyzer and adjust the turns ratio and configuration for the desired response.
(3) Transmission line transformers are often more effective at higher frequencies. They are described in detail in references (4) and (5).