Patent Application
20100302119
The present invention relates to antennas and especially to antennas for mobile devices located close to an object interfering with an antenna's properties.
Wearable electronics are an emerging trend in consumer communication systems. Devices worn on the body communicate with each other via local wireless links like Bluetooth, and to local access points via WLAN. A challenge in these systems is to form the antennas since they need to be small and there is usually a large impact from the body. For handheld devices, it is well known that the influence of the human head or the hand can greatly impact the efficiency of the antenna. For handhelds, careful design and placement of the antenna in the product is essential. For wearable devices including headsets, watches, radio clips, etc., this is even more difficult, since it cannot always be predicted beforehand how the user is going to wear the product. That is, the user decides how to wear the product.
When the antenna is close to the body, the antenna efficiency is greatly degraded, as illustrated in
Antenna diversity is one solution to address the body effect.
The invention addresses the problem of reduced antenna efficiency in wearable devices due to the body effect. Thus, a new method and arrangement is presented that automatically selects the best antenna element in a multi-antenna device to compensate for body effects without a need for additional hardware. With this technique, the average antenna efficiency is improved as compared to only one antenna systems.
For these reasons, an antenna system is provided comprising at least two antenna elements distanced from each other and arranged substantially in parallel and connected to a common input/output for the antenna system. Each of the antenna elements comprises a portion having an impedance such that the impedance is influenced by an external load such that one of the at least two antennas elements closest to the load is influenced by the load and detuned. When the antenna element is detuned it results in an optimal driving of the second antenna element. The object may be a human body portion. Preferably, the antenna elements are configured to operate as a closely coupled antenna array, wherein the antenna elements have same resonance frequency, and their reflection coefficient response coincide when there is no load influencing them.
Aspects of the invention also relate to a communication device comprising a transceiver portion and an antenna system comprising at least two antenna elements arranged distanced from each other and substantially in parallel and connected to a common input/output port for the antenna system. Each of the antenna elements comprises a portion having an impedance such that the impedance is influenced by an external load such that one of the at least two antennas closest to the load is influenced by the load and detuned. When the antenna element is detuned, it results in an optimal driving of the second antenna element. The object may be a human body portion. The antenna elements are configured to operate as a closely coupled antenna array, wherein the antenna elements have the same resonance frequency, and their reflection coefficient response coincide when there is no load influencing them.
Aspects of the invention also relate to a method of enhancing radiation efficiency of an antenna system. The method comprises: providing at least two antenna elements distanced from each other and arranged substantially in parallel and connected to a common input/output and providing the antenna elements with a portion having an impedance such that the impedance is influenced by an external load such that one of the at least two antennas closest to the load is influenced by the load and detuned.
The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended figures where:
Aspects of the invention introduce an alternative solution, which will be described in more detail below. The antenna system consistent with aspects of the invention does not require a switch. Two antenna elements are used as in the diversity scheme, but no switch mechanism is involved. Instead, the selection of the best antenna element is done automatically. The impact of the body will detune one antenna element (the antenna element closest to the body), which will result in a good power transfer to optimal driving (low reflection loss, insertion loss) the second antenna element (the antenna element furthest away from the body). This results in a system which hereinafter is called Autonomous Antenna Select System (AASS)
Radiation resistance as used in this description relates to a part of an antenna's feed point resistance that is caused by the radiation of electromagnetic waves from the antenna. The radiation resistance is determined by the geometry of the antenna, not by the materials of which it is made. It can be viewed as the equivalent resistance to a resistor in the same circuit. Radiation resistance is caused by the radiation reaction of the conduction electrons in the antenna.
Although the method and arrangement has been shown to work for two antenna elements, the method can be extended to three or more antenna elements. The element closest to the body will be detuned and will automatically reflect more energy which is then exploited by the antenna elements further away from the body.
When there is no body (or an object with load) close by, the antenna elements will operate as a closely coupled antenna array. Both elements will have the same resonance frequency, and their S11 (reflection coefficient) response will coincide.
Simulations of a practical antenna have shown that in this case, the overall antenna efficiency can be close to 0 dB at 2.5 GHz depending on the material used directly around the antenna, and the material used to construct the antenna. Material losses such as conductive losses or dielectric losses can be represented as an additive series and/or parallel resistor to the radiation resistance, but without having the radiation properties, thus giving pure loss, resulting in an increase of thermal energy of the antenna system.
If a body is brought close to one of the antenna elements, e.g., antenna 320, this antenna is detuned and its resonance frequency drops. Effectively, this is caused by an increase in the series capacitance. This is because human tissue has a high effective permittivity εr of around 47 which greatly increases the capacitance. The other antenna element, antenna 310, is hardly affected and is not detuned (or at least at a much smaller scale). The S11 response seen by the driving point is a combination of the S11 responses of the two individual S11 responses, as illustrated in
In this case, antenna 320 is distanced 3 mm from the body. The physical distance between the antenna 320 and antenna 310 needs to be at a sufficient level to reach a satisfying total efficiency.
In this case, the antenna 320 shows a detuned resonance at 2.225 GHz. Although the reflection coefficient has increased at the desired resonance frequency, it has not been reduced completely.
Xcap represents the capacitance change due to body load. In free space, both antenna's have about the same capacitance. The inductance of the antenna is less influenced by the body load. According to this example, the inductance may be 60 nH at 2.44 GHz=+j920 Ohm and is a relative high impedance for e.g., a 37.5 Ohm driver. The capacitance may be 0.312 pF. All figures are given as examples and do not limit the invention to the specific figures. The invention may be achieved using components with other values.
If the antenna 320 is displaced to a closer distance to the body, this antenna is then further detuned, but there is still efficiency left at the 2.5 GHz resonance frequency due to a relative good reflection coefficient of antenna 310, as illustrated in
Table 1 show simulation results with aid of a Three Dimensional Electro Magnetic simulator. Simulations show that improvement of 5dB efficiency is easily reached.
The performance is also controlled on a real product in practice and gives the measured results in table 2:
The communication portion comprises parts (not shown) such as a receiver, a transmitter, (or a transceiver), antenna elements 710 and 720 etc., for establishing and performing communication with one or several communication networks (not shown). The antenna elements 710 and 720 each comprise a load portion 715 and 725 with certain impedance and radiating portions.
The radiating portions constituting the series resistance and all energy dissipated in the radiation resistance is radiated as energy in the radio waves. The antenna elements are simply connected together to a driving point of a transmitter-receiver portion 703.
A method according to aspects of the invention for enhancing the radiation efficiency of the antenna system is illustrated in
It should be noted that the word “comprising” does not exclude the presence of other elements or steps than those listed and the words “a” or “an” preceding an element do not exclude the presence of a plurality of such elements. It should further be noted that any reference signs do not limit the scope of the claims, that the invention may be implemented at least in part by means of both hardware and software, and that several “means”, “units” or “devices” may be represented by the same item of hardware.
A “device” as the term is used herein, is to be broadly interpreted to include a radiotelephone having ability for Internet/intranet access, web browser, organizer, calendar, a camera (e.g., video and/or still image camera), a sound recorder (e.g., a microphone), and/or global positioning system (GPS) receiver; a personal communications system (PCS) terminal that may combine a cellular radiotelephone with data processing; a personal digital assistant (PDA) that can include a radiotelephone or wireless communication system; a laptop; a camera (e.g., video and/or still image camera) having communication ability; and any other computation or communication device capable of transceiving, such as a personal computer, a home entertainment system, a television, etc.
The above mentioned and described embodiments are only given as examples and should not be limiting to the present invention. Other solutions, uses, and functions within the scope of the invention as claimed in the below described patent claims should be apparent for the person skilled in the art.
