Compact Internal Multiband Microstrip Antennas for Portable GPS, Cellular and Satellite Phones

Compact Internal Multiband Microstrip Antennas for Portable GPS, PCS, Cellular and Satellite Phones

Mohamed Sanad and Noha Hassan
AMANT
Reno, Nevada

Compact multiband internal integrated microstrip antennas have been developed for different combinations of hand-held portable global positioning systems (GPS), PCS, cellular satellite phones, Personal Computer Memory Card International Associate (PCMCIA) communication cards and wireless modems. Multiband internal microstrip antennas may be integrated in the printed circuit board or plastic case of the communication equipment and, in this form, are not easy to break. The devices are mechanically rigid and reduce the overall size of the portable communication equipment. Internal integrated microstrip antennas are easily shielded to reduce their interaction with the human body. Hence, they provide good performance beside the human body as well as in free space. Multiband internal integrated microstrip antennas may be designed with only one feed point to be used with a single multiband duplexer, or they may have multiple feed points to be used with multiple single-band duplexers.

Conventional GPS and satellite receivers are usually fixed and used only in free space. Therefore, their antennas are required to be right-hand circularly polarized with hemispherical coverage. Combining GPS and satellite phones with hand-held portable PCS and cellular phones has introduced new, more challenging antenna requirements. Antennas for different combinations of portable GPS, PCS, satellite and cellular phones are required to operate over multifrequency bands while remaining small in size and light in weight. Furthermore, portable GPS, PCS, satellite and cellular phones are usually used in urban areas, often inside buildings or vehicles. Thus, they suffer from multipath reflections, which produce fading and/or polarization rotation. Their antennas must be sensitive to two perpendicularly polarized waves rather than vertically or right-hand circularly polarized and one of the antenna diversity techniques must be used. Moreover, portable GPS, PCS, satellite and cellular phones are randomly oriented by their operators and, therefore, their radiation patterns must be quasi-isotropic rather than having hemispherical coverage or any type of directivity.

PCS, satellite and cellular phones always operate while next to the human head. Thus, their antennas are required to fulfill these requirements beside the human body as well as in free space. The effect of the human body on the antenna and also the effect of the antenna on the human body should be as small as possible. Unfortunately, none of the existing portable GPS, PCS, satellite or cellular phone external antennas, such as helical, quadrifilar-helical or monopole antennas, exhibits good performance beside the human body. Since they are external, these antennas get too close to the human head while the phone is in operation and are not easy to shield. It has been determined that some of these antennas lose more than 80 percent of their efficiency beside the human body.¹ Furthermore, they are all sensitive to only one polarization. External antennas, especially helical antennas, are difficult to manufacture in an accurate, repeatable way and cannot provide a 50 W input impedance without using a separate matching circuit, which increases cost and losses. Such external wire antennas cannot be integrated in the PCB or plastic case; they increase the total size of the phone, especially if they are retractable, and are easy to break or bend. Moreover, external antennas require an internal diversity antenna to solve fading problems. It is also difficult to design an efficient multiband external antenna for different combinations of GPS, PCS, satellite and cellular phones.

Internal Integrated Multiband Microstrip Antennas

Probably the most promising technique to significantly reduce the interaction between the hand-held portable GPS, PCS, satellite and cellular phone antennas and the human body is to use multiband internal integrated antennas such as microstrip antennas,² as shown in Figure 1 . Some special configurations of microstrip antennas feature a very small size and good performance beside the human body.³ They are sensitive to both vertically and horizontally polarized waves and their radiation patterns have good isotropic characteristics.⁴ Internal antennas may be integrated with the PCB or plastic case and are easily shielded. Furthermore, microstrip antennas can be designed to provide multifrequency bands for different combinations of GPS, PCS, satellite and cellular phones. Multiband internal integrated antennas are very rigid and reduce the overall size of the phone. In addition, they can use an E- and H-field components diversity technique, which is the only technique that does not require a separate diversity antenna.⁵ Conventional microstrip antennas do not meet any of these requirements.

In this research, new compact multiband internal integrated microstrip antenna configurations having one feed point have been developed to meet such requirements. This compact multiband internal integrated microstrip antenna concept was applied to different combinations of GPS, PCS, satellite and cellular phones, PCMCIA communication cards and wireless modems. It should be noted that designing multiband internal microstrip antennas having multiple feed points is much easier than designing multiband antennas having a single feed point and, hence, the multiple-feed case is not presented here.

Dual-band Internal Integrated Microstrip Antennas for Portable GPS/Cellular Phones

A new internal integrated dual-band microstrip antenna design having one feed point has been developed for portable GPS/cellular phones. It consists of three stacked layers. The total size of the antenna is 53 x 30 x 3.25 mm. Each of the layers has only one common ground plane, which is the ground plane of the lower layer. The layers also have one feed probe connected to the central element of the middle layer. The thickness of the lower layer is 1 mm while the thickness of the middle and upper layers is 1.5 and 0.75 mm, respectively. Figure 2 shows the antenna configuration and details of each layer. The dielectric constant of the lower layer is 2.94 (Duroid 6002); the dielectric constant of the middle and upper layers is 10.2 (Duroid 6010). The lower layer contains only one wide patch while each of the middle and upper layers consists of three narrow elements separated by very narrow gaps. All elements of the antenna have the same 51 mm length. The width of the lower wide element is 30 mm. The middle and upper layers have similar geometries. The widths of their narrow elements are 10, 9 and 8 mm, respectively, and the width of the gap between adjacent elements is 1.5 mm. Only the central element of the middle layer is directly fed by a coaxial cable and connector, while all the other elements are indirectly fed by coupling as parasitic elements. The lower element generates the GPS  frequency band, which is less than one percent at 1.575 GHz. Both the middle and upper layers cover the required band of cellular phones, which is 70 MHz, centered at 925 MHz (890 to 960 MHz for GSM).

The measured return loss of the antenna is shown in Figure 3 . The antenna provides a bandwidth of 70 MHz at the cellular frequency band with a return loss of -10 dB or less. The radiation patterns of the antenna were measured in the principal planes at several frequencies in the cellular and GPS bands. The bandwidths of the radiation patterns were even wider than the bandwidths of the input impedance. For example, the vertically and horizontally polarized radiation patterns in the horizontal plane at 1575 (GPS band) and 900 MHz (cellular band) are shown in Figures 4 and 5 , respectively, while the antenna was mounted on the phone and both the antenna and phone were vertically oriented. As can be seen, the antenna is sensitive to both vertically and horizontally polarized waves and has quasi-isotropic radiation patterns. Similar results were obtained in the other principal planes and at all the frequencies in the cellular and GPS bands.

On the other hand, the effect of the antenna on the human body (as well as the effect of the human body on the antenna) has been studied and determined to be small. The antenna demonstrates good performance in free space as well as beside the human body while held next to the head. The detailed results of the interaction between the antenna and the human body very closely matched results published previously.5

Dual-band Microstrip Antennas for Portable GPS/PCS Phones

Another dual-band internal microstrip antenna design has been developed for portable GPS/PCS phones. The geometry of the antenna is similar to the geometry of the GPS/cellular phone antenna shown previously. The two antenna designs are different only in dimensions and substrate material. The total size of the GPS/PCS antenna is 27 x 20 x 2.5 mm. The thickness of each of the lower and middle layers is 1 mm while the thickness of the upper layer is 0.5 mm. The same substrate material has been used in all three layers (Duroid 6010 with a dielectric constant of 10.2). The lower element covers the GPS frequency band, which is less than one percent at 1.575 GHz. Both the middle and upper layers provide the required PCS band, which is 140 MHz, centered at 920 MHz (1850 to 1990 MHz).

The measured return loss of the antenna is shown in Figure 6 . The antenna provides a bandwidth of 140 MHz at the PCS frequency with a return loss of -10 dB or less. The radiation patterns of the antenna were measured at several frequencies in the PCS and GPS bands, in free space and beside the human body. The radiation patterns were very close to those of the GPS/cellular antenna and, therefore, are not presented here. The antenna demonstrates good performance in free space as well as when held next to the human head.

Multiband Antennas for Portable Satellite/Cellular Phones

Satellite phones require two narrow frequency bands (1610 to 1626.5 MHz and 2483.5 to 2500 MHz for downlink and uplink low earth orbit (LEO) communication, respectively), while cellular phones require one wide frequency band (890 to 960 MHz for GSM). Therefore, each application requires two substrate layers to cover these frequency bands. The microstrip antenna for portable satellite/cellular phones consists of four stacked substrate layers.

The total size of the antenna is 53 x 30 x 3.5 mm. As with the previous design, all layers have only one common ground plane, which is the ground plane of the first (lower) layer. The antenna also has one feed probe connected to the central element of the third layer. The thickness of the first layer is 0.5 mm, the thickness of the second and fourth (upper) layers is 0.75 mm and the thickness of the third layer is 1.5 mm. The dielectric constant of the first and second layers is 2.2 (Duroid 5880) and 2.95 (Duroid 6002), respectively, and the dielectric constant of the third and  fourth layers is 10.2 (Duroid 6010). The first and second layers consist of only one element each. The third and fourth (upper) layers consist of three narrow elements each, separated by very narrow gaps. Only the central element of the third layer is directly fed by a coaxial cable and connector; all other elements are indirectly fed by coupling as parasitic elements. Figure 7 shows the antenna configuration and details of each layer. The first layer covers the lower frequency band of the satellite phone (1610 to 1626.5 MHz for the LEO uplink), while the second layer covers the upper frequency band of the satellite phone (2483.5 to 2500 MHz for the LEO downlink). Both the third and fourth layers cover the required cellular phone band (890 to 960 MHz for GSM).

The measured return loss of the antenna is shown in Figure 8 . The antenna provides the required frequency bands with a return loss of -10 dB or less. The radiation patterns of the antenna were measured at several frequencies in the satellite and cellular bands, in free space and beside the human body. Again, the antenna demonstrated good performance in free space as well as next to the human head.

Antennas for Portable GPS/PCMCIA Pagers and Wireless Modems

PCMCIA cards are small form factor adapters for personal computers, personal communications or other electronic devices.5 They are approximately the size and shape of a credit card and can be used with any personal portable computer system equipped with a PCMCIA slot. These PCMCIA cards are used for input/

output features such as wireless modems and pagers, and are designed to provide messaging capability to laptops, notebooks, palmtops and other portable computer systems. These cards may also work as stand-alone pagers when they are not connected to a computer. Since these pagers may also be held or stored in an operator's pocket, their antennas must have a negligible human body effect. Furthermore, these antennas must have almost the same resonant frequency, input impedance and radiation patterns in free space and inside the PCMCIA type II slot in any portable computer. Some of these PCMCIA communication cards require a wide bandwidth while others require a very narrow bandwidth, and GPS may be combined with either case.

The dual-band GPS/cellular internal integrated antenna could be used with the wideband PCMCIA communication cards combined with GPS. For the case of the narrowband PCMCIA communication cards combined with GPS, the dual-band microstrip antenna is much simpler. It consists of only two layers, as shown in Figure 9 . The total size of the antenna is 53 x 30 x 2 mm, and the thickness of each layer is 1 mm. The dielectric constant of the lower and upper layers is 2.2 (Duroid 5880) and 10.2 (Duroid 6010), respectively. Each layer contains only one wide patch. The lower layer covers the GPS frequency band, while the upper layer covers the required band of the PCMCIA communication card.

Figure 10 shows the return loss of the dual narrowband microstrip antenna contained in a PCMCIA pager card while the pager card was inside the PCMCIA slot of a palmtop computer. It was determined that inserting the antenna into the PCMCIA slot has a negligible effect on the antenna's resonant frequency and return loss. The radiation patterns of the GPS/PCMCIA pager card internal integrated dual-band microstrip antenna were measured in different situations and at different frequencies in both the GPS and PCMCIA communication card bands. The radiation patterns were quasi-isometric and sensitive to both vertically and horizontally polarized waves. Furthermore, the performance of the antenna inside the PCMCIA slot beside the human body was still good.

Conclusion

Compact multiband internal integrated microstrip antennas have been developed for different combinations of portable hand-held GPS, satellite, PCS and cellular phones, PCMCIA communication cards and wireless modems. These multiband antennas are designed with only one feed point in order to be used with one multiband duplexer. They also may have multifeed points in order to be used with different single-band duplexers. When multifeed points are used, the different layers of the antenna may be stacked together or separated and located away from each other to increase the isolation between the different frequency bands. However, designing stacked multiband microstrip antennas having one feed point is much more complicated than designing separate microstrip antennas having multiple feed points.

Single-feed multiband stacked antennas consist of several layers of substrate material having different dielectric constants. With different combinations of substrate material having dielectric constants ranging from 2.2 to 10.2, the overall size of the dual-band GPS/cellular phone antenna is 53 x 30 x 2.25 mm. The size of the GPS/PCS dual-band antenna is 27 x 20 x 2.5 mm and the size of the satellite/cellular multiband antenna is 53 x 30 x 3.5 mm. The overall size of the GPS/PCMCIA communication cards and wireless modem dual-band internal microstrip antenna is 53 x 30 x 2 mm. Of course, all of these dimensions may be further reduced by using substrate materials having higher dielectric constants.

Multiband internal integrated microstrip antennas are sensitive to both vertically and horizontally polarized waves with good radiation pattern characteristics in free space as well as next to the human body. Internal integrated microstrip antennas are relatively easy to shield. The interaction between the shielded antennas and the human body was determined to be less than the interaction between the human body and any external wire antenna, such as monopole, helical and quadrifilar-helical antennas.

Acknowledgment

The Duroid 5880, 6002 and 6010 materials are products of Rogers Corp., Chandler, AZ. 

References

1.         Mohamed Sanad and Noha Hassan, "Evaluation of Cellular Phone Antennas and the Advantages of Using Internal Antennas," IEEE AP-S Symposium Proceedings, Atlanta, GA, June 1998.

2.         Mohamed Sanad, "The Necessity of Using Internal Integrated Cellular Phone Antennas," Wireless and Microwave Technology Conference and Exposition Proceedings, Chantilly, VA, October 1997.

3.         Mohamed Sanad, "Effect of the Human Body on Microstrip Antennas," IEEE AP-S Symposium Proceedings, University of Washington, Seattle, WA, June 1994, pp. 298-301.

4.         Mohamed Sanad, "Microstrip Antennas on Very Small Ground Planes for Portable Communication Systems," IEEE AP-S Symposium Proceedings, University of Washington, Seattle, WA, June 1994, pp. 810-813.

5.         Mohamed Sanad and Noha Hassan, "An Internal Integrated Microstrip Antenna for PCS/Cellular Phones and Other Hand-held Portable Communication Equipment," Microwave Journal, Vol. 41, No. 7, July 1998, pp. 64-77.