**4.3. Four-port circulator utilizing cylindrical ferrite coupled line junction**

The last investigated device is a four-port circulator shown in Fig. 27. The advantage of this circulator is that the signal transmission in the circulation direction requires only one pass through the ferrite coupled line section. As a result, this allows for reduction of losses in the device in comparison to the alternative configuration of the four-port circulator realized with the use of double ferrite coupled line section.

The investigated device is realized as a cascade connection of the magic-T structure, the CFCL junction, and the output section being the transformer from the cylindrical coupled lines to the uncoupled microstrip lines. The magic-T structure (shown in Fig. 28(a) and (b)) allows to excite ferrite section with odd or even mode.

In the case of port (1) excitation, the signal is transmitted directly through the structure ensuring even mode excitation of cylindrical coupled lines. On the other hand, when port (2) is excited, the signal is coupled to slotline, which results in the odd mode signal at ports (3) and (4). Output section, shown in Fig. 28(c), is a multilayer structure, which transforms

**Figure 26.** Measured scattering parameters of double MFCL isolator: (a) transmission with isolation and (b) reflection

**Figure 27.** Top and bottom view of four-port circulator utilizing cylindrical ferrite coupled line junction

the cylindrical ferrite coupled line to microstrip lines. In the case of port (1) excitation of this section, the signal is transmitted to port (3). Similarly, when port (2) is excited, the signal is transmitted to port (4). For a complete understanding of the operation of the device, the excitation in ports (1) or (2) can be represented as a superposition of the even and odd signals, which are of the same amplitude and phase equal to 0 or 180 ◦. Then, for even-mode excitation, the signal is divided equally between the output ports. On the other hand, in the case of odd-mode excitation, part of the signal is guided in the bottom slot of the device. Due to the fact that this slotline is shorted with radial stub, the signal is coupled to a microstrip line and divided between the output ports. In order to reduce the isotropic coupling, which

**Figure 28.** Feeding circuits of CFCL circulator: (a) magic T, top view; (b) magic T, bottom view; and (c) output transformer from coupled slotlines to microstrip lines

<sup>8</sup> <sup>9</sup> <sup>10</sup> <sup>11</sup> <sup>12</sup> <sup>13</sup> <sup>14</sup> <sup>15</sup> <sup>16</sup> <sup>17</sup> <sup>18</sup> −30

2

slotline

A'

microstrip line

microstrip line

A

(a)

Frequency (GHz)

S12 S21

**Figure 26.** Measured scattering parameters of double MFCL isolator: (a) transmission with isolation and (b) reflection

B

B'

dielectric 1

AA' cross-section BB' cross-section

dielectric 3 dielectric 2

the cylindrical ferrite coupled line to microstrip lines. In the case of port (1) excitation of this section, the signal is transmitted to port (3). Similarly, when port (2) is excited, the signal is transmitted to port (4). For a complete understanding of the operation of the device, the excitation in ports (1) or (2) can be represented as a superposition of the even and odd signals, which are of the same amplitude and phase equal to 0 or 180 ◦. Then, for even-mode excitation, the signal is divided equally between the output ports. On the other hand, in the case of odd-mode excitation, part of the signal is guided in the bottom slot of the device. Due to the fact that this slotline is shorted with radial stub, the signal is coupled to a microstrip line and divided between the output ports. In order to reduce the isotropic coupling, which

**Figure 27.** Top and bottom view of four-port circulator utilizing cylindrical ferrite coupled line junction

Magic T-junction CFCL-section ( = /4) Output section

−18 −16 −14 −12 −10 −8 −6 −4 −2 0

Magnitude S (dB)

<sup>8</sup> <sup>9</sup> <sup>10</sup> <sup>11</sup> <sup>12</sup> <sup>13</sup> <sup>14</sup> <sup>15</sup> <sup>16</sup> <sup>17</sup> <sup>18</sup> −20

(b)

Frequency (GHz)

microstrip line

3

slotline

4

ferrite rod dielectric 4 coplanar line

S11 S22

−25

1

microstrip line slotline

microstrip line

−20

−15

Magnitude S (dB)

−10

−5

0

140 Advanced Electromagnetic Waves

could deteriorate the isolation of the circulator, an additional dielectric layer is used in the considered structure.

The simulation results of the circulator are presented in Fig. 29. It can be seen that the device operates in the frequency range from 7.6 to 8.6 GHz. In this frequency range, the device isolation and return losses are better than 18 dB. In the analysis, a lossless section was assumed.

**Figure 29.** Simulated scattering parameters of four-port CFCL circulator: (a) transmission with isolation and (b) reflection coefficients with coupling between neighboring ports

The designed device was manufactured (see the photo in Fig. 30) and measured. The obtained scattering parameters characteristics are shown in Fig. 31.

It can be seen that in the frequency range from 7.6 to 8.6 GHz, the value of transmission from port (1) to (4) and from port (3) to (1) is about −1.5 dB, while transmission from port (2) to (3) and port (4) to (2) is about −3 dB. The isolation between these ports in the considered frequency range is better than 12 dB. Return losses in ports (2), (3), and (4) are better than 17 dB and in port (1) are better than 8 dB. The isolation between ports (1) and (2) is better than 18 dB and between ports (3) and (4) better than 8 dB. A higher level of return losses

**Figure 30.** Photograph of fabricated four-port CFCL circulator: (a) top view and (b) bottom view

**Figure 31.** Measured scattering parameters of four-port CFCL circulator: (a) transmission with isolation and (b) reflection coefficients and coupling between neighboring ports

and a lower level of isolation with respect to simulated results at port (2) result from the inaccuracy of manufacturing process due to technological limitations. The greatest impact on the deterioration of the device parameters had the positioning of the cylindrical ferrite coupled line section with respect to planar structures. Nevertheless, the obtained results of measurements well agree with the results of simulation.
