Section III: Measurement and Results
A BSF prototype using the proposed concept is developed on Rogers 4350B substrate. The BSF is connected by two ports, both connected to the defected ground and also to the 50 ꭥ microstrip line as shown in Figs. 8 and 9. The external SMD capacitors are soldered on the excitation slots of the semi-H shaped DGS. The S-parameters are then measured using Keysight E5063A VNA over a frequency range of 0.4 to 0.95 GHz. The measured results are plotted in Fig. 10 and apparently there are three clear resonant frequencies. The measured result is in agreement with the circuit simulation and EM simulation results given in Fig. 7 and therefore validate the tri-band filter design technique.
Fig.8 -> Fabricated Ground Plane of the DGS showing Semi-H Defect
Fig.9-> Fabricated Top Plane of the DGS showing microstrip feed line and ports.
Fig.10-> Measured Results of the Multi-band DGS filter
It can be seen that slight shift in frequencies has taken place and this can be attributed to the presence of the external SMD capacitors. These SMD capacitors help in size reduction and are relatively easy to solder but suffer from poor tolerances. It has been observed that tolerances of even ∓ 0.25 pF can cause significant shift in the resonant frequency. In brief, the shifting of resonant frequencies can be overcome by using lower tolerance capacitors. Furthermore, the rejection in the measured values at the three frequencies is different when compared to the EM and circuit simulation values due to the port,solder and connector losses.