SAR (specific absorption rate) for users of portable wireless devices (PWDs) is a matter of increasing concern. RF radiation to the user's head results from the free-space generally omnidirectional radiation pattern of typical current PWD antennae. When PWDs equipped with such an antenna are placed near the user's head, the antenna radiation pattern is no longer omnidirectional as radiation in a large segment of the azimuth around the user is blocked by the absorption/reflection of the user's head and hand. An antenna system for PWDs that greatly reduces radiation to the body and redirects it in a useful direction is also desirable.
Prior art antennas for PWDs may cause audio noise in a hearing aid of the user. Referring to FIG. 16, a diagrammatic view of a prior art PWD 400 (in the form of a cellphone) used in the vicinity of a hearing aid 402 is illustrated. Cellphone 400 has a speaker on the keyboard surface near the top of the phone, which is normally aligned with the center of the user's ear 404 during use. Hearing aid 402 may be any type, including in-ear and behind-ear variations. Hearing aid 402 has an amplified audio output port 406, which is inserted into the ear canal of the ear 404. During operation, an electromagnetic field 408 is generated around cellphone 400 by omnidirectional antenna 440. In operation, electromagnetic field 408 illuminates the hearing aid 402, user's ear 404, and the user's head. RF noise is induced in the hearing aid by the field 408, resulting in excessive audio noise being presented to the user.
The planar inverted F antenna or PIFA is characterized by many distinguishing properties such as relative lightweight, ease of adaptation and integration into the device chassis, moderate range of bandwidth, omni directional radiation patterns in orthogonal principal planes for vertical polarization, versatility for optimization, and multiple potential approaches for size reduction. Its sensitivity to both vertical and horizontal polarization is of practical importance in mobile cellular/RF data communication applications because of the absence of the fixed antenna orientation as well as the multi-path propagation conditions.
To assist in the understanding of a conventional PIFA, a conventional single band PIFA assembly is illustrated in FIG. 17. FIG. 17 illustrates a prior art single-band PIFA antenna 440 located on the rear side 442 of a personal wireless device 444. PIFA 440 consists of a radiating element 446, a ground plane 448, a feed conductor 450, and a grounding conductor 452. PIFA 440 is typically positioned near an upper edge of ground plane 448 with the free end of radiating element 446 being closer to a user's hand than the feed conductor 450 and grounding conductor 452. The feed conductor 450 serves as a feed path for radio frequency (RF) power to the radiating element 446. The feed conductor 450 is electrically insulated from the ground plane 448. The grounding conductor 452 serves as a short circuit between the radiating element 446 and the ground plane 448. The resonant frequency of the PIFA 440 is determined by the length (L) and width (W) of the radiating element 446 and is slightly affected by the locations of the feed conductor 450 and the grounding conductor 452. The impedance match of the PIFA 440 is achieved by adjusting the dimensions of the conductors 450, 452, and by adjusting the separation distance between the conductors 450, 452. In operation, ground plane 448 radiates RF energy which is absorbed by a user's hand. Antenna 440 can be configured to reduce the SAR value to 1.6 mw/g with the PWD 444 transmitting at the 0.5 watt cw level. However, even at this level audio noise may be generated in a user's hearing aid by operation of PWD 444. Another limitation of the PIFA is its relatively low front-to-back ratio. Front-to-back ratios of typically PIFAs range from 0 to 2 dB. A 5 dB front-to-back ratio may be achieved by substantially increasing the distance between radiating element 446 and ground plane 448. A need exists for an antenna exhibiting substantially greater front-to-back ratios.
FIG. 18 illustrates a prior art dual-band PIFA antenna 462, which is located on the rear of a personal wireless device 464, and electrically connected to ground plane 466 at one end and capacitively coupled to ground plane 466 at another end. PWD 464 further includes a battery pack 470 positioned away from antenna 462. In normal operation, PWD 464 is oriented in an upright manner so that end 472 is generally above end 474. Ground plane 466 is provided by the ground traces of the printed wiring board (PWB). The portion of antenna 462 indicated by numeral 476 resonates over a higher frequency band, while the entire portion 476, 478 of antenna 462 resonates over a lower frequency band. PIFA antenna 462 is grounded at its upper end at location indicated as numeral 480 to ground plane 466. PIFA antenna 462 is capacitively coupled at pad 482 in a direction away from upper end 472 of PWD. This type of antenna provides some reduction in SAR, but has limited ability to reduce hearing aid noise from a digital PWD.
Despite all of the desirable properties of a PIFA, the PIFA has the limitation of a rather large physical size for practical application. A conventional PIFA should have the semi-perimeter (sum of the length and the width) of its radiating element equal to one-quarter of a wavelength at the desired frequency. With the rapidly advancing size miniaturization of the radio communication devices, the space requirement of a conventional PIFA is a severe limitation for its practical utility.