The design offers the flexibility to fine tune all three resonant frequencies independently without altering the original size. A parametric study of several critical parameters is shown in Figures 4 through 6. The resonant frequency ƒ1 is tuned from 5.6 to 5.73 GHz by varying l1 from 20.7 to 26.1 mm. By varying the position of the shorted vias along the radiating aperture, ƒ2 and ƒ3 are tuned (see Figure 4). As shown in Figure 5, ƒ2 is shifted from 6.5 to 6.9 GHz by varying s1 from 8.6 to 10.2 mm. Similarly, as shown in Figure 6, ƒ3 is shifted from 6.8 to 7.2 GHz by varying s2 from 9.3 to 11.1 mm. This shows each resonant frequency can be tuned independently; however, isolation is affected significantly and must be optimized, which can be accomplished with the aid of a commercial simulator. The optimized antenna dimensions are tabulated in Table 1.

Figure 4 S-parameters vs. changing l1.

Figure 5 S-parameters vs. changing s1.

Figure 6 S-parameters vs. changing s2.

FABRICATION AND MEASUREMENT

A prototype of the design was constructed using the dimensions shown in Table 1. Conventional printed circuit board technology was used to fabricate the radiating element and plated-through via holes. A single-layered Rogers RT/Duroid substrate with a thickness of 0.78 mm and relative permittivity of 2.2 was used as the dielectric material. Figure 7 shows the prototype antenna.

Figure 7 Fabricated antenna.

Figure 8 compares the measured S-parameters and antenna gains with the simulations. With port 1 on and the other ports terminated with matched loads, the measured resonant frequency was 5.64 GHz versus the simulated 5.6 GHz. The measured resonant frequency was 6.61 GHz versus the simulated 6.64 GHz with input port 2 on and the remaining ports terminated with matched loads. With input port 3 on and the remaining ports terminated, the measured resonant frequency was 6.97 GHz versus the simulation of 6.94 GHz. Measured port-to-port isolations were better than 23.8 dB at all operating frequencies.

Figure 8 Simulated and measured results: S-parameters and gain.

The measured gains of the antenna at the three resonant frequencies was 4.48, 3.7 and 4.3 dBi, compared to the simulated values of 6.1, 4.9 and 5.0 dBi, respectively. The small differences between the measured and simulated results may be attributed to fabrication tolerances.

Far-field radiation patterns were measured in an anechoic chamber at a distance much greater than λ/2π. Figure 9 shows the radiation patterns in elevation (φ = 0 degrees) and azimuth (φ = 90 degrees) at the measured resonances. The patterns are oriented in the boresight direction.

 
 

Figure 9 Far-field radiation patterns at 5.64 (a), 6.61 (b) and 6.97 (c) GHz.

CONCLUSION

Table 2 compares the results from this design with previously reported work. This design uses a relatively compact structure to provide good radiation performance with excellent isolation. It uses the desirable features of metallic cavity-backed antennas and planar slot antennas, such as low-cost fabrication and ease of integration with other planar circuits.

References

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8. D. Chaturvedi and S. Raghavan, “Low‐Profile Substrate Integrated Waveguide (SIW) Cavity‐Backed Self‐Triplexed Slot Antenna,” International Journal of RF and Microwave Computer‐Aided Engineering, Vol. 29, No. 7, December 2018.
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