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Home Radiofrequency Transmission Lines Construction of a Circle Diagram  
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Construction of a Circle DiagramAuthor: Edmund A. Laport For fieldengineering purposes it is desirable to have a master chart from which blueprints can be made in any quantity for field use. The following information will enable one to reconstruct a chart like that of Fig. 4.58, the field of which embraces the range of values encountered in all except extreme cases.
a. Starting at 0, progressing upward on the lefthand scale, mark each major division in order 0.2, 0.4, 0.6, etc., to the top of the sheet. This will be the ratio scale for positive reactances. b. Starting at 0, progressing downward along this same scale, mark the major divisions 0.2, 0.4, 0.6, 0.8, etc., to the bottom of the sheet. This will be the ratio scale for negative reactances. c. Starting at 0, mark the horizontal scale so that successive major divisions are 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, etc., to the righthand margin. This will be the ratio scale for resistances, or resistance axis. 2. Locate the point where resistance is 1.0 and reactance zero. This will be the focal point for the circle diagram. 3. With centers on the resistance axis, draw the following confocal circles:
4. Draw the following semicircles from centers on the reactance axis, using a radius that causes the semicircle to pass through the focal point 1.0  j0.
This gives a set of semicircles which all pass through the focal point and are orthogonal with respect to the Q circles and whose ends are marked in progression clockwise from 10 to 170 degrees in 5degree steps. This completes the circle diagram for a transmission line in terms of the ratios of resistance and reactance to that of its characteristic impedance at any distance from a resistive termination in degrees and for any standingwave ratio from 1.00 to 5.0, the same as shown in Fig. 4.58. If drawn with care and skill, the accuracy of this chart will equal or surpass the usual accuracy of measurements.


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