In recent years, Ultra wideband antennas become more notable in wireless communication system because of its appealing advantages, such as high data rate, large bandwidth of 7.5 GHz, low power consumption, low cost, and its uses for short range data and voice communication. On February 14, 2002 Federal Communication Commission unlicensed the Ultrawideband frequency range from 3.1- 10.6 GHz, bandwidth of 7.5 GHz and emission limit of -41.3 dbm.

Ultra wideband communication system coexists with some narrowband communication services like WLAN, WIMAX, etc., which causes unenviable inter ference in Ultrawideband system. To mitigate this problem, filters can be used with Ultra wideband antenna, but this idea increases the cost of Ultrawideband antennas, therefore antennas with the embedded property of filter is suitable for attenuation of unwanted frequencies for Ultra wideband communication.

There are different techniques used for removing unwanted frequencies, such as embedding slots, Split Ring Resonator (SRR), parasitic stub, Complementary Split Ring Resonator (CSRR) in radiating patch of antenna, Electromagnetic Bandgap (EBG) structure, and defected ground plane structure.

UWB communication system become prominence because of its characteristics therefore the requirement of UWB of antenna design also enhances. UWB antenna is categorized in two ways-

• Omidirectional Ultra Wideband Antenna.
• Directional Ultra Wideband Antenna [9].

Omnidirectional antenna radiates electromagnetic wave in all direction whereas Directional antenna radiates energy in a particular direction. Cost of directional Antenna is higher than the Omnidirectional antenna. Generally used UWB antenna are Omnidirectional antenna such as Planar Monopole Antenna Printed Monopole Antenna as shown in Figure 1.

Figure 1. Circular Planar Monopole Antenna [2]

Simplest structure of UWB monopole antenna is square monopole antenna. In Figure 2, beveling is done for impedance matching.

Figure 2. Square Monopole Antenna [13]

Printed planar monopole antenna is also a type of Omnidirectional UWB Antenna which is more compact than that of planar monopole antenna.

There are another types antenna, directional UWB Antenna such as printed-wide slot antenna and dielectric resonator antenna.

Ultra wideband antennas with bandstop characteristics is a good solution to reduce the effect of unwanted frequencies in wireless communication system or to reduce the interference effect from narrow band. Commonly used bandstop techniques are embedding slot, use of parasitic stub, and use of resonator as shown in Figure 3.

Figure 3. CSRR for Bandstop Characteristics [14]

Etching slot on the radiating patch or on the ground of antenna is the simplest method. The slots which are generally used are U-shaped, Y-shaped, or nested Cshaped. These structures have a length of λ/2, calculated at resonant frequency. Likewise, slot method stub is placed in the aperture area of antenna or nearby radiator, which forms a resonant structure is given in Figure 4. A line resonator is used generally in CPW fed antenna which acts as a band stop filter. Hence it rejects the undesired band within the pass band of antenna.

Figure 4. E-Shaped and U-Shaped Slotted Antenna [8]

Band stop properties for WLAN (5-6 GHz) and WIMAX (3.3- 3.6 GHz) is achieved by embedding C-shaped slot in radiating patch [12]. So its easy to tune notch frequency by adjusting the total length of C-shaped slots. In slot method current is concentrated around the edge of slot and the direction of current is directed opposite which results in attenuation at resonant frequency. Defected ground plane structure is used to mitigate the effect of unwanted frequencies 5.2-5.35 GHz, 5.8-6.0 GHz, and 3.7-4.2 GHz for designing UWB planar monopole antenna [12]. Two L-shaped slots placed in CPW ground and tuning stub in the middle of fork like patch removes the undesirable frequency[8]. Length of slot and stub is usually depending on the resonant frequency of antenna.

Two U-shaped slot in radiating patch and H-shaped slot in the CPW ground is used to mitigate the WLAN (5-6 GHz) and X- band (7.7-8.5 GHz) [12].

(1)

In above equation, F_notch is the resonant frequency of antenna c is the speed of light, L is the length of stub, and , ∈_eff dielectric constant[9].

Closed loop ring resonator is used to achieve triple stop band frequency for 3.3-3.7 GHz, 5.75-5.825 GHz, and 5.15- 5.35 GHz [11].

(2)

In above equation, F₀ is the resonant frequency, c is the speed of light, D is the mean diameter of ring, and ∈_eff is the effective dielectric constant[2].

Electromagnetic Band Gap (EBG) can also be used to eliminate undesired frequency by placing this structure close to the feed line. Edge located via mushroom type band gap structure produce high rejection band in comparison of Conventional mushroom type bandgap structures [3].

Removal of undesired frequency is done by placing pair of SRR on the opposite side of CPW fed circular monopole antenna. Mitigation effect comes into picture due to strong magnetic field coupling effect between propagating electromagnetic signal and SRR[5]. The SRR circuit acts as a cascaded LC parallel circuit which gives high input impedance corresponding to input signal at its resonance frequency, hence causing reflection. Here a band stop techniques table is given, as in Table 1.

Table 1. Different Techniques of Bandstop Characteristics

This study is concluded with the different techniques used for developing bandstop characteristics in UWB antenna, which is advantageous for removing the undesired frequency in UWB communication system. Simplest method is to insert parasitic stub and embedding slot in the radiating patch or in ground plane of the antenna. Different types of antenna studied in this paper mainly focuses on the removal of narrow band services like WLAN, WIMAX, Xband, etc. Using different techniques like embedding slot, parasitic stub, and spilt ring resonator band-stop characteristics is achieved without compromising the performance of antenna.