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A printed loop antenna is designed to produce three distinct bands to cover six different frequency bands from UWB groups. Loop is fed using a printed BALUN transformer. The first band is available from 3.09 GHz to 4.44 GHz, the second band is available from 6.11 GHz to 7 GHz and the third band is available from 8.85 GHz to 10.5 GHz. The measured results are in good agreement with the simulated results. Frequency band from 3.0 GHz to 4.44 GHz covers UWB BAND-1 and BAND-2, the frequency band from 6.11 GHz to 7 GHz covers UWB BAND7 and the last band from 8.85 GHz to 10.5 GHz covers UWB BAND-12, BAND-13, and BAND-14. Antenna rejects IEEE 802.11 Wi-Fi / WLAN band at 5.8 GHz.
BALUN Transformer, Coplanar Stripline, Loop Antenna, Ultra Wideband, Wavelength.
There is a continuous increase in the demand for more and more band with high data rate; therefore, UWB antennas have dragged attention of antenna researchers. Ultrawideband technology occupies a very large bandwidth and thus ensures the transmission of the larger data rate in the range of Gbps. In order to cover larger bands researchers have proposed different antennas to cover UWB frequency range from 3.1 GHz to 10.6 GHz. An UWB antenna senses all the frequency falling in its band but in order to reject busy channel and receive only free channels, multiband UWB antennas are preferred. Multiband antennas are antennas which generates multiple bands of interest. There can be different approaches to generate multiple bands: a) it is well known that the resonant frequency depends on impedance which in turn depends on the distribution of current in the antenna. Therefore multiband antennas can be designed using current reconfiguration techniques. Antenna current can be reconfigured by using electronic or electromechanical switches. b) Every antenna resonates at the designed fundamental frequency and its multiple frequencies. This approach can be used to design a multiband antenna and is simpler than the approach (a). There are several approaches available in the literature to generate multiple bands in the loop antenna. A 3-D loop antenna with three tuning strips produces five different bands for the application in smartphones. These bands are considered for -6 dB reflections coefficient . A similar loop antenna is proposed to cover GSM 850/900, DCS 1800, PCS 1900, UMTS 2100 and LTE 2300/2500 bands by introducing reconfigurability technique. These bands are generated with the help of RF switch which is connected to reactive elements to influence the bands . Dual asymmetric Loop antennas are integrated to work on WLAN/WiMAX. To have a larger impedance bandwidth a nonuniform loop width is used along with ground traces . Loop antenna with outer SRR and inner strip can be operated as multi-mode and wideband antenna. Outer SRR loop radiates for odd modes and inner Loop radiates for even modes thus the multimode operation is achieved . A loop antenna added with capacitive gaps and a passive strip along with a passive loop of smaller dimension than the main loop improves the gain and bandwidth of conventional loop antenna. By optimizing the gap between the main loop and passive strip and passive loop the upper cut-off frequency can be improved . A small non-planar dual-meander folded loop antenna along with a disc loaded monopole produces omnidirectional radiation pattern with radiation efficiency of 65% and bandwidth of 10.8%. Bandwidth is enhanced due to proximity coupling between meander loop and monopole . All the antennas proposed in these literatures either have complicated 3-D shape of loop or array of the loop and other passive structures are used to generate multiple bands.
It is shown that multiple bands can be achieved using simple printed loop antenna without using the reconfigurable technique. A new folded feed is proposed to feed the balanced loop. This feed can be applied to any balanced antenna. The advantage of the feed is that it reduces onboard space. The proposed multi-band loop is capable to generate four distinct bands which cover six different bands from UWB group and one GSM band when interfaced with broadband printed BALUN transformer. The first band is available from 1.7 GHz to 1.85 GHz and covers upper GSM. The second band is from 3.09 GHz to 4.44 GHz and covers BAND-1 and BAND-2. The third band ranges from 6.11 GHz to 7 GHz and covers BAND-7, the fourth band ranges from 8.85 GHz to 10.5 GHz and it covers BAND-12, BAND-13, and BAND-14. It is also found that the antenna rejects IEEE 802.11 Wi-Fi / WLAN at 5.8 GHz. Complete analyses of the current distribution along the loop reveal that at higher frequencies antenna current does not remain uniform and the radiation pattern fluctuates. Therefore the proposed antenna cannot be used for all the six bands at a time but can be used only for three different bands at a time i.e. from 3 GHz to 7 GHz together and from 8.85 GHz to 10.5 GHz together as the second group. To improve the antenna performance to have constant radiation pattern, RIS (reactive impedance surface) or other metamaterials can be introduced along with the proposed loop antenna.
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