What are the items that represent the characteristics of circulator and isolator?
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A. The three main factors of circulator and isolator are "Insertion loss (forward loss)", "Isolation (reverse direction loss)" and "V.S.W.R." as shown in Fig.
Since the circulator and isolator have a frequency characteristic, you will need to specify a particular frequency range, when you actually use it.
Circulator is evaluated by connecting a terminal resistor to 1 port and making it operate as isolator. Since the circulator is a three-fold symmetry circuit, its characteristics will be basically the same when seen from any port. However, strictly speaking, this will be slightly different. So you may have to check the characteristics of all three ports, if necessary.

Fig. Isolator characteristics example