A Compact Dual Band-notched Circular Ring Printed Monopole Antenna for Super wideband Applications

In this article, a simple and compact dual bandnotched (DBN) super wideband (SWB) printed monopole antenna (PMA) has been proposed. The proposed antenna composed of a circular PMA, which is connected through a 50-Ω triangular tapered microstrip fed line (TTMFL) and a round-cornered finite ground plane (RCFGP). It exhibit s a very wide frequency band from 1.6–25 GHz (ratio bandwidth of 15.63:1) with a voltage standing wave ratio (VSWR) ≤2. By employing a U-shaped parasitic element (USPE) near the RCFGP and a T-shaped protruded stub (TSPS) inside the radiating patch, a single band-notched (SBN) characteristic in the frequency band of 3.2–4.4 GHz (WiMAX/C-band) is generated. In order to realize the second band-notched function for X-band satellite communication systems (7.2–8.4 GHz), a U-shaped slot (USS) has been inserted in the RCFGP. The overall dimension of the proposed antenna is 24×30×0.787 mm and occupies a relatively small space compared to the existing DBN antennas. Good agreement has been attained between predicted and measured results.


1.Introduction
Now-a-days there are increasing demand of super wideband (SWB) radios in the modern wireless communication systems, owing to its extremely large bandwidth (BW) and very high data transmission rate.Printed monopole antenna (PMA) is an ideal candidate for SWB applications due to its several fascinate features such as small size, low cost, planar structure, operation over extremely large impedance BW and easy to accommodate with small space provided by hand held gadgets .Existing ultra wideband (UWB) PMA (BW of 3.1 to 10.6 GHz) [1] also have features like that of SWB except that its ratio bandwidth (RB) fall short of SWB antennas and which is just 3.42:1.SWB antennas should offer RB of more than 10:1 and was first supposedly developed by Rumsey et al. in the late 1950 and early 1960 and was called frequency-  independent antennas [2].
Recently, few SWB antennas have been investigated by many researchers in the published literature [3]- [7].Unfortunately, it is to be noted that some of the existing narrow band systems such as WiMAX (3.3-3.6 GHz), Cband (3.7-4.2GHz) and X-band satellite communication systems operating in 7.25-8.395GHz (for down link: 7.25-7.745GHz and uplink: 7.9-8.395GHz) may cause electromagnetic interference (EMI) to the SWB system.So, to mitigate this electromagnetic interference issue, SWB antennas with band-notched characteristics is required.Several dual band-notched (DBN) UWB antennas have been investigated by many researchers across the globe [8]- [13].
However above mentioned antennas have been designed to have notch within the UWB BW and occupies a relatively large space.But to author's knowledge, there are no DBN SWB antennas reported in the literature.In this communication, a novel and compact DBN SWB PMA is presented.The designed antenna offers a wide impedance BW from 1.6 to 25 GHz with a VSWR ≤ 2 except in the DBN of 3.2-4.4GHz and 7.2-8.4GHz.The proposed antenna offers wide BW with DBN characteristics and is small in size compared to the DBN UWB antennas reported in [8]- [13].By tuning the dimensions of U-shaped parasitic element (USPE) and U-shaped slot (USS), two suitable notches are obtained.Good agreement has been obtained between simulation and experimental results of the proposed DBN SWB PMA.Simulation results have been carried out with the finite element method (FEM) based commercial software Ansoft high-frequency structure simulator (HFSS).

2.Antenna Configuration
The configuration of the proposed DBN SWB circular ring (CR) PMA is illustrated in Fig. 1.The CR PMA is printed on the RT/Duroid 5870 substrate of thickness (t) = 0.787 mm with dielectric constant (  ) of 2.23, which is excited through a 50-Ω triangular tapered microstrip fed line (TTMFL).For the signal transmission, a two hole Sub Miniature version A (SMA)-connector has been utilized.We have specifically chosen the above mentioned substrate since FR4 substrate becomes highly lossy at higher frequencies.The proposed antenna (overall dimension of about 24×30×0.787mm3) consists of a CR radiating patch with a T-shaped protruded stub (TSPS) inside CR radiating patch.The rear side of the RT/Duroid substrate composed of a round-cornered finite ground plane (RCFGP) with embedded USS and USPE, which is placed under the CR patch.The TTMFL is used for broadband impedance matching and the reflection coefficient (|Γ(θ)|) response of triangular taper has minima for βl = 2mπ where m = 1, 2, 3,… and corresponding frequencies.A closed form solution of the Riccati equation for triangularly tapered feed line is given by [14] where Γ ( θ ) is the reflection coefficient, β is the phase constant and   = H is the triangular tapered feed line length.To get the widths at the two ends of the tapered lines, method reported in [15] is followed.

Results and Discussions
Fig. 2 presents design evolution of the proposed SWB DBN PMA which illustrates the steps for enhancing the impedance BW as well as formation of DBN characteristics.The corresponding simulated VSWR characteristics comparison of Antennas 1-5 are shown in Fig. 3. Antenna 1 consists of simple monopole disc connected with a rectangular feed line and rectangular finite ground plane that provides BW of 2.8-9.9GHz (refer to Fig. 3).To improve the impedance BW of the Antenna 1, rectangular feed line has been replaced by a triangular tapered microstrip fed line (TTMFL) in Antenna 2 and it offers wide BW of 1.6-23.5GHz without increasing the dimension as depicted in Fig. 3.
To further increase the BW of the Antenna 2, we modified the rectangular ground plane of the Antenna 2 into round-cornered finite ground plane (RCFGP) in Antenna 3. In Antenna 3, ground plane also participates in radiation [16]- [17], additional resonance is excited at the higher frequencies and hence the impedance matching characteristics tend to improve over the entire band, resulting in an extremely large BW of 1.To know more details about dual band-notched functionality of the proposed SWB antenna, parametric study has been carried out.Dual band-notched characteristics of the SWB antenna can be controlled mainly by the following parameters such as R, W p and L s for the radiating ring patch, U-shaped parasitic element and U-shaped slot respectively.Fig. 5 depicts the simulated VSWR curves for different values of outer radius R of circular ring patch.It can be observed that by increasing the value of R from 8 to 9 mm, the first notch band blocks deeper (amplitude of VSWR is about 6.5) within WiMAX/C-band, however center frequency of the second notch band (X-band) is shifted from 7.8 to 7.4 GHz and also there is an impedance mismatch at higher frequencies (12-13 GHz and 16.5-18 GHz).When the position of R = 7 mm, the impedance matching within SWB region is excellent, the center frequency of the first (WiMAX/C-band) and second (X-band) notched band is also appropriate.So the optimum value of R = 7 mm.The simulated VSWR for different values of W p (width of U-shaped parasitic element) is plotted in Fig. 6.In this arrangement by varying the value of W p from 13.5 to 15.5 mm, center frequency of the second (Xband) notched band characteristics is in sensitive; however bandwidth of the first notch band (WiMAX/C-band) is decreased significantly.Therefore, the optimum value of W p is decided to take 14.5 mm.Fig. 7 shows the effect of the length L s of U-shaped slot on the simulated VSWR curves.From the Fig. 7 it can be found that, when the length L s of U-shaped slot changes fro m 4.7 to 5.7 mm, the center frequency of the second notched band is varied from 9 GHz to 6.5 GHz, however there is no variation within the first notched band region.Therefore the value of L s has been selected to be 5.2 mm, which controls the variable notched band with center frequency of 3.8 GHz.    8 (a) it is clearly seen that high current is gathered on the USPE around the 3.8 GHz notched frequency and they are opposite in direction (out-of-phase) to the current flowing on the CR patch as well as inverted TSPS, which cancel out the effective radiation.Similarly, at frequency 7.8 GHz, it can be observed that strong current is crowded on USS and very small current is concentrated on the ground plane, which offers strong attenuation near the notch frequency (7.8 GHz).It can be observed that small surface currents are flowing on both USPE and USS element at other frequencies such as 7 GHz and 10 GHz as depicted in Fig. 8(c) and (d) respectively.
Image of the fabricated antenna is shown in Fig. 9(a).The measurement is carried out with a Rohde and Schwarz ZVA24 vector network analyzer.Simulated and measured VSWR versus frequency graph for the proposed DBN SWB antenna is depicted in Fig. 9(b).Good agreement has been obtained up to 19 GHz between simulation and experimental VSWR results, while small deviation can be observed between simulated results and measured one as  frequency higher than 19 GHz.This may be owing to the maximum suitable frequency for the SMA-connector is up to 18 GHz.Fig. 10 shows the simulated and measured gain radiation pattern of the proposed SWB PMA.It indicates that at low frequencies (2.5 and 10 GHz), antenna has omni-directional radiation pattern in the H-plane and bidirectional (eight shaped) patterns in the E-plane with negligible cross polarization.However, at the higher frequencies 15, 20 and 25 GHz, the cross-polarization level rises in the E and H-planes, which is probably owing to the rising horizontal component of the surface currents on the radiating patch at higher frequencies.The simulated efficiency () of the antenna varies from 82% -95% throughout SWB frequency except at the DBN frequency (1 st BS  ≅ 65% and 2 nd BS  ≅ 45%).The simulated and measured peak gain versus frequency graph for the proposed DBN SWB PMA is shown in Fig. 11.From the graph, it can be observed that the peak gain of the proposed antenna is 5.8 dBi, while gain sharply drops in the vicinity of 3.8 and 7.8 GHz BS.Table I depicts the dimension and RB of the proposed DBN SWB antenna with existing DBN UWB antennas.It can be observed that our proposed antenna has significantly larger RB than the antennas present in the published literature [8]- [13].It also has smaller size than the DBN UWB antennas [8]- [11] except that of [12] and [13].Note that DBN antennas of [12]- [13] are designed on FR4 substrate which are meant for low frequency applications up to 2 GHz [18].FR4 substrate also has higher relative permittivity of 4.4 and size also depends on relative permittivity.

Time Domain Performance
The Time domain performance is the significant characteristics of UWB antennas in which that is required to have least distortion in transmitting and receiving scenarios.To explore the time-domain characteristics of a     band-notched SWB antenna, two identical antennas were employed as transmitter and receiver.They are estranged from each other by a distance of 60 cm and placed in faceto-face and side-by-side orientations.To act as a transmitter, one antenna is excited by a time-domain transient pulse, while the other one is a receiver.
A 5 th order derivative Gaussian input pulse waveform (generated in Tektronix AWG 7122B arbitrary signal generator) in the time domain with a width of 300 ps and its power spectral density in frequency domain (dBm/MHz) with reference to federal communication commission (FCC) spectral mask are displayed in Fig. 12 and Fig. 13.The UWB short pulse reported in [19] is used to excite the proposed band-notched antennas is a fifth-order derivative of the Gaussian pulse and is given by where A is the amplitude parameter, t is the time, and σ is the standard deviation, whose value is 51 ps to ensure that the shape of the spectrum fit inside the FCC spectral mask.The measured results suggest that the chosen fifth derivative of Gaussian pulse fits into the FCC spectral mask reasonably well, indicating that this UWB pulse complies with the FCC spectral mask.
Figs. 14 and 15 plot the received pulse for face-toface and side-to-side configurations.It is seen that ringing distortion is observed in both the cases face-to-face and side-by-side scenarios, owing to the impedance mismatching at the notched bands.This confirms that the antenna does not response at all at the band-notched region.Moreover, it is also observed that the received signal amplitude is higher when two identical antennas are placed face-to-face (200 mV) with respect to side-by-side (100 mV) orientation.Overall ringing distortion is better than the existing UWB antennas in the literature [20].To authenticate the similarity between received pulse  2 (  ) and transmitted or excited pulse  1 (  ) of the proposed antenna, the fidelity factor  is calculated which is given by equation 3 where,  is a delay which is varied to make numerator in (3) a maximum.Using equation (3), the fidelity factor is calculated to be 0.8521 when the two identical antennas were placed face-toface scenario, and the value becomes 0.8132 when they were placed side-by-side scenario.It has been reported that the fidelity factor for UWB antenna [8]

Fig. 4 .
Fig. 4. Simulated VSWR versus frequency graph for the proposed compact DBN SWB PMA with and without inverted T SPS.

Fig. 5 .
Fig. 5. Simulated VSWR versus frequency graph for the proposed compact SWB dual band-notched antenna with different value of R.
6-25 GHz or above.It is capable of supporting SWB radios without increasing the antenna dimensions.Now to generate the first band-stop (BS) characteristics for X-band satellite communication systems (7.2-8.4GHz), a U-shaped slot (USS) is inserted in the RCFGP (Antenna 4).By employing a U-shaped parasitic element (USPE) on the rear side of substrate and an inverted T-shaped protruded stub (TSPS) within the circular ring (CR) shaped radiation patch we generate the second BS characteristics for WiMAX/C-band (3.2-4.4GHz) application (Antenna 5).The effect of inverted TSPS on the WiMAX/ C-band (3.2 -4.4 GHz) and X-band satellite communication systems (7.2-8.4GHz) is shown in Fig. 4. It is found that without inverted TSPS, the upper edge frequency of WiMAX/ C band increases from 4.4 GHz to 4.9 GHz, while upper edge frequency of the X-band satellite communication systems decreases from 8.4 to 8.1 GHz, which do not cover the uplink frequency (7.9-8.395GHz) of X-band satellite communication systems.The widened frequency BW of WiMAX/ C band notched characteristics (3.2-4.9GHz) unnecessarily blocks useful frequencies from 4.4 to 4.9 GHz.

Fig. 6 .Fig. 7 .
Fig. 6.Simulated VSWR versus frequency graph for the proposed compact SWB dual band-notched antenna with different value of

Fig. 9
Fig. 9 (a).Image of the fabricated proposed DBN SWB PMA (b) simulated and measured VSWR versus frequency graph.

Fig. 8 (
Fig.8(a) and (b) illustrates the simulated surface current distribution for the proposed SWB PMA at bandnotch center frequencies of 3.8 and 7.8 GHz.From the Fig.8(a) it is clearly seen that high current is gathered on the USPE around the 3.8 GHz notched frequency and they are opposite in direction (out-of-phase) to the current flowing on the CR patch as well as inverted TSPS, which cancel out the effective radiation.Similarly, at frequency 7.8 GHz, it can be observed that strong current is crowded on USS and very small current is concentrated on the ground plane, which offers strong attenuation near the notch frequency (7.8 GHz).It can be observed that small surface currents are flowing on both USPE and USS element at other frequencies such as 7 GHz and 10 GHz as depicted in Fig.8(c) and (d) respectively.

Fig. 10 .
Fig. 10.Simulated (red) and measured (black) gain radiation pattern of DBN SWB PMA (solid line is co-polarization and line is cross-po lar iz atio n) .

Fig. 11 .
Fig. 11.Simulated and measured peak gain versus frequency graph for the proposed compact DBN SWB PMA.
is 91.2 % for face-to face-link.