**4.1. Three-port circulator utilizing ferrite coupled slotline junction**

The first investigated circuit is three-port circulator realized in coupled slotline technology, which is shown in Fig 19(a).

The device is realized as a cascade connection of a T-junction, ferrite coupled slotline (FCSL) junction, and the output section being the transformer from the coupled slotlines to the microstrip lines. The cross section of the FCSL junction is shown in Fig. 19(b). The FCSL junction is a four-layer structure, where the coupled slotlines are realized on a thin laminate situated above the ferrite material. The T-junction and the output section have the same cross section as ferrite section, although instead of ferrite, a dielectric material with relative

**Figure 19.** FCSL circulator: (a) schematic view of the device and (b) cross section of FCSL junction

permittivity *ε<sup>r</sup>* = 9.6 is used. T-junction is realized as a transformer from coplanar line into coupled slotlines. Due to the fact that the coplanar line is fed from the coaxial connector, the junction provides even excitation of FCSL section. On the other hand, the odd mode, which is transmitted from the ferrite section, is totally reflected in the plane of the coaxial connector. The output structure allows for the transmission of the signal from the specific slot of ferrite junction to the corresponding output microstrip port, while the remaining ports of the circulator are isolated.

In order to determine the scattering parameters of the circulator shown in Fig. 19(a), the scattering matrices of the ferrite junction, T-junction, and output section were first calculated. The scattering parameters of the FCSL junction were simulated with the use of our own software based on the methods described in section 2. The feeding circuits (T-junction and output section) were designed with the use of commercial software. The circulator scattering parameters were obtained by cascade connection of the calculated scattering matrices of individual sections. The simulated frequency characteristics of the scattering parameters are shown in Fig 20.

From the obtained results, it can be seen that the investigated configuration has return losses and isolation better than 10 dB over a wide frequency range from 13 to 20.5 GHz. In the considered frequency band, the average transmission losses are about 1 dB in the case of a single pass of the signal through the ferrite section and about 2 dB in the case of double pass of the signal through the ferrite section. In the analysis, the material losses were not taken into consideration, and the resulting level of losses is due to return losses and the lack of a perfect isolation between the ports of circulator.

A photograph of the manufactured prototype of the circulator is shown in Fig. 21. The measured characteristics of its scattering parameters are depicted in Fig. 22.

Measurements were performed in the frequency range from 10 to 21 GHz. The results show that the investigated device operates in a wide frequency band. In the frequency range from 12 to 19 GHz, the average transmission losses are about 2.5 dB, and the level of isolation and return losses is about 10 dB for a single pass of the signal through the ferrite

T-junction FCL = /4 Output section

Ferrite

(a)

perfect isolation between the ports of circulator.

3

permittivity *ε<sup>r</sup>* = 9.6 is used. T-junction is realized as a transformer from coplanar line into coupled slotlines. Due to the fact that the coplanar line is fed from the coaxial connector, the junction provides even excitation of FCSL section. On the other hand, the odd mode, which is transmitted from the ferrite section, is totally reflected in the plane of the coaxial connector. The output structure allows for the transmission of the signal from the specific slot of ferrite junction to the corresponding output microstrip port, while the remaining ports

In order to determine the scattering parameters of the circulator shown in Fig. 19(a), the scattering matrices of the ferrite junction, T-junction, and output section were first calculated. The scattering parameters of the FCSL junction were simulated with the use of our own software based on the methods described in section 2. The feeding circuits (T-junction and output section) were designed with the use of commercial software. The circulator scattering parameters were obtained by cascade connection of the calculated scattering matrices of individual sections. The simulated frequency characteristics of the scattering parameters are

From the obtained results, it can be seen that the investigated configuration has return losses and isolation better than 10 dB over a wide frequency range from 13 to 20.5 GHz. In the considered frequency band, the average transmission losses are about 1 dB in the case of a single pass of the signal through the ferrite section and about 2 dB in the case of double pass of the signal through the ferrite section. In the analysis, the material losses were not taken into consideration, and the resulting level of losses is due to return losses and the lack of a

A photograph of the manufactured prototype of the circulator is shown in Fig. 21. The

Measurements were performed in the frequency range from 10 to 21 GHz. The results show that the investigated device operates in a wide frequency band. In the frequency range from 12 to 19 GHz, the average transmission losses are about 2.5 dB, and the level of isolation and return losses is about 10 dB for a single pass of the signal through the ferrite

measured characteristics of its scattering parameters are depicted in Fig. 22.

**Figure 19.** FCSL circulator: (a) schematic view of the device and (b) cross section of FCSL junction

slot lines

2

microstrip line

microstrip line

**Hi**

(b)

h0

h1 h2 h3

h4

w1 s w2

ferrite

dielectric r3

dielectric r2

dielectric r1

1

coplanar line

136 Advanced Electromagnetic Waves

of the circulator are isolated.

shown in Fig 20.

**Figure 20.** Simulated scattering parameters of FCSL circulator: (a) transmission with isolation and (b) reflection

**Figure 21.** Photograph of the fabricated FCSL circulator: (a) top view and (b) bottom view

section. The best isolation is observed in the frequency range from 12 to 15 GHz and is better than 16 dB. In the case of a double pass of the signal through the ferrite section, the transmission losses are two times higher and are about 5.5 dB in the frequency range from 12 to 19 GHz. The isolation is better than 14 dB in the entire frequency range. In the measured transmission characteristics, small periodically repeating resonances occur, which result from the inaccuracies in the manufacturing process.

### **4.2. Double isolator utilizing microstrip coupled line section**

Another investigated configuration is a double isolator shown in Fig. 23. This arrangement is composed of two interconnected and magnetized in the same direction three-port circulators (see Fig. 2(c)), in which the appropriate ports are terminated by matched loads. The advantage of this configuration is the ability to achieve high isolation. Unfortunately, due to the fact that the device uses two ferrite sections, the losses in the system are twice as high as in the case of isolator with a single section of FCL.

**Figure 22.** Measured scattering parameters of FCSL circulator: (a) transmission with isolation and (b) reflection

**Figure 23.** Double isolator utilizing microstrip ferrite coupled line junction

Utilizing own software, as well as commercial simulator, the double isolator utilizing ferrite junction from Fig. 8 was designed. The simulation results are shown in Fig. 24.

From the obtained results, it can be seen that in the frequency range from 10 to 16 GHz, the isolation is better than 20 dB with return losses better than 10 dB.

The designed structure was manufactured. A photograph of the prototype is shown in Fig. 25. To obtain the double isolator, ports (3) and (4) of the structure were terminated with the matched SMA connectors. The obtained experimental results are presented in Fig. 26.

It can be seen that the device works in the frequency band from 9 to 16 GHz. In the given frequency range the isolation and the average return losses are better than 15 dB. Furthermore, the average transmission losses are 3.5 dB and they change from 3 dB at 9 GHz to 4 dB at 16 GHz. Based on these results, it can be estimated, that the losses for a single pass of the signal through the investigated microstrip ferrite section are about 1.8 dB. The losses are lower by about 1.5 dB in comparison to the results published for a single section configuration in [4].

**Figure 24.** Simulated scattering parameters of double MFCL isolator: (a) transmission with isolation and (b) reflection

**Figure 25.** Photograph of the manufactured double MFCL isolator

<sup>11</sup> <sup>12</sup> <sup>13</sup> <sup>14</sup> <sup>15</sup> <sup>16</sup> <sup>17</sup> <sup>18</sup> <sup>19</sup> <sup>20</sup> <sup>21</sup> −22

(a)

T-junction

Frequency (GHz)

FCL-section

microstrip line

**Figure 23.** Double isolator utilizing microstrip ferrite coupled line junction

isolation is better than 20 dB with return losses better than 10 dB.

S13 S21 S32 S12 S23 S31

**Figure 22.** Measured scattering parameters of FCSL circulator: (a) transmission with isolation and (b) reflection

**Hi Hi**

Utilizing own software, as well as commercial simulator, the double isolator utilizing ferrite

From the obtained results, it can be seen that in the frequency range from 10 to 16 GHz, the

The designed structure was manufactured. A photograph of the prototype is shown in Fig. 25. To obtain the double isolator, ports (3) and (4) of the structure were terminated with the matched SMA connectors. The obtained experimental results are presented in Fig. 26. It can be seen that the device works in the frequency band from 9 to 16 GHz. In the given frequency range the isolation and the average return losses are better than 15 dB. Furthermore, the average transmission losses are 3.5 dB and they change from 3 dB at 9 GHz to 4 dB at 16 GHz. Based on these results, it can be estimated, that the losses for a single pass of the signal through the investigated microstrip ferrite section are about 1.8 dB. The losses are lower by about 1.5 dB in comparison to the results published for a single section

junction from Fig. 8 was designed. The simulation results are shown in Fig. 24.

−20 −18 −16 −14 −12 −10 −8 −6 −4 −2 0

( = /4) T-junction

FCL-section ( = /4)

Magnitude S (dB)

<sup>11</sup> <sup>12</sup> <sup>13</sup> <sup>14</sup> <sup>15</sup> <sup>16</sup> <sup>17</sup> <sup>18</sup> <sup>19</sup> <sup>20</sup> <sup>21</sup> −22

(b)

Frequency (GHz)

S11 S22 S33

2

−20 −18 −16 −14 −12 −10 −8 −6 −4 −2 0

1

configuration in [4].

Magnitude S (dB)

138 Advanced Electromagnetic Waves
