coax transformer
Hi there,
i frequently used transmission line transformers (1 to 9 and 1 to 4) and i never had problems. Basic guidelines for design those devices can be found, as RayEngine said, on Motorola AN749 and Philips ECO6907, ECO7703, ECO7213. I have all of these, but not in electronic format, so, if someone is interested, i could scan and "PDFize" them (mail me).
In my experience, transmission line transformers, provide large bandwidths (up to 3-4 octaves but my designs cover the 88-108 MHz band) and with appropriate core selection, low loss.
Remeber always that power transferring in rf transformers is not through the core, that is the core is only useful to rise the inductance of windings at lowest working frequency. For this reason, you might use a coax wound on the core, then arrange the shield and center conductor connections to make the 4:1 transformation you need (look at the sketch).
For a 4:1 transformer you can use a 25 ohm coax or two 50 ohm coax
in parallel. Higher impedances will result in poor bandwidth.
Last consideration is that you 'll have to cancel the primary and secondary winding inductances, compensating them with capacitors in parallel to the primary and secondary side. Often, if your active device has a capacitive reactance at its ouput (in the frequency band of your interest), the primary winding inductance is designed to cancel exactly the capacitive reactance of mosfet or transistor output.
For a deeper sight in the world of transformers, i 'll, if someone is interested, scan those very interesting application notes.
Bye
Tomass