Radiation pattern of reconfigurable antenna design for portable device applications

Today, wireless technology is the main areas of research in the world of communication systems and communication system study is incomplete without an understanding of the fabrication and operation of antennas. This is the main reason for fabricating antenna. The area of antenna study is a huge and vast, so, grasping the fundamentals, two pronged approach by dividing the process into steps as follows are adopted. One is the design and optimization of antenna–mechanical/geometric structures, in particular, the orientation and shape of the radiators, the feeding network, as well as loading. This conventional approach most often employed in antenna design. Top-loaded dipole antennas, Inverted-F antennas, and slotted planar antennas all fall into this category. The use of non-conducting material - Antennas loaded with high-permittivity or ferrite dielectric materials (for instance, ceramics) are examples of this type of technology, as is the dielectric resonant antenna. The application of special fabrication processes - The fabrication of low-temperature co-fired ceramics printed circuit boards has made co-planar and multiple-layer microstrip patch antennas popular. Such technologies are conducive to the mass production of miniaturized antennas at low cost. The portable mobile devices will include - mobile phone handsets; RFID tags; laptops with embedded wireless local area network (WLAN) access points; medical devices for microwave thermal therapy; sensor installed on or above the human body; and ultra-wideband (UWB) based high-data-rate wireless connectors such as the wireless USB dongle. All of these portable mobile devices are widely used. The antennas used in them have become a bottleneck in the miniaturization of portable devices in terms of performance, size, and cost. The increasing design challenges have made the antenna design for portable devices much more critical than before.

Antenna having rectangle shape and rectangle shape with semi circle at top offered -18dB Return loss, -15.7dB to -31.4dB Insertion loss, -25dB Isolation and Omni directional radiation pattern using Varactor diodes and PIN diodes as switch for frequency reconfiguration (Husseini et al., 2013). In ref. (Munir and Ludihargi, 2013) a simple circular microstrip patch antenna with insertion loss of 1.87dB and 200MHz bandwidth is noted. S. Singh et al. (2013) presented Reconfigurable circular patch antenna having -10dB return loss and 1GHz Bandwidth using PIN diodes for polarization reconfiguration. A meander antenna with 0 to -5dB return loss, -30dB to -20dB Insertion loss, 2:1 SWR, Gain of the order of 1.1dBi, 95MHz and Omni directional radiation pattern using copper link for switching purpose for polarization reconfigurable antenna is obtained here (Raman et al., 2013). Helix antenna with-10dB to -14dB Return loss, 6.2dB gain, 10% of resonance frequency and isotropic radiation pattern with polarization reconfiguration is studied (Alaa et al., 2014). In this paper, Meander shaped dipole antenna used which offers -15dB Return loss, 2.21dBi Gain and 9.95% Bandwidth is noted. To achieve these parameters PIN diode for switching is used. Polarization and Frequency reconfiguration is shown here (Husna et al., 2014). L shaped antenna with -28dB to -30dB Return loss, 25dB Isolation, 3dB Insertion loss and 196MHz Bandwidth is obtained in ref. (Sodre et al., 2014). Dipole antenna divided into slots for frequency variation and offers -20dB Return loss, Gain of 2.82 to 4.11dBi and Radiation pattern same as Dipole antenna. Using PIN diode slot size changed to get Frequency Reconfiguration (Borakhade and Pokle, 2015). E shaped antenna shifting is carried out using Photo conductive switches. With this technique Return loss of -15dB, 12.5dB Gain and Omni directional Radiation pattern is obtained (Kang and Juang). Rectangle antenna with Return loss -7dB, Insertion loss 0.7dB, Gain 8.1dBi and 850MHz Bandwidth is reported in ref. (Adami et al., 2018). Pentagon Antenna for frequency reconfiguration is used here. Return loss of -15.8dB and -18.5dB, SWR between 1.06 to 1.26, Gain of 2.14dBi, 290MHz and 370MHz and Omni directional radiation pattern is reported in ref. (Sahu, 2015) with help of Loop and Beam steering antenna is used for Beam variation using PIN diode. Return loss under 6dB, SWR less than 3, Gain between 5.9 to 6.6dBi, Bandwidth of 130MHz and Omni directional radiation pattern is obtained in (Takacs et al., 2015). In this paper polarization reconfiguration is obtained using square shaped antenna and PIN diode for switching purpose. Return loss -18.69dB, Insertion loss -14dB, SWR 1.03, Gain 6.5dB, Bandwidth of 32.5MHz and Isotropic radiation pattern is reported (Trong et al., 2015). Helix antenna with -6.2dB Return loss, 5.9dBi Gain and 123MHz Bandwidth is reported in (Cai et al., 2016). Polarization shifting from circular polarization to linear polarization with Varactor diode (Switch) and using Square shaped antenna is reported here. In this work -9.8dB Return loss, Gain 7.3dB, 40%Bandwidth and Omni directional radiation pattern is obtained (Chatterjee and Parui, 2016). Polarization reconfiguration is obtained using Slotted circle in this paper. PIN diodes are used for polarization diversity. In this paper Return loss of -10dB, Gain of 2.5dBic, Bandwidth 19.8% and Conical shaped Omni directional radiation pattern is reported (Gao et al., 2016). Monopole antenna with Return loss < -10dB, Gain 12.5dBi, Insertion loss <-5dB, Bandwidth -10dB (38%) and Omni directional radiation pattern is noted (Heydari et al., 2017). Polarization variation with the help of Triangle connected in circular shape is done here. The return loss >-10dB, Isolation >20dB, large Bandwidth of 400MHz and Omni directional radiation pattern is reported. The reconfiguration is obtained by changing feeding technique to antenna (Bharathi et al., 2017). Frequency Reconfiguration of circular patch antenna and PIN diodes for switching is used. Return loss -5dB, SWR <2, Gain 16dBi, Omni directional radiation pattern and Bandwidth 1.2GHz is obtained using this technique. Polarization and frequency reconfiguration using single Square patch antenna is obtained in this paper. 6 PIN diodes are used for this purpose. Return loss <-10dB, Gain 6.3dBi, 60MHz Bandwidth and Omni directional radiation pattern is obtained in ref. (Kadam and Kulkarni, 2015).

A number of factors affect antenna performance in hand held i.e. portable computing devices in wireless communication. Antenna performance is defined or accounted by many parameters. Above survey shows that the researchers have considered parameters like return loss, gain, impedance, isolation, insertion loss, radiation pattern and SWR for evaluating their antenna. But the effect of cover is also important for measuring parameters as all portable devices are now a day integrates antenna inside the box.         Due to this it is important to study the effect of different covers on antenna performance and also on the radiation pattern of antenna which is important parameter for evaluation. The aim of this paper is to test and compare the radiation pattern of reconfigurable antenna. Here frequency and polarization reconfiguration is used for evaluation purpose. In the laboratory, initially measure radiation pattern of polarization and frequency reconfigurable antenna without any cover. In next step plastic, paper and cloth are used as cover for both antennas. By changing the cover of antenna, radiation pattern is measured in laboratory with the help of antenna measurement kit. The objective of this process is to find the effect of cover on radiation pattern of reconfigurable antenna and also observe the effect on radiation pattern due to change in reconfiguration parameter from frequency to polarization.

Table1 shows the parameters of Meander antenna. Number of meander sections can be calculated using Eq. 1.

$A=\frac{Z0}{L}\sqrt{\frac{\varepsilon r+1}{2}}+\frac{\varepsilon r-1}{\varepsilon r+1}\left( 0.23+\frac{S}{\varepsilon r} \right) $                     (1)

where, A= Number of folds.

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Figure 1. Methodology

The antenna is designed, simulated, optimized and results are verified using HFSS. The optimized parameters used to fabricate antenna as shown in Figure 2 with FR4 as a dielectric and Tin plating used for tracks. The basic need of portable device antenna is antenna should be small size. The proposed antenna design is a miniature in size with performance stability. This is achieved by co axial feed along with capacitive de coupling structure. 6PF/16V NPO SMD capacitor connected to give good ESR (Equivalent Series Resistance) at microwave frequency. Another requirement of portable device antenna is Bandwidth. Portable device has a limited bandwidth. In many applications, bandwidth up to 25MHz is observed and focus is given on increasing their band efficiency but not bandwidth. The designed antenna satisfies existing channel Bandwidth (130MHz) requirement. Along with these parameters, Impedance, far field requirement and constraints i.e. in which device antenna is placed, (the adjacent devices and circuitry) antenna performance should not degrade due to vicinity of other components. As the direction of signal coming to antenna is not fixed, the radiation pattern of antenna should be omni directional whereas for point to point communication, directional pattern with high directivity is preferred. In case of portable computing devices, omni directional radiation pattern is expected which is obtained from the designed antenna. In portable devices, co axial feed is not feasible to antenna because as per size the feed itself is bigger which can deteriorate signal due to coaxial pin reactance and losses that are taking place in the cable. So inset fed arraignment is done. As PCB mountable compact feed, it gives good performance whereas if feed size is large, performance gets deteriorated. Thus antenna co axial feed can be easily converted into inset feed to achieve high performance.

Table 1. Parameters of meander antenna

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Figure 2. Fabricated antenna structure

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Figure 3. Reconfigurable antenna with Raspberry Pi

Another need of reconfigurable antenna is to carry fast switching between antennas to match high frequency signal. The PIN diode (HSMP3864) is used for switching purpose. The operating frequency of this diode is 3.4GHz, 100nS switching time (Current and Capacitance between two plates will decide switching time). For testing purpose, and to observe the switching signal on DSO for analysis, the cycle period 100 Sec is adjusted in the program and later it can be changed to achieve high switching speed. Initially reconfigurable antenna is fabricated with gap and to switch between two antennas copper strip used where switching is carried out manually. In later step, PIN diode is connected in the gap and auto switching is done. Two separate PIN diodes are used for the purpose and for switching purpose, required square wave obtained from Raspberry Pi as shown in Figure 3. Python is used to program the board. In future when the overall system is transferred to SoC (System on chip) antenna, Raspberry can give a better solution.

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Figure 4. Geometry of proposed antenna

The overall dimensions of antenna are such that the whole space is utilized properly as shown in Figure 4. Initially no gap is added at the point of feed and antenna is simulated. In further step, RF switches (PIN diodes) are added for reconfiguration purpose, which can be fitted in the top right corner, on the same PCB in the fabrication process. The length and width of substrate is 0.30λ and at 2.4GHz (λ = 76mm) i.e. the size of the substrate is selected as 23mm × 23mm. The feed line strip has width of 6mm. The overall length of monopole antenna is 30.76mm and 2 folds are used to fit it into given size of board as shown in Figure 4(a). The other side of Board contains 10mm × 10mm ground plane as shown in Figure 4(b).

In this paper, in case of frequency reconfiguration, radiation pattern is less affected. But if the signal is suffering from noise at a particular frequency, then one has to shift to lower band because lower band is more immune to noise. Polarization reconfiguration is suggested than frequency reconfiguration as the antenna for portable device is suffered from noise and strength of signal (Generally it is weak). If noise has to be avoided, then frequency band has to be shifted but then lower band is already crowed. Whereas polarization reconfiguration employs shift from vertical to horizontal or vice versa which result to overcome both problems at same frequency. Even in WiFi module and other wireless devices polarization change is suggested to improve the performance. Frequency reconfiguration utility is less useful than polarization reconfiguration. That is why when noise is more polarization plane change is best. Frequency reconfiguration change band is not allowed. In a particular band polarization reconfiguration is allowed. No money for polarization is charged. But if one needs to shift from 1 band to other, he has to pay the service provider. In wireless communication, 802.11, 802.11a, 802.11.b, 802.11g, and 802.11n protocol, polarization change allowed. Thus, it is observed that the plastic cover is better for antenna to avoid deterioration in radiation pattern.

Conflict of interest: The authors declare that there is no conflict of interests regarding the publication of this paper.