In Search of an Impedance Standard for Microwave Frequencies

F i r s t , i t i s necessary t o c a r e f u l l y review t h e measurement ob jec t ives , which u s u a l l y i nc lude ob ta in ing values f o r t h e parameters o r p roper t i es of c e r t a i n hardware items. To t h e ex ten t t h a t these parameters are l oose ly o r i l l d e f i n e d , a l i m i t a t i o n w i l l be imposed on t h e meaningful accuracy one can c l a i m f o r t h e network analyzer, i r r e s p e c t i v e o f i t s own i n t e r n a l characteri s t i c s . I n t h i s context it i s impor tant t o r e c a l l t h a t microwave c i r c u i t theory owes i t s ex is tence t o t h e form taken by t h e s o l u t i o n o f Maxwell 's equations i n an assumed un i fo rm propagat ing s t r u c t u r e (e.g., waveguide), and t h i s i n t u r n prov ides t h e bas is f o r t h e usual d e f i n i t i o n o f impedance a t microwave frequencies. Although one's i n a b i l i t y t o completely r e a l i z e t h e un i fo rm i t y cond i t i on may be p a r t i a l l y compensated i n coax ia l systems (prov ided t h a t t h e rad ius i s small i n comparison t o t h e wavelength), t h e usual r e s u l t i s t h a t t h e impedance, which i s a common measurement ob jec t ive , i s not p r e c i s e l y de f i ned .