Design of a Compact UWB Antenna with Triple Band-Notched Characteristics

S 11 < −10 dB. By inserting two arc-shaped slots in the radiation patch, two sharp bands of 3.3–3.7 GHz and 5.15–5.35GHz are notched. The notch band of 7.25–7.75GHz is achieved by etching a U-shaped slot in the ground plane. The notched bands can be controlled, respectively, while the characteristics of the proposed UWB antenna almost keep completely unchanged at the unnotched frequencies. Equivalent circuit models, surface current distributions, and input impedance are applied to analyze the principle of the proposed UWB antenna. Parametric studies are given. Simulated and measured results show that the proposed antenna has good impedance matching, stable radiation patterns, and constant gain.

In this paper, we propose a simple microstrip-fed UWB antenna with triband notched characteristics.The proposed antenna is simulated and optimized by the high-frequency structure simulator (HFSS).A modified ground with two fillets and three steps is introduced to produce smooth transition from one resonant mode to another as this structure changes the inductance and capacitance of the input impedance.These measures are useful to decrease the discontinuities and the reflections.Hence, the impedance bandwidth can be effectively improved.By etching two arcshaped slots in the radiation patch, the notched bands of 3.3-3.7 GHz and 5.15-5.35GHz are produced.A U-shaped slot is cut in the ground plane to generate the third notched band in 7.25-7.75GHz for the downlink of X-band satellite communication systems.It should be noted that the notched bands can be controlled independently by adjusting the location and length of slots mentioned above.We present equivalent circuit models, surface current distributions, and input impedance to discuss the proposed UWB antenna.Details of the antenna designs are given.The main parameters of the proposed antenna are discussed.The proposed antenna was fabricated and measured with a vector network analyzer Agilent E8363B.Simulated and measured results are given below to illustrate the performance of the proposed antenna.

Antenna Design and Analysis
Figure 1 illustrates the geometry and configuration of the proposed antenna, which is printed on the FR4 substrate with a thickness of 1.4 mm, relative permittivity   = 4.4, and loss tangent tan  = 0.02.A circular patch with a radius of 8.8 mm is printed on the top side of the substrate.The circular patch is connected to the microstrip line.To achieve 50 Ω characteristic impedance, the width of the microstrip feed line is fixed at 2.7 mm. On the bottom of the substrate is a modified rectangular ground plane with two fillets and a stepshaped slot.
The outer arc-shaped slot is introduced to achieve the lower notched band of 3.3-3.7 GHz.The inner arc-shaped slot is used to obtain the middle notched band of 5.15-5.35GHz.To make the design work and discussion much simple, the width and angle of the two arc-shaped slots are set to be 1.4 mm and 60 ∘ , respectively.We use the U-shaped slot in the ground plane to perform the higher notched band of 7.25-7.75GHz band. Figure 2 shows the photograph of the proposed antenna, which is connected to a 50 Ω SMA connector for excitation and measurement.
The main parameters of the proposed antenna are studied by changing one parameter at a time and the others are International Journal of Antennas and Propagation  fixed.Figure 3 shows the effect of the modified ground plane.We can find that the modified ground plane broadens the impedance bandwidth significantly, particularly in the high frequencies.Figure 4 shows the simulated effects of radius  1 of inner arc slot on the simulated  11 of the proposed antenna.It is observed that the lower notch band shifts toward higher frequencies as  1 decreases.The simulated  11 curves of the proposed antenna with different values of radius  2 of outer arc slot are illustrated in Figure 5.We can find that the longer the outer arc slot, the lower the middle notch band.The effects of length  1 on  11 of the proposed antenna are shown in Figure 6.It is found that the higher notch band shifts to the lower frequencies with the increase of length

Results and Discussion
To analyze the principle of the proposed UWB antenna, the surface current distributions at 3.6, 5.2, 7.3, and 8.9 GHz are    shown in Figures 8(a)-8(d).From Figure 8(a)-8(c), we can find that the majority of the currents flow around the arcshaped slots and the U-shaped slot at the notched frequencies, respectively.It implies that the impedance of the radiation patch is quite small, just like about shorted, at the notched bands.It can be seen in Figure 8(d), at a passband frequency of 8.9 GHz, that the currents mainly concentrate on the feed line and the edge of radiation patch, whereas, the currents around the arc-shaped slots and the U-shaped slot are weak.This suggests that the slots do not have a large impact on the proposed antenna performance at the unnotched bands.
For further discussion, the microstrip feed line given the dimensions can be expressed as a transmission line with characteristic impedance  0 (50 Ω).As mentioned above, the proposed antenna is connected to a 50 Ω SMA connector for excitation, so we can select the feeding point as the reference plane.Based on the transmission line theory, the input impedance given the reference plane is where  0 is the characteristic impedance, and Γ is the reflection coefficient.
As is shown in Figure 7, at the notched frequencies, the  11 of the proposed antenna is much higher, and at the unnotched frequencies the  11 is small.That means the reflection coefficient Γ is large (close to 1) at the notched frequencies and small (nearly 0) at the unnotched frequencies.Inserting the Γ in formula (1), the input impedance should be about 50 Ω at the unnotched frequencies and a big difference to 50 Ω at the notched frequencies.Figure 9 shows the input impedance of the proposed antenna.We can find the input impedance changes around 50 Ω at the unnotched band.It is also observed that the input impedance changes greatly at the notched frequencies.This demonstrates the principle discussed above.
It is also observed from Figure 7 that the wide matching bandwidth is the result of several resonances at 4, 6, and 8 GHz and each one can be represented by an RLC circuit.Based on the  11 curves and the input impedance, the radiation patch can be seen as several RLC cells in series at passband frequency.On the other hand, the currents mainly concentrate on the half-wavelength slots and the input impedance is singular at the notched bands.This is equal to reflection coefficient Γ closing to 1 in formula (1).So the radiation patch can be modeled as short circuit at the notched band.The introduced equivalent circuit model is shown in Figure 10.
Figure 11 shows the radiation patterns at 4.5, 6.0 and 8.9 GHz.The antenna displays a good omnidirectional radiation pattern in the H-plane (yz-plane) and bidirectional radiation pattern in the E-plane (xz-plane).The radiation pattern is rather stable.
Figure 12 shows the antenna gain of the proposed antenna.At the notched band, the gain of the proposed antenna drops sharply, which implies the effectiveness of band-notched feature of the proposed antenna.However, the gain keeps stable at the un-notched frequencies.
Time-domain characteristics are also investigated as flat group delay and small signal distortion is a primary requisite for UWB communication systems.In order to obtain time domain characteristics, a pair of proposed antennas is placed face-to-face with a distance of 30 cm.Figures 13(a

Figure 2 :
Figure 2: Photograph of the fabricated UWB antenna.

Figure 3 :
Figure 3: The effect of the modified ground plane.

Figure 4 :
Figure 4: The effect of  1 on  11 of the proposed antenna.

Figure 5 :
Figure 5: The effect of  2 on  11 of the proposed antenna.

Figure 6 :
Figure 6: The effect of  1 on  11 of the proposed antenna.

Figure 7 :
Figure 7: Simulated and measured  11 of the proposed antenna.

𝑙 1 .
Note that the others almost keep unchanged when we change any of the notched bands.This phenomenon suggests that the slots can be controlled independently for the desired notched bands.The optimum parameter values are  1 = 2.8 mm,  2 = 4.6 mm, and  1 = 5.9mm.The simulated and measured results of  11 are shown in Figure7.A good agreement between measured and simulated  11 results is observed.

Figure 8 :
Figure 8: Surface current distributions.(a) The first notched band at 3.6 GHz, (b) the second notched band at 5.2 GHz, (c) the third notched band at 7.3 GHz, and (d) a passband frequency of 8.9 GHz.

Figure 9 : 6 International
Figure 9: Real and imaginary part of the input impedance of the proposed antenna.

Figure 10 :
Figure 10: Equivalent circuit models of the proposed antenna.(a) The first notched band, (b) the second notched band, (c) the third notched band, and (d) the unnotched band.

Figure 11 :
Figure 11: The measured radiation patterns in the H-planes and the E-planes (a) at 4.5 GHz, (b) at 6.0 GHz, and (c) at 8.9 GHz.