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The number of applications utilizing ultrawideband (UWB) electronic systems has increased, but these applications all rely upon effective antenna designs. For practical use, UWB antennas should also coexist with narrowband applications within the UWB frequency range, and this can be done via the inclusion of band notches within the antenna’s UWB frequency coverage range.

Fortunately, an equivalent circuit has been developed for an UWB antenna with a pair of frequency notches to minimize interference from narrowband applications. The equivalent circuit was used to produce a printed-ellipse-slot (PES) UWB antenna with a frequency range of 3.0 to 10.8 GHz, in addition to band-notched functions at 3.30 to 3.89 GHz and 5.6 to 6.1 GHz.

Equivalent circuit

These band-notched functions can be realized by adding two “braids” to the antenna design—braids which are equivalent to two series inductance-capacitive (LC) resonances in the antenna circuitry. The notched bands can be used to cover working bandwidths of 5.725 to 5.850 GHz for wireless local area networks (WLANs) and 3.30 to 3.70 GHz for WiMAX, minimizing interference from those applications for UWB systems.

UWB systems are gaining attention for their capabilities to support high transmission rates at low power levels and small-sized circuits.1,2 Because of existing applications operating within the UWB frequency range, such as WLAN and WiMAX systems, the interference from these more narrowband applications must be resolved before UWB circuits can be used. One possible solution is the addition of a bandstop filter to the UWB system, although the filter adds complexity. A more practical approach is the use of an antenna with band-notched functions.

Various methods can be employed to realize band-notched functions in an antenna with UWB coverage. These include adding parasitic elements,3-5 using matching units,6 working with a slot antenna,7-11 and using a fractal antenna.12 Methods for achieving band-notched functions in an antenna are based on adding a parallel or series LC resonator to the antenna.

VSWR characteristics

For a slot antenna with band-notched functions, the slot in the radiating unit serves as a parallel or series LC resonator.13 Adjusting the spectral position of the band notch involves changing the size of the slot. Also, the A band-notched antenna function can also be realized by means of split-ring resonators, although this requires a typically artificial electromagnetic (EM) structure.

As a practical solution, a wideband planar monopole antenna was developed with two braids to achieve the two band notches. The approach yields effective results without adding undue complexity to the antenna. To reinforce the validity of the approach, simulation results based on the High Frequency Structure Simulator (HFSS) electromagnetic (EM) simulation software from Keysight Technologies (formerly Agilent Technologies) will be presented along with experimental results.

The basic principle of a dual-band notched antenna is somewhat equivalent to adding two bandstop filters to an antenna. In the process of designing such an antenna, the bandstop filters can be alternated by using special structures, much like the braids described in this report. The braids within the antenna play a role much like filters to reject two target frequency bands.

The proposed antenna

The antenna operates in a transmission-line-like mode. The UWB antenna and feeder are equivalent to a 50-Ω load in the equivalent circuit, with two simple series LC branches added to the circuit (Fig. 1). Using the Advanced Design System (ADS) simulation software from Keysight to obtain simulation data (Fig. 2) for use as a reference, the results shows that two obvious band-notched functions are realized at 3.6 and 5.7 GHz, when L1 = 28 nH, C1 = 0.292 pF, L2 = 3.567 nH, and C2 = 0.586 pF.

The simulation results confirmed that adding two series LC resonances to the antenna circuit is a feasible way to obtain the dual-band-notched characteristics. The LC resonances can be realized by means of special structures, but a number of steps should be taken beforehand. First, to confirm whether a special structure will provide the desired resonance, the input impedance of the proposed antenna should be well known. According to resonance theory, the imaginary part of the input impedance should have two downward trends at the notch frequency points.

Second, the notch frequency spectral responses should be somewhat adjustable, which requires that the structures used for the resonances should be isolated and adjustable independently. Finally, any added structures should not impact the directional and/or impedance-matching characteristics of the original UWB antenna design.

Based on this analysis, an UWB antenna with standard band-notched functions at 3.3 to 3.89 GHz and 5.6 to 6.1 GHz was developed (Fig. 3). It was fabricated on high-frequency substrate material with dielectric constant of 4.5, loss tangent of 0.0035, and thickness (h) of 1 mm. The antenna features an elliptically shaped radiating patch.

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