Abstract:
A wireless communication system includes a portable, typically hand-held transmitter which includes RF absorbing material and/or one or more dissipative parasitic antennas for reducing RF fields in close proximity to the operator to such an extent as to substantially preempt the effect of the operator presence, specifically the operator&#39;s hand, in field operation. As a result, the RF absorbing material and/or one or more dissipative parasitic antennas reduces the proportionate effect upon the RF radiation level caused by the operator&#39;s hand or other body part. The RF absorbing material may comprise a layer of such material upon a surface of a ground plane within a housing of the transmitter, and/or on an interior surface of the housing, and/or on an exterior surface of the housing. The dissipative parasitic antenna may comprise a printed antenna and resistive load integrated on the transmitter&#39;s printed circuit board or elsewhere in or on the housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to U.S. Provisional patent application No. 60/898,703 filed Feb. 1, 2007 and U.S. Provisional patent application No. 60/960,294 filed Sep. 24, 2007. The entire contents of each of these Provisional patent application are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]    1. Technical Field 
         [0003]    This invention relates to wireless communications systems and methods and wireless transmitter units for use therein and is especially concerned with radiated power and hence range of such wireless transmitter units in field operation. 
         [0004]    2. Background Art 
         [0005]    Radio frequency (RF) transmitters for control/communications/signaling systems are used in a wide variety of applications. In some applications, the transmitter is a hand-held, standalone unit, i.e., not part of a transceiver. One particular application is the monitoring of persons, such as hydro workers, forestry workers, oilfield workers, and so on, who work alone in remote or isolated locations outside of their vehicle. It is known for such workers to use a hand-held transmitter to communicate periodically with a receiver in the worker&#39;s vehicle, for example, to convey status or other signaling messages. The receiver in the vehicle is associated with a satellite transceiver whereby the status or signaling messages, along with position information, can be communicated to a central location or monitoring station via a satellite communications network. 
         [0006]    It is generally desirable to have a longer operating range allowing the user to be farther from the receiver (vehicle) and still be able to use the transmitter to send an “OK” or “PANIC” signal back to the receiver. For systems employing unlicensed bands in the vicinity of 400 MHz, typical ranges achieved for safety systems vary from hundreds to thousands of feet. 
         [0007]    Regulatory bodies, such as the FCC in the United States of America, have certain frequency bands allocated for unlicensed operation. These bands are popular choices for remote signaling systems because of the cost savings associated with unlicensed operation. To avoid spectral interference between systems, the regulations prescribe limits to RF radiation levels. In order to maximize range, RF radiation levels of the transmitters are set just below regulatory limits. 
         [0008]    Since transmitted power cannot be increased beyond the prescribed limit, other approaches have been used to increase range. One approach is to increase receiver antenna gain. For vehicular applications requiring 360 degree azimuthal coverage, however, practical increases in receive antenna gain are limited. 
         [0009]    Inadequate range may also result from the manner in which a particular transmitter is tested to determine compliance with the prescribed limit. More particularly, it is usual to operate the transmitter in a suitable chamber and measure its output using a field strength meter. In field operation, however, RF radiation levels are reduced by the presence of the operator&#39;s hand near the antenna. The hand absorbs RF radiation and potentially detunes the transmitter&#39;s antenna, thereby reducing the RF radiation level available at the receive antenna. 
         [0010]    This effect might be mitigated to some extent by keeping the operator&#39;s hand as far away as possible from the antenna. For example, increasing the size of the transmitter housing, or providing a separate antenna, causes the controls and handgrip to be further away from the antenna. This approach would be undesirable, however, where a compact transmitter is required, as is usually the case. 
         [0011]    Another approach might be to discourage handheld operation, by providing a wrist band, but this would be impractical or undesirable for most applications and, because the wrist would still be in close proximity to the antenna, performance would still be negatively affected. 
         [0012]    To summarize, maximum radiation levels are limited by the regulations, which determine the maximum allowable RF radiation level for the transmitter in isolation. In field operation, the actual maximum radiation level is reduced by the proximity of the operator. 
       SUMMARY OF INVENTION 
       [0013]    An object of the present invention is to mitigate the deficiencies of known such wireless communications systems, and transmitters, or at least provide an alternative. 
         [0014]    According to one aspect of the present invention, there is provided a wireless communications system comprising a portable transmitter means for transmitting signals and receiver means for receiving said signals, the transmitter means having an operating frequency for which restricted radiation levels are regulated and in normal operation requiring the presence of an operator in such close proximity to the transmitter means, or contact therewith, as to cause a significant reduction of the RF radiation produced thereby, wherein the transmitter means includes absorber means for reducing RF field strength in close proximity to the exterior of transmitter means so that the effect upon the output radiation attributable to the presence of the operator is proportionately reduced. 
         [0015]    According to a second aspect of the present invention, there is provided portable transmitter means for use in a wireless communications system, the portable transmitter unit having means for transmitting RF signals and the system having means for receiving said RF signals, the transmitter means having an operating frequency for which restricted radiation levels are regulated and whose normal operation requires the presence of an operator in such close proximity to the transmitter means or contact therewith, as to cause a significant reduction of the RF radiation produced thereby, wherein the transmitter means includes absorber means for reducing RF field strength in close proximity to the exterior of the transmitter unit, such that the effect upon the output radiation attributable to the presence of the operator is proportionately reduced. 
         [0016]    According to a third aspect of the present invention, there is provided a method of compensating for deleterious effects of operator presence upon maximum radiation level of a portable transmitter means of a wireless communications system, maximum radiation level of the transmitter means being restricted by regulations, the method including the steps of including in or on the transmitter means during calibration of its maximum radiation level an absorber means for reducing the RF field strength in close proximity to the exterior of the transmitter means, the absorber means remaining in or on the transmitter means during normal operation so as to reduce the effect upon the output radiation attributable to the presence of the operator is proportionately reduced. 
         [0017]    Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]    In the drawings, identical or corresponding elements in the different Figures have the same reference numeral. 
           [0019]      FIG. 1 , labelled PRIOR ART, illustrates components of a typical unidirectional wireless signaling/communications system; 
           [0020]      FIG. 2 , labelled PRIOR ART, illustrates a known transmitter unit of the kind used in the system of  FIG. 1 ; 
           [0021]      FIG. 3 , labelled PRIOR ART, illustrates the transmitter unit and its surroundings during regulatory testing; 
           [0022]      FIG. 4 , labelled PRIOR ART, illustrates the transmitter unit with an operator&#39;s hand present in field operation; and 
           [0023]      FIGS. 5A and 5B  are front and rear views, respectively, of a first preferred embodiment of the present invention in the form of a transmitter unit including preemptive absorption. 
           [0024]      FIG. 6  is a front view, of a second preferred embodiment of the present invention in the form of a transmitter unit including preemptive absorption. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]      FIG. 1  illustrates a known kind of unidirectional wireless signaling/communications system with a portable, specifically hand-held, transmitter unit  100  and a vehicle-based receiver  101  for a safety application, specifically for use by a person  102  carrying the transmitter unit  100  while working in an isolated location to transmit status signals periodically to the receiver  101 , which relays reports, including status signals of interest and position information, to a monitoring station  103  via a satellite transceiver  104  and a satellite communications network  105 . 
         [0026]    As shown in  FIG. 2 , the transmitter unit  100  includes operator controls  202 , a monopole antenna  203 , a battery  204 , and the usual electronics, which include RF electronics  205  and other electronics  206 , and an attenuator  207 , all located on a printed circuit board (PCB)  208  which also acts as a ground plane for the monopole  203 . This attenuator  207  can be used to adjust the radiation level as required, typically from 0-20 dB. It should be noted that in a monopole configuration, the transmitter unit&#39;s antenna actually comprises the monopole antenna  203  and its ground plane  208 . All of these components  202 - 208  are located in or on the housing  209 . 
         [0027]    The operator controls  202  would typically include “PANIC” and “OK” buttons. Operation of the “PANIC” button by the operator causes an emergency signal to be transmitted to the receiver and relayed immediately to the monitoring station. The operator is required to operate the “OK” periodically to cause the transmitter unit to send a “Status OK” signal to the monitoring station, to confirm that he is “OK”. The absence of an “OK” signal after a predetermined amount of time triggers a “NOT OK” report, generated at the receiver  101 , to be sent to the monitoring station. In either case, the personnel at the monitoring station would then send help to the operator, determining his location from position information embedded in the “PANIC” or “NOT OK” report. 
         [0028]    Because the system is unidirectional, the only range-affecting performance metric for the transmitter unit  100  is its radiation level, the maximum radiation being limited by applicable regulations. RF radiation levels are determined by a combination of the antenna ( 203 ,  208 ) performance and the output power of the RF electronics  205 , for a given setting of the attenuator  207 . Thus, degraded performance of the transmitter antenna ( 203 ,  208 ) can be compensated for by increasing the output power of the RF electronics  205  to maintain the same RF radiation levels. This is to be contrasted with a receiver where the receive sensitivity is directly affected by the receiver&#39;s antenna and cannot be compensated for by other means to maintain the same level of performance. 
         [0029]    As discussed hereinbefore, the RF radiation levels from the transmitter  100  are limited by applicable regulations and established by measurements conducted upon a sample transmitter unit  100  in isolation. A typical configuration for regulatory compliance measurements is illustrated in  FIG. 3 . The transmitter  100  is supported by a non-invasive support and transmits without an operator in close proximity, while a neighbouring calibrated test receiver  300  with a receive antenna  301  measures the radiation levels at a prescribed distance  302 . The test environment  303  is shielded and anechoic to create conditions that approximate free-space operation. 
         [0030]    This calibrated test setup is used to set the maximum RF radiation level of the transmitter  100 , specifically by adjusting the attenuator  207  while monitoring the radiation level measured by the receiver. It should be noted that this is done without an operator&#39;s hand in the near field and in close proximity to the antenna or ground plane. 
         [0031]      FIG. 4  shows the transmitter  100  in use, i.e., in the hand  400  of an operator. The operator&#39;s hand  400  is wrapped around the transmitter  100 , but more particularly, wrapped around the ground plane  208 , causing a reduction in the radiation level of the transmitter unit  100 . Because the operator&#39;s hand was not present during regulatory compliance testing, no allowance was made for the resulting reduction in RF radiation level. 
         [0032]    Thus, when the transmitter unit  100  is held in the operator&#39;s hand and actually being used in the field, the maximum radiation level attainable will be significantly lower than the maximum radiation level prescribed by the regulations as established during the above-described calibration process. 
         [0033]    A first preferred embodiment of the invention, which addresses this problem, will now be described with reference to  FIGS. 5A and 5B . The transmitter  500  illustrated in  FIGS. 5A and 5B  is similar to that illustrated in  FIG. 2  so components common to both have the same reference numbers. The transmitter  500  differs from the transmitter  100  of  FIG. 2 , however, in that it includes RF absorbing material  501  placed on the ground plane  208  below the monopole  203 , in areas where the operator&#39;s hand  400  would be in close proximity. The RF absorbing material  501  is therefore present during regulatory testing. 
         [0034]    A second preferred embodiment of the invention, which also addresses this problem, will now be described with reference to  FIG. 6 . The transmitter  600  differs from the transmitter  100  of  FIG. 2 , however, in that it includes a parasitic antenna  601  integrated on the printed circuit board  208 , in areas which would be proximate the operator&#39;s hand  400  when the transmitter was in use. Power received by the parasitic antenna  601  is then dissipated in a resistive load  602 . Together, the parasitic antenna  601  and resistive load  602  form a dissipative parasitic antenna. The dissipative parasitic antenna  601 , 602  is therefore present during regulatory testing. 
         [0035]    The way in which this substantially preempts the effect of the operator&#39;s hand  400  can be interpreted in two ways. In one interpretation, the RF absorbing material  501  or the dissipative parasitic antenna  601 , 602  can be seen to simulate the presence of the hand  400 , thereby effectively including part of the hand when the RF radiation level is set as part of regulatory testing. In a second interpretation, the RF absorbing material  501  or the dissipative parasitic antenna  601 , 602  limits the RF fields in close proximity to the operator&#39;s hand  400 , thereby reducing the hand&#39;s  400  effect. Both interpretations are valid and correct. 
         [0036]    Referring also to  FIG. 3 , during regulatory compliance testing of the transmitter  500 , 600 , the attenuator  207 , between the RF electronics  205  and monopole  203 , will be adjusted (reduced as compared with a typical known transmitter  100 ), so as to increase RF radiation levels to a maximum, within the regulatory limits, while compensating for the presence of the RF absorbing material  501  or the parasitic antenna  601  with resistive load  602 . Then, in field operation, the effect of the operator&#39;s hand  400  is substantially preempted, with higher RF radiation levels and therefore longer range than would have been available without the use of RF absorbing material  501  or the dissipative parasitic antenna  601 ,  602 . 
         [0037]    Selection of either the RF absorbing material  501  approach or the dissipative parasitic antenna  601 , 602  approach, or both approaches in combination, depends on a number of factors including cost, space available, the configuration of the transmitter&#39;s communications antenna  203 , the frequency of operation, absorber effectiveness at the frequency of operation and the likely operator hand  400  positions. The effectiveness and optimal configuration are best determined experimentally. 
         [0038]    It should be noted that the selection and placement of the absorbing material are configuration dependent. Placement locations could include not only the ground plane  208  of the antenna, but the inside surface of the housing, or the outside surface of the housing, or the volume between the ground plane and the inside of the housing, or any combination of these locations. 
         [0039]    Absorbing material characteristics must be carefully considered when determining placement. For example, for direct application to the ground plane  208 , magnetically-loaded elastomeric absorber sheets, such as Wave-X-A020, from ARC technologies, have high losses in some of the frequency bands of interest and are effective because of inductive coupling. This has the effect of attenuating RF currents in the areas where the absorber material is applied. Wave-X-is also sufficiently non-conductive as to not affect the low-frequency performance of components and traces, thereby allowing direct application to a populated PCB/ground plane  208 . 
         [0040]    Internal parasitic antenna  601  and resistive load  602  selection and placement are configuration dependent. Parasitic antenna topologies could include, monopoles, dipoles, patch antennas, grounded line (example: planar inverted-f antenna, PIFA) antennas, or chip antennas. Resistive load configurations could include chip resistors, printed resistors, or a lossy transmission line. It is further envisaged that the parasitic antenna and load could combined in the form of a lossy antenna. Placement locations could include not only the PCB  208 , but the inside surface of the housing, or the outside surface of the housing, or the volume between the ground plane and the inside of the housing, or any combination of these locations. The use of multiple dissipative parasitic antennas could be advantageous in configurations where one dissipative parasitic antenna cannot approximate the effect of the hand. 
         [0041]    In the first preferred embodiment, the RF absorbing material  501  is used to cover all portions of the ground plane  208  (front and back) other than the portion of the ground plane in the closest proximity to the antenna. The RF electronics  205  are placed inside a shielded compartment  502  to avoid being covered with RF absorbing material  501  which could affect their performance. 
         [0042]    In the second preferred embodiment, the parasitic antenna  601 , is located in close proximity to the likely hand  400  position and is integrated in the PCB  208 . The resistive load  601 , is a chip resistor, mounted on the PCB  208 . 
         [0043]    In certain applications, a combination of RF absorbing material ( 501 ) applied to one or more areas, and one or more dissipative parasitic antennas ( 601 , 602 ) might be advantageous. The effectiveness and optimal configuration are best determined experimentally. 
         [0044]    It should be noted that, although the preferred embodiment described herein is concerned with increasing radiated power in field operation for a safety application, especially where operators working in isolation, especially in remote locations that are generally inaccessible to road vehicles, the present invention comprehends a number of alternative RF remote control/signaling applications including remote keyless entry, door and gate openers, among others. It is also envisaged that the system could be used by lone mountaineers to report their status to a receiver located at a stage or base camp. 
         [0045]    Although embodiments of the invention have been described and illustrated in detail, it is to be clearly understood that the same are by way of illustration and example only and not to be taken by way of limitation, the scope of the present invention being limited only by the appended claims.