WO2006042562A1

WO2006042562A1 - Compact single feed quad band antenna for wireless communication systems

Info

Publication number: WO2006042562A1
Application number: PCT/EG2004/000042
Authority: WO
Inventors: Hala A. Elsadek; Dalia M. Nashaat; Hani A. Ghali
Original assignee: Electronics Research Institute
Priority date: 2004-10-23
Filing date: 2004-10-23
Publication date: 2006-04-27

Abstract

The antenna comprises a ground plane, a radiating surface having a single feed point, wherein a shorting wall connects the radiating surface to the ground plane. The- rectangular radiating surface comprises three U-slots at appropriate positions, whereby the outer dimension of the radiating surface determines the first resonance frequency (0.9 GHz) and the length and the width of the additional three U-slots matches the second (1.8 GHz) to fourth resonance frequency (5.2 GHz), respectively. By adding a capacitive load in the vertical direction between the ground plane and the radiating surface and by applying a fractal concept to the edges of the antenna, the resonance frequencies and thus the outer dimension of the antenna can be further reduced. A foam dielectric substrate is used for shielding purposes and for providing a rigid structure. The reduced size quad-band antenna provides a low specific absorption rate (SAR) and is compatible with wireless and portable communication systems.

Description

Compact Single Feed Quad Band Antenna for Wireless

Communication Systems

Technical Field:

Wireless communication systems and multi-band mobile or portable sets.

Background Art:

Nowadays, there is a rapid progress in the wireless communication technology. Different Frequency bands are used in last years for mobile/cellular/non cellular applications. Using a device for each band is not practical solution. So there is an increasing demand for multi-band systems or multi-system handset. Recently there is an enhanced trust in internal antennas because of its inherent advantages over other types as external antennas. PIFA has proved to be the most widely used internal antenna in commercial applications of wireless communication. The most research achievement in the multi-band PIFA has been in the design of single feed with dual resonant frequencies. Depending upon the wide bandwidth around the resonant frequencies, the dual resonant PIFA can potentially cover more than two bands or more. However, system applications such as Bluetooth (IEEE 802.11) or WLAN have frequency bands that are significantly far from the cellular bands (GSM/DSC). So, enhancing the bandwidth of a dual band PIFA to additionally cover the Bluetooth/WLAN applications can prove to be a very difficult or impossible task. The multi-band functionality is not the only required demand in such antenna systems for wireless communication applications but, also other characteristics should be satisfied as small size, light weight, omni directional radiation pattern, reasonable gain and acceptable bandwidth. Unfortunately, none of external antennas satisfies these requirements. It was found that some of external antennas loose more than 80% of their efficiency beside the human body. The most promising technique to reduce the interaction between the portable device and the human body is to use internal integrated antenna that can be shielded easily. Probably, the most suitable candidate is the PIFA. In this invention, a new method to design multi-band PIFA with independent frequency bands and good characteristics is developed. Disclosure of the Invention

(Objective and Description of the Invention)

A practical method to design a single feed multi-band PEFA that covers both the cellular and non-cellular bands is developed. From the commercial point of view, there are now different frequency bands for portable cellular/non cellular devices as the conventional 0.9GHz GSM band for mobile phones, 1.8GHz DSC band for wireless Laptops. Furthermore the Bluetooth wireless technology at 2.4 GHz is already applied nowadays in many portable devices and soon it will be the standard in most wireless communication systems as mobile phones, laptops, PDAS, car stereos, audio speakers, toys, etc. Moreover the band of WLAN at 5.2GHz is being applied in several applications as WI-FI. The proposed structure is as shown in figure 1. It is a quad band PIFA with single coaxial probe feeding. The PIFA with a shorting wall is a quarter wavelength structure that the resonating frequency is approximately related to the dimensions by the following equation:

(1) Where λ, resonance wave length at band ( i );

L₁ and W₁ length and width of the radiating surface at operating band ( i ) and ε r dielectric constant of the substrate

Foam substrate is used for light weight, rigid structure and easy shielding purposes. Adding U- slots reduces the size of the antenna by 30% from the conventional /L /4 PIFA. Additional reduction by 15% is achieved by adding a capacitive load in the vertical direction. More 5% reduction is achieved by applying the fractal concept to the edges of the antenna as in figure 2(b). In figure 1, the length L₁ and width Wi are the PIFA rectangular radiating surface to determine the first resonance frequency fj (0.9GHz). While, the dimensions of the largest U-slot (L₂, W₂) are used to generate the second resonance frequency f₂ (1.8GHz). The length L₃ and width W₃ of the middle U-slot is to get the third resonance frequency f₃ (2.45GHz). Finally, the dimensions (L₄ ,W₄) of the smallest U-slot is to have the fourth resonance frequency at U₄ (5.2GHz). This multi-band antenna has approximately the same size as a single-band PIFA operating at the lowest frequency band. The radiating element is grounded with a shorting wall. It is found that the widest bandwidth is achieved when the width of this wall is equal to the width of the PIFA radiating plate. The antenna is fed at the appropriate matching point for the four bands of operation. The antenna impedance can be matched to 50Ω by controlling the distance between the feed point and the shorting wall. The PIFA antenna is fabricated on a foam substrate with dielectric constant 1.07 and height H=8mm in order to have rigid structure that can be easily shielded as mentioned. It is worth to mention that, more frequency bands can be added by inserting additional U-slots with appropriate dimensions for the required resonating frequencies since all the bands are independent on each other except the small mutual coupling between the slots. Since when the antenna is resonating at one frequency, the other slots act as parasitic. The photo of the fabricated antenna in figure 2(a) is without fractal concept while in figure 2(b) is with fractal.

The operation of the proposed design is investigated through simultaneous steps that start first with a conventional PEFA (the U-shaped slots and the parasitic capacitive plate are removed). The antenna dimensions are (Lj, Wi) = (61, 40) mm. The coaxial feed is connected to the top plate at a distance equals 22mm from the shorting edge. The antenna ground plane is with length L = 100mm and width W=60mm. The resonance frequency is IGHz, at reflection coefficient Sn= -18dB. The bandwidth is 10%. The second step is to add the first U-slot with dimensions (L₂, W₂) = (23, 30) mm on the center of radiating surface at distance equals 18.5mm from the shorting edge. The length and width of this slot forms the size of the obstacle that the input current is forced to propagate around to create the second resonance. The antenna resonance frequencies are 0.95GHz and 2GHz with reflection coefficients -27dB and -13dB, respectively. The bandwidths are 6% and 5% for the lower and upper bands, respectively. The third design step is adding the second U-slot. to generate the antenna third resonance at f₃=2.4GHz. The dimensions of the middle U-slot are (L₃, W₃) = (18, 20) mm. The simulated frequencies of this tri-band antenna are 0.95GHz, 1.8GHz and 2.45GHz, respectively. The impedance bandwidths are 6%, 5% and 5%; respectively. In the final step, the third U-shaped slot is added with dimensions (L₄, W₄) = (9.5, 8) mm. The resonance frequencies are 0.95GHz, 1.8GHz, 2.45GHz and 5.2GHz with measured reflection coefficients -2IdB, -2OdB, -18dB and -28dB, respectively. The four bandwidths are around five percent. Figure 3 presents the simulation and experimental results of the quad band PIFA. From the figure, one can notice that there is small discrepancy between the simulated and measured results of 2^nd, 3^rd and 4^th resonance frequencies. This is attributed to the fabrication tolerance and the bonding material distribution between the copper and foam layers. In addition, there is coupling between the different U-slots which may add an inductive load, since at resonance frequency of one slot, the other two slots act as parasitic. Adding the fractal concept as in figure 2(b) reduces the previous dimensions by about 5% : 8% more.

The antenna gain is about 9dBi which is accepted for most mobile and wireless applications. Matching of the upper three resonant frequencies can be controlled by slot widths, G₂, G₃ and G₄. The best result of impedance matching is obtained when the gab widths are equal. The impedance bandwidth can be affected by changing G₁ for all slots. When Gi=G₂=G₃=G₄, acceptable bandwidth at all bands is reached. Table 1 illustrates the effect of the geometrical parameters on the four resonant frequency bands and their corresponding bandwidths. From this table, we notice that the resonating bands are independent on each other. This is a clear distinct advantage of the design since it has more three degrees of freedom. It is found that the extension to more than four bands of frequencies has no limit and can be easily adjusted by adding U slots with appropriate dimensions according to equation (1), if there is available space on the radiating surface.

Antenna size reduction methodology

The second advantage of adding U-slots over the multi-band operation is the antenna size reduction. The inserted U-slots acts as obstacles to increase the current path length. The second approach is achieved by adding a capacitive load in the vertical direction between the radiating surface and the ground plane. The reduction of the resonance frequency depends on the capacitor plate dimensions. This size reduction is at the expense of the operating bandwidth. The effect of decreasing the bandwidth can be compensated by optimizing the width of the PIFA shorting plate as well as the width of the gaps of the U-shaped slots. Table 2 illustrates the effect of the capacitive load value on the antenna size reduction ratio as well as its effect on the bandwidth for all the four bands of operation. From the table, it is clear that the capacitor load effect on the bandwidth can be neglected. Figure 4 illustrates the relation between the antenna's percentage reduction ratio from conventional PIFA and the value of the equivalent capacitance load, which is proportional to (A/t), where A is the area of the capacitor plate (L_c x W_c) and t is its separation from the ground plane. Table 2 and figure 4 are considered as designer's aids to detect the capacitor plate dimensions for certain required reduction ratio. Figure 5 illustrates a comparison between the simulated and measured reflection coefficients of the quad band PIFA at two different capacitance load values. The antenna provides good far field radiation pattern in the four bands of operation as shown in figure 6.

The effect of the antenna on the human head is studied. The simulation is done for the head model with the proposed PIFA design in one radiation box with the PIFA back toward the head. The head model is taken form the head phantom of commercially used Ansoft HFSS software package. The Specific Absorption Rate (SAR) is calculated over the average of I g of the head tissue. The SAR value is 0.48W/Kg at 5mm separation between the head and the antenna while it is reduced to O.lW/Kg at 20mm separation as shown in figure 7. It is worth to mention here that the IEEE safety standard SAR limit is 1.6W/kg. Our proposed antenna introduces lower effect on user's head than many antennas used in commercial mobile systems. For example the Alcatel-300 mobile handset has SAR=I .02 W/Kg and the Nokia-8210 has SAR=IWZKg.

Table 1 : The effect of the geometrical parameters of the proposed antenna on its resonance frequencies and their corresponding bandwidths.

Table2: The effect of the capacitance load value on the size reduction ratio and the impedance bandwidth of the quad band PIFA.

Brief Description of the figures:

The aforementioned of objectives and advantages represented by this invention will be easily apparent by the accompanying figures where:

Figure 1 : is the three dimensional geometrical structure of the proposed quad band PIFA with capacitive load. The figures illustrate the rectangular plate of PDFA with its shorting wall connected to the ground plane. The three inserted U-shaped slots are shown with appropriate dimensions. The coaxial feeding point is illustrated. The capacitor load plate is also shown in the vertical direction.

Figure 2: (a) is the photo of the fabricated antenna at three different design steps:

-with one U-slot for dual band PIFA

-with two U-slots for tri band PIFA

-with three U-slots for proposed Quad band PIFA

Figure 2: (b) is the photo of the quad band PIFA with fractal concept applied to the antenna and the U-slots edges.

Figure 3: are the reflection coefficients of the quad band PIFA with three U-shaped slots at four commercial bands of frequencies at GSM 0.95GHz, DSC 1.8GHz, Bluetooth ISM 2.45GHz and WLAN 5.2GHz, respectively. The figure is the comparison between the simulation and the measured results.

Figure 4: Shows the second technique for antenna size reduction by adding capacitor load in the vertical direction. The curve shown is the relation between the capacitor load in PF and the antenna percentage reduction ratio. The curve is considered as a designer aid to determine the capacitor load dimensions for certain reduction ratio required.

Figure 5: is the reflection coefficient of the proposed quad band PIFA after adding the capacitor plate with two different values of 0.34PF and 1.5PF to give reduction ratios of 13% and 23%, respectively. The figure is a comparison between the simulation and measured results.

Figure 6: is the radiation pattern of quad band PIFA with IOPF capacitor plate load. The figure is the radiation pattern at the four bands of operation at 0.9GHz, 1.8GHz, 2.45GHz and 5.2GHz, respectively. It illustrates both the radiation pattern at the E-plane which is parallel to the antenna surface and H-plane which is perpendicular to the antenna surface. The figure shows that at both planes, the radiation pattern is almost omni directional that satisfies most of the wireless communication systems requirements.

Figure 7: Shows the relation between the calculated SAR for the proposed quad band antenna at different distances between the head and the antenna. The figure shows that the maximum value is 0.48WTKg at 5mm separation which is much lower than the IEEE safety standard limit (1.6W/Kg).

Claims

With the aforementioned description of this invention, the following are claimed as exclusive property and rights:

1-PIFA operates at the most usable four commercial bands of wireless communication systems which are: 0.9GHz GSM band, 1.8GHz DSC band, 2.4GHz Bluetooth ISM band and 5.2 GHZ WLAN band, respectively.

2- The quad band PIFA design with appropriate dimensions of : Radiating rectangular plate (40mmx48mm),

Largest U-slot (28mmx 30mm)

Middle U-slot (14mmx20mm)

Smallest U-slot (5mmx7mm)

The antenna base is from foam dielectric with dielectric constant

£_r =1.07.

The antenna height is 8mm divided to h_L =3mm between the ground plane and shorting capacitor plate and h_v =5mm between the shorting capacitor plate and radiating surface.

The U-slots are with equal widths G₁ = G₂ = G₃ = G₄ =lmm.

The shorting wall width is equal to the rectangular radiation plate width. The feeding is with single coaxial cable at distance d=22mmfrom the shorting wall.

3- The proposed antenna is single feed multi-band structure with operating frequencies far from each other on the spectrum.

4-Each antenna operating band is independent on the other bands of operation. Which means that optimizing one band doesn't have (or have negligible) effect on the other bands. This adds number of degrees of freedom to the structure design that are equal to the number of operating bands- 1.

5 -Design aid for compact size PEFA which is a curve between the capacitor load value -that is inserted in the vertical direction between the radiating plate and the ground plane- and the percentage size reduction ratio from conventional λ_Q /4 PEFA. This curve gives the designer the capacitor plate dimensions for certain reduction ratio required.

6-The antenna bandwidth at the four bands of operation is 5% in average which is accepted for most wireless communication devices. Wider bandwidths can be achieved by making the U-slot's arms with unequal widths. This creates two near staggered resonant modes which increases the bandwidth up to 20% in average.

7-The antenna size is 50% from the size of conventional PIFA. The reduction techniques are as follows:

30% reduction due to inserting U-slots in the lateral direction

15% reduction due to inserting capacitor load plate in the vertical direction

5% reduction due to applying the fractal concept on the edges of the antenna radiating plate and on U-slots.

8- The SAR value for the proposed quad band antenna is 0.48W/Kg at 5mm separation between the antenna back and the user's head. This value is lower than the IEEE safety standard limit (1.6W/Kg) and is also lower than many commercially used antennas in wireless and cellular systems.