TABLE OF CONTENTS More recent experiments by the author (since the publication of the first edition of this book) revealed some interesting results concerning low-impedance applications. Foremost was the design of closely wound rod transformers as compared to toroidal transformers for matching 12.5 ? unbalanced to 50 ? unbalanced (matching a short vertical antenna to 50- ? coaxial cable). Recent measurements showed that, with a tightly wound rod transformer, a characteristic impedance of 25 ? was easily obtained and the optimum high-frequency response did approach very closely the theoretical prediction by Ruthroff (ref 9). [1]
An early assumption that a tightly wound toroidal transformer had the same characteristic impedance of a rod transformer led to the conclusion that the optimum characteristic impedance for best high-frequency response should be considerably lower than predicted. Actual measurements of the characteristic impedances on toroids (before the transformers were connected as ununs) showed the values to be greater by about 30% over the rods. This is attributed to the difficulty of achieving as tight a winding on a toroid and the different effect of the fringing field on toroidal cores. Transmission loss measurements on toroids again showed that the optimum impedance level for maximum high-frequency response was well predicted by Ruthroff. Some other interesting effects from the length-to-width ratio of the coiled winding on rod transformers were also discussed in the previous chapter.
When matching 50- ? coaxial cable to impedances of 12.5 ? and lower using toroidal cores,...
