Some Questions and Answers Concerning Air Lines as Impedance Standards

This paper attempts to answer a number of questions that arise when using one or more lengths of precision coaxial transmission line to calibrate a dual 6-port automatic network analyzer, questions such as: How important is the quality of the test port relative to that of the line? What type connectors should the line standards have? What are the advantages of using two lines instead of one line and a through connection when test port imperfections are considered? How many lines are optimum from a quality control point of view? What should the lengths be? The answers to these questions appear to be: ¿ The quality of the line is much more important than that of the test port. A perfect line will calibrate out most imperfections in the test port. An example is given where 75-¿ test ports are calibrated with 50-¿ lines, and then used to measure reflection coefficient relative to 50 ¿ with very little error. ¿ Greatest accuracy is achieved with line standards having male connectors. ¿ Two lines get rid of many test port imperfections that one line cannot. Three lines will show up a problem if one line is bad. Five lines will identify which line is bad. Five is probably optimum. ¿ There may not be an optimum for the actual lengths of a set of lines, but there does appear to be an optimum difference in the lengths.