Practical Consideration

1. Transmission line transformers are basically low-impedance devices. In practice, characteristic impedances as low as 5 ohms and as high as200 ohms are obtainable. Thus broad bandwidths in the impedance-ratio range of 2.5:50 ohms and 50:1000 ohms are possible
   
   
2. Transmission line transformers are basically unilateral devices. For example, a 4:1 transformer (with 50 ohms at one terminal) is only designed to match 50 ohms to 12.5 ohms or 50 ohms to 200 ohms. It cannot handle both conditions. Only when the impedance-ratio is low, say 1.5:1, is bilateral operation practical. Even then the bandwidth in the favored direction is usually twice as great as in the other direction.
   
   
3. Since high-impedance transformers require higher choking reactances (and hence more turns) and characteristic impedances, they are generally larger and more difficult to construct. Their power ratings are not any greater than their low-impedance counterparts. In fact, their losses (which are dielectric) could be greater!
   
   
4. Broadband baluns operating at high-impedance and high-power levels are generally easier to design and construct than ununs (unbalanced-to-unbalanced transformers). Important considerations in either type of transformer requires an understanding of their low-frequency circuit models and trade-offs in efficiency for bandwidth.
   
   
5. In power applications where efficiency is an important consideration, only low-permeability nickel-zinc ferrites (40-300 range) have been found (by the author) to be required. Powdered iron, because of its very low permeability is not recommended for any transmission line transformer applications.
   
   
6. Transmission line transformers are completely different from their conventional transformer counterparts. They are a combination of RF chokes and a configuration of transmission lines. Therefore, their designs and applications involve conventional transmission line theory, RF choke limitation, core losses (dielectric) and parasitics.
   
   
7. Since one of the objectives in designing these transformers is to obtain the optimized characteristic impedance of the transmission lines (for maximum high-frequency response), the spacing between the conductors is generally a critical parameter. Therefore the thickness of the coatings on the wire as well as the use of other dielectrics like Teflon tubing and polyimide tape, play important roles. The electrical insulation properties which determine the voltage-breakdown capability, are only of secondary importance.
   
   
8. Because transmission line transformers are so efficient, they can be combined in many ways offering applications heretofore untried. These combinations could be in series (even on the same core) on in parallel. Their flexibility has yet to be explored.