Circulators, Isolators, Nomograph

Channel Microwave Corporation

A junction circulator is a 3-port device formed by a symmetrical Y Junction stripline coupled to a magnetically biased ferrite material (see Fig. 1). When one of the ports is terminated, in either an internal or external termination, (see Fig. 2) the unit then becomes an isolator which isolates the incident and reflected signals.

FIG. 1
CIRCULATOR

FIG. 2
ISOLATOR

circulator isolation
Figure 3.
Circular isolation as a result of third port VSWR
As you can see in Fig. 2 the isolation is actually the return loss due to the terminated port mismatch. This basic relationship is shown in Fig. 3.

Figure 4.
Input power rating versus VSWR

When an isolator has the output leg either open or shorted all the incident power will be reflected into the termination where it is dissipated as heat. Therefore, the power power rating of termination must be equal to or exceed the maximum incendent level. Figure 4 gives the input power rating (as a multiplier of termination rating) versus load VSWR.

The most common use of three-port circulators is as duplexers. Connecting a transmitter to Port 1, antenna to Port 2 and a receiver to Port 3 permits simultaneous transmission and reception of signals while protecting the transmitter from reflection due to antenna mismatch. See nomograph below.

nomograph use USE OF THE NOMOGRAPH
As examples in utilizing the nomograph, three cases are illustrated:

No. 1
What specifications (with matched loads on other ports) must a circular have to provide an input VSWR of less than 1.5 for an the input VSWR of less than 1.5 for an antenna mismatch of 3.0 to 1?
The Answer is found by drawing a line between 3.0 on the antenna VSWR scale and 1.5 on the input VSWR scale. The intersection with the circulator scale indicates that an 18 dB isolator with a VSWR of 1.3 will suffice.

No. 2
What is the effective input VSWR of a three port 25dB junction circulator with the antenna port short circuited?
The Answer is that all conditions where the antenna is short circuited are given by a horizontal line through the ^pC and ^pA scales. Thus a horizontal line through the 25 dB point on the circulator scale intersects the input VSWR scale at a VSWR of 1.27. Note also that the 25 dB circulator will have matched load VSWR of 1.12.

No. 3
What VSWR will a transmitter see when connected to a 20 dB three port circulator looking into a 2.0 to 1 antenna mismatch?
The Answer of 1.3 to 1 is indicated by the intersection of line 3 with the ^pE scale.

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