Communications: radio wave antennas – Antennas – With radio cabinet
Reexamination Certificate
1999-09-27
2001-03-20
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
With radio cabinet
C343S742000, C343S867000
Reexamination Certificate
active
06204817
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a portable radio communication device, comprising: a housing; antenna means for transmitting and receiving RF signals; transmitting and receiving circuits arranged in the housing; at least a conductive portion; a power source; and, a user interface.
Further it relates to an antenna system for transmitting and receiving RF signals from and to a portable radio communication device, comprising: a first antenna, being a transmitting antenna, and being connectable to transmitting circuits of the radio communication device; and, a second antenna, being a receiving antenna, and being connectable to receiving circuits of the radio communication device. Specifically, it relates to an antenna device for a mobile radio communication device, e.g. a hand-portable telephone.
RELATED ART AND BACKGROUND OF THE INVENTION
Antenna systems of the type mentioned above are previously known from U.S. Pat. No. 5,231,407 and WO-A1-91/01048. One advantage of the separation between transmitter and receiver is that the requirements for duplexing filters will decrease. However, a problem is the coupling between the transmitting and the receiving antennas. To decrease said coupling the antennas used in U.S. Pat. No. 5,231,407 are tunable narrow band antennas, while the antennas described in WO 91/01048 are arranged at different ends of the telephone.
At first sight, the use of more than one antenna can be seen as waste of space etc., but nevertheless, a number of inventors have pointed out numerous advantages. The present invention can be said to be a new and inventive way to utilise the concept of two or more antennas or antenna functions for operation in respect of a single system transmitting/receiving band, to save space and decrease losses in human tissue. Some further examples of the use of more than one antenna that are known, used for achieving diversity or directional properties, for minimising the influence of a users hand, and for satellite telephones.
To achieve diversity, more than one receiving antenna is used together with one transmitting antenna (usually the same as one of the receiving antennas). 5-10 dB fading reduction is reported as a result of the use of diversity reception. EP-B1-0 214 806 and EP-A1-0 648 023 disclose two examples thereof. A further example is shown in WO-A1-95/04386. Diversity is standard in the Japanese PDC system and typically one whip antenna combined with one PIFA (Planar Inverted F Antenna) are used.
Directional properties have been suggested in order to improve antenna gain in the direction of the base station (i.e. in a variable way) and to suppress interfering sources. EP-A1-0 649 227 is one example.
EP-A1-0 752 735 discloses the use of multiple antennas in order to minimise the influence of the users hand, simply by using one of the antenna elements which are not covered by the hand (as detected by the VSWR).
Satellite telephones generally have strong requirements on the antennas, such as big difference between transmitting and reception frequencies or extreme requirements on low losses (i.e. filters should be avoided). WO-A1-97/26713 and WO-A1-98/18175 are two examples hereof, where separate transmitting and receiving antennas with the same circular polarisation are used.
Modern mobile phones are small and thus the interaction between antenna, phone body and user will become more important than earlier. There is also normally a requirement for two or more frequency bands and a recent trend is to integrate the antenna function into the telephone body making it invisible from the outside, which is customary named built-in antenna. According to the present invention, a number of benefits can be achieved by using separate antennas for transmitting and receiving if they are implemented according to special principles of the present invention, which will be described below. The requirements for transmitting and receiving antennas are quite different and with the diminishing size it becomes more and more important to optimise each of them separately. It is well known that antenna performance will go down when the antenna is made smaller.
Since the mobile telephones today are very small and the antennas, during telephone calls, will be located close to the head of a user, much attention is paid to the effects on the human body when exposed to electric fields. An issue especially discussed is the SAR (Specific Absorption Rate) values, which preferably should be low. In the documents mentioned above, no efforts are shown how to decrease the SAR values.
SAR (Specific Absorption Rate) is used to quantify electromagnetic fields in respect of influence to the human body, and is also applicable in the near field. SAR is defined as the power loss per a certain unit of body tissue, and for instance FCC (Federal Communications Commission) in the US requires less than 1.6 mW per gram. The phone systems require a certain power level (such as 2 W peak and 0.25 W average for GSM in highest power level). It should however be noted that, the field near the antenna can be different for different types of antennas, even if the field far from the antenna should be the same. SAR is measured inside a dummy head, or can be calculated. Due to SAR's nature of power density, a smaller antenna structure carrying the same power as a bigger structure is more likely to be close to the limit value. This is the case for most phones using small antennas. The general development of the phones thus calls for SAR optimised solutions. Bigger antenna structures will generally cause lower SAR values, but modern telephone design requirement do not support increasing size. Antenna efficiency is another important characteristic and efficiency and SAR are somewhat correlated as high SAR obviously means extra losses. The term SAR will be used herein when reference to existing limits (stated by FCC, CENELEC etc.) or corresponding measuring methods is relevant but otherwise the more general expression “losses in human tissue” will be used.
To define some terms reference is made to
FIG. 1
a
, which shows a typical telephone with a helical antenna, which is one of the most common types of antennas today. The user
1
holds the telephone body
2
, provided with an antenna
3
, to the ear
4
. The radiated power Prad has to comply with the requirement of the telephone system in question. Prad is smaller than the power Pin fed by the transmitter, and the quotient between them gives the efficiency. A part of the loss in the human tissue (head, hand, etc.), causes a (very small) heating
5
of the human tissue close to the antenna, and many times more heating occurs at locations as
6
along the phone. For the subsequent discussion it should be noted that the telephone configuration in
FIG. 1
a
can be understood as a very asymmetric electric dipole as shown in
FIG. 1
b
. The asymmetric dipole
1
b
differs from the common symmetric dipole in
FIG. 1
c
only by its feeding impedance. The currents along the dipoles
1
b
and
1
c
are the same, which is the reason for the occurrence of the current and loss maximum at
5
in
FIG. 1
a.
For a transmitting antenna, both SAR and efficiency are important. For losses in human tissue it can be shown that various small antennas radiating the same power and located on the same distance from the ear can give values differing more than 100 times. Should much lower SAR values be required than those that can be achieved by the typical antenna of today, it will be necessary to use some of the more efficient antenna principles with regard to the losses in human tissue. Naturally magnetic type antennas (loops etc.) will give less SAR in their near field as compared to antennas of the electric dipole type. This can be exemplified by studying the fields from a electric dipole and a magnetic dipole, radiating the same power. When r decreases, the electric fields are increasing as 1/r
3
and 1/r
2
, respectively, and thus the magnetic dipole (1/r
2
) will have much lower E-field (corresponding to SAR) at very small distances, in
Allgon AB
Chen Shih-Chao
Jacobson Price Holman & Stern PLLC
Wong Don
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