TABLE OF CONTENTS When matching a 50- ? coax down to a balanced load of 5.56 ? or up to a balanced load of 450 ?, the Guanella balun is the transformer of choice. There is little doubt that his baluns offer the widest bandwidths under these two very different conditions. Further, his modular concept (that is, adding transmission lines in parallel-series arrangements) offers the highest efficiency at high impedance levels. Experiments have shown (see Chapter 11) that efficiency, at least with the Ruthroff unun, decreases as the impedance level increases. With Guanella's approach, each transmission line shares a portion of the load; therefore, his transmission lines can work at lower impedance levels. Also, the longitudinal gradients are less with his transformers.
Figs 9-15 and 9-16 are photographs of three 1:9 baluns designed for these two extremes in impedance levels. The balun in Fig 9-15 is designed to match 50 ? to 5.56 ?. This transformer could be used to match 50- ? coaxial cable directly to short-boom, four-element Yagi beams with resonant impedances of about 6 ?. Fig 9-16 is a photograph of two 1:9 baluns capable of broadband operation from 450 ? to 600 ?. By using four transmission lines, instead of three, baluns capable of matching 50 ? to 3.125 ? or to 800 ? become readily available. The parameters for the three baluns in Figs 9-15 and 9-16 are as follows:
