Patent Publication Number: US-2012026009-A1

Title: Medical device having a multi-element antenna

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to provisional U.S. patent application Ser. No. 61/369,184 entitled “MULTI-LOOP ANTENNA FOR A BODY WORN DEVICE” (Attorney Docket No. P0036139.00) filed Jul. 30, 2010, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure relates generally to wireless communications between medical devices such as implantable medical devices (IMDs) and, in particular, to multi-element antennas for wireless communications of the devices. 
     BACKGROUND 
     A wide variety of implantable medical devices (IMDs) that deliver a therapy to or monitor a physiologic or biological condition of a patient, or both, have been clinically implanted or proposed for clinical implantation in patients. An IMD may deliver therapy to or monitor a physiological or biological condition with respect to a variety of organs, nerves, muscles or tissues of the patients, such as the heart, brain, stomach, spinal cord, pelvic floor, or the like. The therapy provided by the IMD may include electrical stimulation therapy, drug delivery therapy or the like. 
     The IMD may exchange communications with another device. The IMD may exchange communications with a body worn device that is either attached to (e.g., worn by) the patient or otherwise located near the patient. The information exchanged may be information related to a condition of the patient, such as physiological signals measured by one or more sensors, or information related to a therapy delivered to the patient. This information may be previously stored or real-time information. The IMD may also receive information from the body worn device, such as information that may be used to control or configure a therapy to be provided to the patient. 
     The IMD and the body worn device (collectively “medical devices”) may exchange information using any of a variety of communication techniques, such as radio frequency (RF) communications. For example, the IMD and the other device may communicate in the 402-405 megahertz (MHz) frequency band in accordance with the Medical Device Radiocommunications Service (MEDRADIO) band regulations. As another example, the IMD and the other device may communicate over the 401-402 MHz or 405-406 MHz frequency bands in accordance with the MEDRADIO band regulations. 
     In RF communications, the reliability and efficiency of wireless communication systems are often affected by the environment in which the communications occur. Yet, the medical devices will typically operate in a variety of environments each of which may impact the tuning of the RF antenna. The body worn device will operate in both wearable and non-wearable environments. As for the IMD, the device will typically be operational in an external environment prior to implantation, while the implant environment will vary depending on such things as implant depth, orientation etc. Therefore, the need remains for a tunable antenna for optimizing communications of the medical devices in a variety of operating environments. 
     SUMMARY 
     In general, the exemplary embodiments of the present disclosure provide medical devices comprising a tunable antenna. In an embodiment, the antenna comprises a multi-element assembly including at least a first inner element nested within at least a first outer element. In an implementation, the first inner element and first outer element may share a central axis with at least a portion of the first inner element overlapping with the outer element to enable magnetic coupling between the elements. In other implementations, the first inner element and the first outer element may be configured in an offset axis. 
     In an embodiment, one of the at least first inner element and the first outer element may be formed in a loop configuration. In another embodiment, one of the at least first inner element and the first outer element may be formed in a square configuration. In another embodiment, the at least first inner element and the first outer element may be formed in a rectangular configuration. In another embodiment, the at least first inner element and the first outer element may be formed in a helical configuration. In another embodiment, the at least first inner element and the first outer element may be formed in a spiraling configuration. 
     In an exemplary embodiment, a tuning network may be coupled to the outer antenna element. The tuning network may be a component that will facilitate varying of the impedance of the antenna in response to changes to one or more electrical properties of the tuning network. Such changes may include but are not limited to varying an input such as a voltage or current to the tuning network. In one or more embodiments, the tuning network may be a variable capacitor, or a variable inductor, or a switched network implemented with a plurality of capacitors or inductors or a combination of both a capacitor and an inductor. 
     According to yet another embodiment, an apparatus comprises a telemetry module, and an antenna coupled to the telemetry module. The antenna may comprise a first antenna element that is selectively electrically-coupled to the telemetry module via a feed point, a second antenna element that is coupled to the first antenna element, and a tuning network that is selectively tuned in response to an operating environment of the apparatus. In some implementations, the antenna may further comprise a third antenna element that is selectively electrically-coupled to the telemetry module via the feed point, wherein one of the first antenna element and the third antenna element is coupled to the telemetry module via the feed point in response to the operating environment of the apparatus. Yet further, the tuning network may comprise a configurable network having at least a first switchable component and at least a first non-switchable component. In other embodiments, the tuning network may comprise a passive network having a tuning element that passively adjusts an electrical characteristic in response to a change in the operating environment of the apparatus. 
     According to an alternate embodiment, the antenna may comprise a tuning module that generates tuning signals to apply to the tuning network. The tuning module may selectively apply the tuning signals to adjust an electrical characteristic of the tuning network to a first value in response to the apparatus being disposed in a first operating environment and to adjust the electrical characteristic of the tuning network to a second value in response to the apparatus being disposed in a second operating environment, wherein the first value is different than the second value. For example, the first value may be smaller in comparison to the second value. 
     In another aspect of the disclosure a method comprises determining the operating environment in which a device is disposed selectively tuning an antenna of the device based on the determination. 
     According to another embodiment, an antenna is provided comprising a plurality of antenna elements, wherein two of the elements are selected to define a first outer antenna element and a first inner antenna element, providing optimum antenna performance (e.g. impedance matching and efficiency). 
     According to another aspect of the disclosure, the antenna may have a feed-point through which the inner antenna element or the outer antenna element or both may be coupled to a telemetry module. For example, the power applied to the inner antenna element induces a magnetic field that couples to the outer antenna element. The coupling can effect an impedance transformation to impedance match the antenna&#39;s feed-point, and in turn may optimize the performance of the antenna by allowing the larger outer element to operate at higher tuning element Q. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings are illustrative of particular embodiments of the present disclosure and therefore do not limit the scope of the disclosure. The drawings (not to scale) are intended for use in conjunction with the explanations in the following detailed description, wherein similar elements are designated by identical reference numerals. Moreover, the specific location of the various features is merely exemplary unless noted otherwise. 
         FIG. 1  is a conceptual diagram illustrating an example medical system. 
         FIG. 2  is a block diagram illustrating an example body worn device in further detail. 
         FIG. 3  is a schematic diagram illustrating an example multi-element antenna in accordance with one aspect of this disclosure. 
         FIG. 4A  is a schematic diagram of a wrist-worn device. 
         FIG. 4B  is a schematic diagram illustrating a top view of one example antenna configuration for the wrist worn device of  FIG. 4A . 
         FIG. 4C  is a schematic diagram illustrating a side view of another example antenna configuration for the wrist worn device of  FIG. 4A . 
         FIG. 5  is a schematic diagram illustrating another example multi-element antenna. 
         FIG. 6  is a schematic diagram illustrating another example antenna in accordance with this disclosure. 
         FIG. 7  is a schematic diagram depicting an alternative embodiment of a multi-element antenna  70 . 
         FIG. 8  is a schematic block diagram illustrating a system for real-time automatic self-tuning of antennas of the present disclosure. 
         FIG. 9  is a flow diagram illustrating example operation of a body worn device in accordance with one aspect of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the disclosure or the application and uses of the disclosure. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
       FIG. 1  is a conceptual diagram illustrating an example medical system  10 . Medical system  10  includes an implantable medical device (IMD)  14 , a body worn device  16  and an external device  18 . Medical system  10  may, however, include more or fewer devices. IMD  14 , body worn device  16  and external device  18  communicate with one another using any of a number of wireless communication techniques. 
     IMD  14  may be any of a variety of medical devices that provide therapy to patient  12 , sense one or more parameters of patient  12  or a combination thereof. In some instances, IMD  14  may be a device that provides electrical stimulation therapy in the form of cardiac rhythm management therapy to a heart of patient  12  via one or more electrodes. In one example, IMD  14  may include one or more implantable leads (not shown) with one or more electrodes that extend from IMD  14  for delivering therapy to and/or sensing physiological signals of a heart of patient  12 . The leads may be implanted within one or more atria or ventricles of the heart of patient  12  or a combination thereof. In this manner, IMD  14  may be used for single chamber or multi-chamber cardiac rhythm management therapy. The cardiac rhythm management therapy delivered by IMD  14  may include, for example, pacing, cardioversion, defibrillation and/or cardiac resynchronization therapy (CRT). In other instances, IMD  14  may be a device that provides electrical stimulation to a tissue site of patient  12  proximate a muscle, organ or nerve, such as a tissue proximate a vagus nerve, spinal cord, brain, stomach, pelvic floor or the like to treat various conditions, including movement and affective disorders such as chronic pain, Parkinson&#39;s disease, tremor and dystonia, urinary storage and voiding dysfunction, digestion dysfunction, sexual dysfunction or the like. 
     In other instances, IMD  14  may be a device that delivers a drug or therapeutic agent to patient  12  via an implantable catheter (not shown). IMD  14  may, for example, be implanted within a subcutaneous pocket in an abdomen of patient  12  and the catheter may extend from IMD  14  into the stomach, pelvic floor, brain, intrathecal space of the spine of patient  12  or other location depending on the application. IMD  14  may deliver the drug or therapeutic agent via the catheter to reduce or eliminate the condition of the patient and/or one or more symptoms of the condition of the patient. For example, IMD  14  may deliver morphine or ziconotide to reduce or eliminate pain, baclofen to reduce or eliminate spasticity, chemotherapy to treat cancer, or any other drug or therapeutic agent to treat any other condition and/or symptom of a condition. 
     In further instances, IMD  14  may be a leadless IMD. In this case, the IMD is implanted at a target site with no leads extending from the IMD. The leadless IMD may provide therapy and/or sense various parameters via one or more electrodes or sensors on the device, thus avoiding limitations associated with lead-based sensors. For example, IMD  14  may be a leadless pacer. As another example, IMD  14  may be a leadless pressure sensor placed within or near the heart, such as in the pulmonary artery. In some instances, IMD  14  uses the sensed signals to monitor a condition of patient  12  or provide therapy to patient  12  as a function of the sensed physiological signals. Alternatively, or additionally, IMD  14  transmits the sensed physiological signals to another device, such as body worn device  16  or external device  18 , which may in turn monitor the condition of patient  12  or provide therapy to patient  12  as a function of sensed physiological signals. IMD  14  may sense, sample, and process one or more physiological signals such as heart activity, muscle activity, brain electrical activity, intravascular pressure, blood pressure, blood flow, acceleration, displacement, motion, respiration, or blood/tissue chemistry, such as oxygen saturation, carbon dioxide, pH, protein levels, enzyme levels or other parameter. 
     Body worn device  16  is illustrated in  FIG. 1  as being a watch. However, body worn device  16  may be any of a variety of body worn devices, such as a necklace, armband, belt, ring, bracelet, patch, or other device that is configured to be attached to, worn by, placed on or otherwise coupled to a body of patient  12  or placed in close proximity to patient  12 . As will be described in further detail in this disclosure, body worn device  16  is capable of wireless communication while on patient  12  and while off patient  12 . 
     External device  18  may be a programming device or monitoring device that allows a user, e.g., physician, clinician or technician, to configure a therapy delivered by IMD  14  or to retrieve data sensed by IMD  14  or body worn device  16 . External device  18  may include a user interface that receives input from the user and/or displays data to the user, thus allowing the user to program the therapy delivered by IMD  14  or display data retrieved from IMD  14  and/or body worn device  16 . External device  18  may be a dedicated hardware device with dedicated software for programming or otherwise communicating with IMD  14  and/or body worn device  16 . In one example, external device  18  may be a programmer, such as a CareLink® programmer, available from Medtronic, Inc. of Minneapolis, Minn. CareLink® is a registered trademark of Medtronic, Inc. Alternatively, external device  18  may be an off-the-shelf computing device running an application that enables external device  18  to program or otherwise communicate with IMD  14  and/or body worn device  16 . 
     IMD  14 , body worn device  16  and external device  18  may communicate with one another by any of a number of wireless communication techniques. In some instances, IMD  14 , body worn device  16  and external device  18  may be communicatively coupled with each other as well as other medical devices (not shown) to form a local area network, sometimes referred to as a body area network (BAN) or personal area network (PAN). Each device may therefore be enabled to communicate wirelessly along multiple pathways with each of the other networked devices. As such, IMD  14 , body worn device  16  and external device  18  may represent a distributed system of devices that cooperate to monitor a condition of and/or provide therapy to patient  12 . 
     Example wireless communication techniques include RF telemetry, but other techniques are also contemplated, including inductive telemetry, magnetic telemetry, or the like. In one instance, IMD  14 , body worn device  16  and/or external device  18  may communicate in accordance with the Medical Device Radiocommunication Service (MEDRADIO)—Core and Wing—band regulation, MEDRADIO WING. As is known in the art, in countries other than the United States, MEDRADIO core band is known as Medical Implant Communications Service (MICS) and MEDRADIO wing band is known as Medical Data Service (MEDS). The MEDRADIO band regulation defines communication requirements for the 402-405 MHz frequency band. In accordance with the MEDRADIO band regulations, the frequency band may be divided into ten channels with each channel corresponding to a 300 kilohertz (kHz) sub-band. The MEDRADIO band regulation defines a split channel band with a portion of the MEDRADIO band occupying the 401-402 MHz frequency band and a portion of the MEDRADIO band occupying the 405-406 MHz frequency band. The MEDRADIO band may be divided into 20 channels with each channel corresponding to a 100 kHz sub-band, with the first ten channels being located in the 401-402 MHz frequency band and the second ten channels being located in the 405-406 MHz frequency band. The devices of medical system  10  may, however, communicate using any frequency band regulation in addition to or instead of the MEDRADIO Core and MEDRADIO Wing band regulations. 
       FIG. 2  is a block diagram illustrating example components of body worn device  16  in further detail. Body worn device  16  may be a watch, armband, belt, ring, bracelet, patch, or other device that is attached to, worn by, placed on or otherwise coupled to a body of patient  12 . Body worn device  16  includes a telemetry module  22 , antenna  23 , antenna impedance matching network  123 , user interface  24 , sensor  25 , processor  26 , memory  28  and power source  29 . The components of body worn device  16  are shown to be interconnected by a bus  34 , but may be interconnected by other means including direct electrical or non-electrical connections. 
     As described above, body worn device  16  may wirelessly communicate with an IMD  14  and/or an external device  16  via telemetry module  22  and antenna  23 . To this end, telemetry module  22  includes suitable hardware, firmware, software or any combination thereof for communicating with IMD  14  and/or external device  16 . For example, telemetry module  22  may include appropriate modulation, demodulation, frequency conversion, filtering, and amplifier components for transmission and reception of data. In some instances, telemetry module  22  may include two or more sets of components, e.g., one for inductive communication and one for RF communication. 
     Antenna  23  may be a multi-element antenna as described in detail in this disclosure. Antenna impedance matching network is depicted coupled to antenna  23  in dashed line. The dashed-line coupling is intended to clarify that the matching network  123  may be included as part of antenna  23  or as a separate component. As such, in embodiments where antenna impedance matching network  123  is a separate component, the matching network  123  is coupled to telemetry module  22  rather than direct coupling of telemetry module  22  to antenna  23 . 
     Body worn device  16  is typically worn by patient  12 . In the case of a watch, for example, the watch is typically worn around a wrist of patient  12 . There may be instances in which body worn device  16  is not worn by patient  12 , but instead is placed nearby patient  12 , such as in a pocket or a handbag. Conventional body worn devices may have an antenna designed or tuned for operation in response to being worn by the patient. However, in response to the conventional body worn devices not being worn by the patient, the antenna is no longer tuned correctly due to the environmental change (e.g., change in location). The antenna of conventional body worn devices may also be affected by other environmental changes, such as a temperature changes (e.g., worn in summer vs. winter, for example), dampness changes (e.g., worn in water or not), or the like. In accordance with this disclosure, body worn device  16  has a multi-element antenna that may be selectively tuned based on such environmental changes, as described in further detail below. 
     In one embodiment, body worn device  16  may be selectively tuned based on whether body worn device  12  is on patient  12 . Body worn device  16  determines whether body worn device  16  is on patient  12 . In one example, body worn device  16  may determine whether body worn device  16  is on patient based on a characteristic of a received signal, such as a received signal strength indication (RSSI), bit error rate (BER), bit rate, or other channel or signal quality metric. After the communication link is set up between body worn device  16  and IMD  14 , telemetry module  22  may determine the RSSI of the received signals from IMD  14 . Processor  26  may determine whether the RSSI is strong enough and if not, re-tune antenna  23 . If antenna  23  is tuned for communication while body worn device  16  is worn on patient  12 , an RSSI below a threshold may indicate that body worn device is no longer on patient  12  and processor  26  may re-tune antenna  23  for communication while body worn device  16  is not worn on patient  12 . Similarly, if antenna  23  is tuned for communication in response to body worn device  16  not being worn on patient  12 , an RSSI below the threshold may indicate that body worn device is on patient  12  and processor  26  may re-tune antenna  23  for communication while body worn device  16  is on patient  12 . In this manner, processor  26  may determine whether or not body worn device is on patient  12  based on the RSSI (or other channel or signal quality metric). Tuning antenna  23  based on the determined RSSI may also account for other operating environment changes in addition to whether body worn device  16  is worn by patient  12 , such as temperature of the environment, whether the environment is wet or dry, or the like. 
     In another example, sensor  25  may detect whether body worn device  16  is on or proximate to patient  12 . Sensor  25  may, for example, be a proximity sensor that is capable of detecting when body worn device  16  is on patient  12 . The proximity sensor may be a capacitive proximity sensor, inductive proximity sensor, ultrasonic proximity sensor, infrared (IR) proximity sensor or any other type of proximity sensor. In the case of a capacitive proximity sensor, the proximity sensor may be composed of one or more capacitive elements that measure the change in capacitance due to the proximity of the sensor to skin of patient  12 . In another example, sensor  25  may be one or more electrodes located on body worn device  16 . The electrodes may detect cardiac electrical signals of patient  12  to identify when body worn device  16  is on patient  12 . 
     In a further example, patient  12  may interact with user interface  24  to indicate whether body worn device  16  is on patient  12 . Body worn device  16  may, for example, include an input mechanism (such as one or more buttons, touch screens or the like) that the user may interact with to indicate whether body worn device  16  is on patient  12 . Patient  12  interact with the input mechanism upon placing body worn device  16  on the body of patient  12  and interact with the input mechanism upon removing body worn device  16  on the body of patient  12 . An output mechanism may indicate to patient  12  whether the antenna is tuned to function on the body or off the body to confirm correct antenna configuration. As will be described in detail below, a tuning module may selectively tune antenna  23  based on the determination as to whether patient  12  is wearing body worn device  16 . The tuning module may be telemetry module  22 , processor  26  or other component of body worn device  16 . 
     In addition to detecting whether body worn device  16  is on patient  12 , sensor  25  may also measure one or more parameters of patient  12 . In the example in which sensor  25  includes one or more electrodes, sensor  25  may measure cardiac electrical signals of patient  12 . Alternatively or additionally, body worn device  16  may include one or more separate sensors for measuring the one or more parameters of patient  12 . In either case, processor  26  may store the measured signals in memory  28  for later processing or transmission to another device, e.g., external device  18  or a remote server (not shown). 
     Power source  29  delivers operating power to the components of body worn device  16 . In one example, power source  29  may include a battery and a power generation circuit to produce the operating power for the components of body worn device  16 . Power source  29  may be rechargeable or non-rechargeable. In the case of a rechargeable power source, power source  29  may be recharged using any of a variety of techniques. 
     Body worn device  16  of  FIG. 2  is provided for purposes of illustration. Body worn device  16  may include more or fewer components than those illustrated in  FIG. 2 . As such, the techniques of this disclosure should not be considered limited to the example described in  FIG. 2 . 
       FIG. 3  is a schematic diagram illustrating an example multi-element antenna  30  in accordance with one aspect of this disclosure. Antenna  30  may correspond with antenna  23  of  FIG. 2 . Antenna  30  includes a first inner antenna element  32  disposed proximal to an outer antenna element  34 . The inner and outer antenna elements may comprise an electrically conductive material. In the illustrative embodiment of  FIG. 3 , Antenna  30  is a dual loop antenna, with at least a loop segment of the outer antenna element  34  overlapping with a loop segment of the inner antenna element  32 . Inner antenna element  32  and outer antenna element  34  are illustrated as including one turn of a conductive material. In other embodiments, however, inner antenna element  32 , outer antenna element  34 , or both may include more than one turn of the conductive material. 
     Inner antenna element  32  is electrically coupled to telemetry module  22  of body worn device  16  via feed points  36 A and  36 B (collectively “antenna feed-points  36 ”). Telemetry module  22  feeds signals to be transmitted to inner antenna element  32  via feed points  36 . Outer antenna element  34  is magnetically coupled to inner antenna element  32 . In other words, a change in current flow through inner antenna element  32  generates a magnetic field that couples the energy to outer antenna element  34 . The multi-element structure, such as the nested dual loop, of antenna  30  increases the impedance of antenna  30  at the antenna feed point, thus providing a better impedance match (e.g., close to 50 Ohms) Moreover, because the inner antenna element  32  is electrically coupled to the transceiver and may be located anywhere, such as in an offset axis, near outer antenna element  34 , antenna  30  provides flexibility as to the location of antenna feed-points  36 . 
     Outer antenna element  34  may also include a tuning network  38  for tuning a resonant frequency of antenna  30 . The tuning network  38  may include one or more components that facilitate the varying of the impedance of the antenna in response to changes to one or more electrical properties of the tuning network. Such changes may include but are not limited to varying an input such as a voltage or current to the tuning network. Such changes may also include providing digital control inputs/signals to the tuning network. 
     As described above, a tuning module of body worn device  16  may selectively apply or adjust tuning signal  39  to tuning network  38  based on whether body worn device  16  is on patient  12 . In other examples, the antenna may be tuned as a function of one or more criteria. For example, the criteria feeding the tuning selection such as application of the tuning signal  39  by the tuning module may include the RSSI, a quality of service indicator, output of sensor  25  ( FIG. 2 ) or input received from user interface  24  ( FIG. 2 ). As described above, the tuning module may be processor  26 , telemetry module  22  or other component of body worn device  16 . 
     In one example embodiment, the tuning network  38  may comprise, for example, of a varactor, variable capacitor, or other element having a capacitance that can be controlled or adjusted. In the case of a varactor, tuning signal  39  may be a direct current (DC) bias voltage applied to the varactor. The impedance of the antenna may be tuned by selectively adjusting the tuning signal  39  to achieve a predetermined capacitance and therefore resonance frequency for optimal impedance matching. This may include varying the tuning signal  39 , such as the DC bias voltage applied across the voltage-variable capacitor (varactor). In doing so, the multi-element antenna can be tuned to an impedance that is closely matched to the impedance of the transmission line. Although described in this example in the context of the tuning network  38  being a varactor and tuning signal  39  being a DC bias voltage for purposes of illustration, tuning network  38  may be any tunable element and tuning signal  39  may be any signal that adjusts the tuning network  38 . 
     In another example, tuning network  38  may, in another example, comprise a configurable network of an array of switchable (selectable) capacitors and the tuning signal may activate one or more switches to switch all or a subset of the capacitors into and out of coupling with outer antenna element  34 . The impedance of the antenna may be tuned by selecting the appropriate capacitor(s) to achieve a predetermined capacitance and therefore resonance frequency to achieve impedance matching. The tuning may also include varying the tuning signal  39 , such as the voltage applied across the selected capacitor(s). In some implementations, the configurable network may include one or more non-switchable capacitors may be additionally coupled to the tuning network  38  in conjunction with the switchable capacitors. 
     In another embodiment, the tuning network  38  may comprise an inductor, such as a variable inductor, that is coupled to the outer antenna element. The impedance of the antenna may be adjusted by varying the inductance of the variable inductor to achieve a predetermined resonance frequency and thereby achieve impedance matching. The tuning may be achieved by selectively varying the tuning signal  39 , such as a current (or voltage) applied to the variable inductor element. 
     In another embodiment, the tuning network  38  may comprise a switching network utilizing inductors. Tuning with the switched capacitor network may be achieved by varying the tuning signal  39  by, for example, implementing a switching scheme to determine which one or more inductors of the switched inductor network are coupled to a power source. A non-overlapping signal may be used to control the switches, so that not all switches are closed simultaneously. In another embodiment, the tuning network  38  may be implemented as a switching network utilizing a combination of capacitors and inductors. 
     In yet another embodiment, the tuning network  38  may be implemented as a configurable network including at least one switchable component (e.g., inductor or capacitor) and at least one non-switchable component (capacitor or inductor). 
     The tuning network  38  may be adjusted to optimize the operation of the antenna  30  based on its operating environment. In one example, the tuning module may apply a DC bias voltage to the varactor in response to determining that body worn device  16  is on patient  12  and remove the DC bias voltage in response to determining that body worn device  16  is not on patient  12 . In the context of an implantable medical device, such as IMD  14 , the tuning network  38  may be tuned to optimize the operation of antenna  30  for a range of implant environments. For example, the antenna  30  may be optimized for pre- and post-implant environmental operation. In other examples, the post implant operation may further be optimized as a function of the implant depth. 
     In additional embodiments, the tuning network may be implemented as a passively tuned network for self-tuning In other words, the tuning network  38  may comprise one or more components that permit passive self-tuning of the antenna based on changes in the dielectric medium. For instance, the passively-tuned network may comprise an inductor, interdigital capacitor and fixed capacitor coupled to the outer antenna element  34 . In the implementation, the capacitance of the interdigital capacitive component would vary depending on the dielectric medium in which the antenna is exposed. As such, in response to the operating environment being air, which has a relative dielectric constant of approximately 1, the capacitance would increase primarily as a function of the dielectric constant where as in response to the operating environment in which the antenna is disposed comprises contact with the body, which has a dielectric constant approximately between 60 and 80, the network overall capacitance would decrease primarily as a function of the dielectric constant. With regard to the operating environment being air, the antenna will typically be a predetermined distance away from the body so as to eliminate or substantially minimize any contributory effects associated with the body&#39;s effective capacitance and dielectric constant. 
     The criteria employed to tune antenna  30  and thereby optimize its operation in the context of a body worn device  16  may include the output of sensor  25  or input from user interface  24 . As such, the tuning module may apply a bias voltage having a first level in response to an indication that the body worn device  16  is worn on patient  12  and apply a bias voltage having a second level in response to an indication that the body worn device  16  is not worn on patient  12 . Accordingly, applying the tuning signal (e.g., bias voltages in this example) changes the capacitance or inductance of tuning network  38  as a function of a given environment in which the antenna  30  is disposed to optimize the antenna operation for the given environment. For example, the capacitance or inductance of tuning network  38  may be decreased in response to IMD  14  being implanted or body worn device  16  being worn by patient  12 . Conversely, the capacitance or inductance of tuning network  38  may be increased in response to IMD  14  being outside the body of patient  12  or body worn device  16  not being worn by patient  12 . 
     In situations where the body worn device  16  is adjacent to the body or the IMD  14  has been implanted, the body of patient  12  may present an impedance that may manifest in electrical terms as, for example, a parallel capacitor increasing the overall capacitance. The capacitance or inductance of tuning network  38  is thus varied to compensate for the increased capacitance of the body. As such, removing the device from patient  12  causes an environmental change that is accounted for by the selective tuning of antenna  23 . 
     In another example, the tuning module may apply the tuning signal to tunable tuning network  38  without determining the environment in which antenna  30  is operating (e.g., whether it is on a device that is adjacent to the body of patient  12 , implanted in the body or away from the body). For example, the tuning module may apply the tuning signal based on the determined RSSI. The tuning module may receive the determined RSSI from telemetry module  22  and compare the RSSI to a threshold level. If the RSSI is below the threshold, the tuning module may apply the DC bias voltage to the varactor (if currently removed) or remove the DC bias voltage to the varactor (if currently applied). In this example, antenna  23  has two tuning selections: a first tuning selection in response to a DC bias voltage not being applied and a second tuning selection in response to the DC bias voltage being applied. Processor  26  may, however, be unaware of the operating environment of the antenna  30 . In this manner, processor  26  tunes antenna  30  to account for any environmental changes, including whether body worn device  16  is worn by patient  12 , but also including a temperature change of the environment, a change in moisture content of the environment, or any other environmental change that may affect the tuning of antenna  30 . 
     Although the examples described above include two tuning selections (e.g., tuning signal applied and removed), body worn device  16  may be capable of including more than two tuning selections. In the case of a varactor or other tunable element, each of the tuning selections may correspond with a control signal(s). In the case the tuning network  38 , including a plurality of fixed capacitors, each of the tuning selections may correspond with a different number of the capacitors being switched into outer antenna element  34 . The tuning selections may be further used to achieve more optimal tuning of antenna  30 . In one example, processor  26  may periodically scan all of the tuning selections and select the tuning selection resulting in the best RSSI. In another example, upon determining that the RSSI is below a threshold level, processor  26  may automatically switch tuning selections until the RSSI increases above the threshold level. 
     In embodiments where two or more tuning selection criteria are available, one criterion may be used as the primary criteria and a second or subsequent criteria may be used as the secondary tuning criteria. For example, processor  26  may initially tune antenna  30  based on whether body worn device  16  is on patient  12  (as described in detail above) and adjust the tuning to further refine antenna operation. For example, processor  26  may initially tune antenna  30  based on whether the output of sensor  25  or input from user interface  24  indicates that body worn device  16  is on patient  12  and then enhance the tuning of antenna  30  based on the RSSI. 
     In some instances, inner antenna element  32  may have either a capacitive tuning network or an inductive tuning network (not shown in  FIG. 3 ). The capacitive or inductive tuning network on inner antenna element  32  may function as an additional tuning mechanism to tune the impedance of antenna  30 . The capacitive or inductive tuning network on inner antenna element  32  may also be tunable based on the measured RSSI, output of sensor  25  or based on input received from user interface  24 . In other instances, the tuning network on inner antenna element  32  is not tunable, i.e., fixed. 
     In other embodiments, additional criteria may be utilized for tuning the tuning network  38  of the antenna  30  to optimize its operation. In one example, the transmitted power may be monitored while tuning the antenna in order to maximize power transfer or, the tuning network  38  may be tuned to minimize reflections from the tuning network or, the ratio between the transmitted power and the reflected power may be monitored to determine the optimal impedance match of the antenna  30  to a transmission line (not shown) coupling the telemetry module  22  to inner conductive loop  34 , for example, of antenna  30 . The transmitted power and the reflected power may be sensed and a ratio of the two power levels used to indicate when a minimum standing wave ratio has been achieved. The tuning of the antenna  30  is deemed to be optimal when the standing wave ratio is at its lowest value. As such, during a transmitting operation by the antenna  30 , the tuning network  38  is tuned until the standing wave ratio reaches a minimum value. The tuning of tuning network  38  may be performed prior to each transmission, each telemetry session, every hour, in response to a user input or at any other desired interval or input. 
     In another example, the reflected power may be measured and compared to a threshold value. Adjustments of the tuning network may subsequently be performed based on the results of the comparison of the reflected power to the threshold value. For instance, if the measured reflected power is greater than or lower than the threshold value, the capacitance or inductance of tuning element(s) within the tuning network  38  may be adjusted downward or upward to reduce or eliminate the impedance mismatch. 
     In other embodiments, the criteria used for tuning network  38  may include a quality of service metric. Such service metrics may include but are not limited to a bit error rate, a packet error rate, preamble errors, or a rate of forward error correction. As such, the tuning network  38  may be tuned in response to the quality of service metric to ensure that the most optimal value for the metric is achieved during the antenna  30  communication. For instance, the capacitance of tuning network  38  may be down or upward adjusted to reduce or eliminate the impedance mismatch as a function of the quality of service metric. In one example, a threshold value for the quality of service metric may be predetermined and the capacitance or inductance of tuning network  38  adjusted based on the result of the comparison between the threshold value and a monitored quality of service metric. In this manner, impedance matching of antenna  30  may be realized based on the quality of service metric thereby improving power transfer efficiency for the transmission and reception path. 
     Each of the above additional criteria for tuning the antenna  30  may suitably be used as the primary criteria for tuning or secondary criteria for fine-tuning as discussed in more detail above with respect to the capacitance or inductance of tuning network  38  and the RSSI-based tuning. 
     In the example illustrated in  FIG. 3 , inner antenna element  32  and outer antenna element  34  have a circular shape. However, inner antenna element  32  and outer antenna element  34  may be formed in any of a variety of shapes, including square, rectangle, triangle, oval or any other shape. The antenna elements may also be formed in a single dimensional configuration such as that illustrated in  FIG. 3  or any other configurations such as spiraling or helical. Moreover, inner antenna element  32  and outer antenna element  34  need not be formed in the same shape. In other words, inner antenna element  32  and outer antenna element  34  may be of different shapes. The shapes of inner antenna element  32  and outer antenna element  34  may be dependent on a size and shape of body worn device  16  or other factor. 
     Likewise, the sizes of inner antenna element  32  and outer antenna element  34  may depend on the size and shape of body worn device  16 , the frequency at which communication occurs, or the like. In one example, outer antenna element  34  is less than or equal to approximately one twentieth ( 1/20) of a wavelength at 400 MHz, e.g., approximately 3.75 centimeters (cm), and inner antenna element  32  is less than or equal to approximately one one-hundredth ( 1/100) of a wavelength at 400 MHz, e.g., approximately 7.5 millimeters (mm) As such, the antenna configuration described in this disclosure may provide a small size antenna that maintains a high radiation efficiency. 
     In the example illustrated in  FIG. 3 , inner antenna element  32  is located within outer antenna element  34 . However, inner antenna element  32  may not be located within outer antenna element  34 . Inner antenna element  32  may be located anywhere in which there is sufficient magnetic coupling between inner antenna element  32  and outer antenna element  34  to couple the signals between loops  32  and  34 . Additionally, inner antenna element  32  and outer antenna element  34  may be coplanar or non-coplanar, coaxial or non-coaxial, collinear or non-collinear, or any combination thereof. Inner antenna element  32  and outer antenna element  34  may be located in parallel planes. In other embodiments, inner antenna element  32  and outer antenna element  34  may be located in different planes that are not parallel with one another, but are oriented such that there is sufficient magnetic coupling between inner antenna element  32  and outer antenna element  34 . 
     Moreover, inner antenna element  32  and outer antenna element  34  may be separated by one or more layers of material. For example, inner antenna element  32  may be located within a housing of body worn device  16  while outer antenna element  34  is located outside the housing of body worn device  16  (e.g., on an outer surface of the housing or integrated as part of the housing). Alternatively, both inner antenna element  32  and outer antenna element  34  may be located within the housing of body worn device  16  or both located outside the housing of body worn device  16 . 
       FIG. 4A  is a schematic diagram of a wrist-worn device  40 . In the example of  FIG. 4A , wrist worn device  40  is a watch. Wrist worn device  40  includes a band portion  42  and a face portion  43  that includes a display  44  and a button  46 . Display  44  and button  46  may form all or a portion of user interface  24  ( FIG. 2 ). Band portion  42  may extend at least partially around a wrist of patient  12  and, in some instances, may include an attachment mechanism (not shown) to secure the two ends of band portion  42  together and keep wrist worn device  40  on patient  12 . Band portion  42  may be made of a conductive material (e.g., metal) or a non-conductive material (e.g., rubber, plastic, leather, cloth or the like). 
     Display  44  is illustrated as a digital display that shows a number of different types of information. Display  44  may be a light emitting diode (LED) display, a liquid crystal display (LCD), or other suitable digital or electronic display. In the example illustrated in  FIG. 4A , display  44  shows a time indicator  48 , wear indicator  50  and power source indicator  52 . In other examples, however, display  44  may show more or fewer types of information as well as other types of information. For example, display  44  may show one or more parameters that are measured by wrist worn device  40  (e.g., heart rate), one or more parameters that are measured by IMD  14  and transmitted to wrist worn device  40 , or other information. The different types of information may be shown using any combinations of numbers, letters, symbols, icons or other indicia. In other instances, display  44  may be an analog display or a combination analog and digital display. 
     Wear indicator  50  indicates whether wrist worn device  40  is on patient  12 . In the example illustrated in  FIG. 4A , wear indicator  50  includes the words “ON BODY” and a pair of LEDs  56 A and  56 B. LED  56 A may be a green LED that is lit up to indicate that antenna  30  is tuned for operating when wrist worn device  40  is on patient  12  and LED  56 B may be a red LED that is lit up to indicate that antenna  30  is tuned for operating when wrist worn device  40  is not on patient  12 . Such an indicator may be particularly useful when patient  12  is required to manually indicate when wrist worn device  40  is worn by patient  12 , e.g., via button  46  or other input mechanism. However, wear indicator  50  may also be included on a wrist worn device  40  that senses when wrist worn device  40  is worn by patient  12  to provide a confirmation that such detection is successful. Although illustrated as a pair of LEDs, wear indicator  50  may take on any of a number of forms. For example, wear indicator  50  may be set to “ON BODY” when antenna  30  is tuned for operating when wrist worn device  40  is on patient  12  and set to “OFF BODY” when antenna  30  is tuned for operating when wrist worn device  40  is not on patient  12 . For example, wear indicator  50  may be an icon of a human body (e.g., stick figure or more sophisticated icon) and turn on when antenna  30  is tuned for operating when wrist worn device  40  is on patient  12  and turn off when antenna  30  is tuned for operating when wrist worn device  40  is not on patient  12 . Wear indicator  50  may be any sort of indicator that indicates whether antenna  30  is tuned for operating when wrist worn device  40  is on patient  12 . 
     Power source indicator  52  provides an indication as to how much power is left in power source  29 . Power source indicator  52  may enable patient  12  to recharge, change or otherwise take appropriate action prior to wrist worn device  40  running out of power. 
       FIG. 4B  is a schematic diagram illustrating a top view of one example antenna configuration for wrist worn device  40 . In the example of  FIG. 4B , antenna  30  is located within face portion  43  of wrist worn device  40 . Outer antenna element  34  of antenna  30  may extend substantially around a perimeter of face portion  43 . A portion or all of outer antenna element  34  may be located outside a housing of wrist worn device  40  or form part of the housing of wrist worn device  40 . For example, an outer portion of face portion  43  may be made from a conductive material to form outer antenna element  34  and/or tuning network  38 . Alternatively, a majority of outer antenna element  34  may be outside of or part of the housing of wrist worn device  40  while tuning network  38  is located within the housing. In other instances, both outer antenna element  34  and tuning network  38  are located within the housing. 
     Inner antenna element  32  may also be located inside or outside of the housing of wrist worn device  40 . Additionally, inner antenna element  32  may be located anywhere within face portion  43  depending on the configuration of other components of wrist worn device  40  (e.g., telemetry module  22 , user interface  24 , sensor  25 , processor  26 , memory  28 , and power source  29  illustrated in  FIG. 2 ). Inner antenna element  32  and outer antenna element  34  are oriented with respect to one another such that there is a sufficient magnetic coupling to couple signals between the loops. Inner antenna element  32  and outer antenna element  34  may be in the same plane (i.e., coplanar) or may be in different planes. In the case of different planes, the planes may be parallel planes or non-parallel planes that are oriented with respect to one another such that there is an adequate magnetic coupling between the loops. 
     As described with respect to  FIG. 2 , inner antenna element  32  is electrically coupled to a telemetry module (including a transmitter and/or receiver) located within the housing of wrist worn device  40  via feed points  36 . Tuning network  38  is electrically coupled to a tuning module that provides tuning signal  39 , such as a voltage source that provides a voltage signal. 
       FIG. 4C  is a schematic diagram illustrating a side view of another example antenna configuration for wrist worn device  40  of  FIG. 4A . In the example of  FIG. 4C , outer antenna element  34  of antenna  30  may extend substantially around band portion  42  and a part of face portion  43 . A portion or all of outer antenna element  34  may be located outside a housing of wrist worn device  40  or form part of the housing of wrist worn device  40 . For example, band portion  43  may be made from a conductive material to form outer antenna element  34  and/or tuning network  38 . As another example, band portion  43  may be made from a non-conductive material but include a wire or other conductive material embedded within the non-conductive band. Tuning network  38  may be located within face portion  43  or within a housing of face portion  43 . 
     Inner antenna element  32  may be located within face portion  43 . Inner antenna element  32  and outer antenna element  34  are oriented such that there is a sufficient magnetic coupling to couple signals between the loops. Inner antenna element  32  and outer antenna element  34  may be coplanar, in parallel planes or in non-parallel planes that are oriented with respect to one another such that there is an adequate magnetic coupling between the loops. 
     Wrist worn device  40  illustrated in  FIGS. 4A-4C  is one example of a body worn device in accordance with this disclosure. Other body worn devices that are either worn on the wrist or elsewhere are also within the scope of this disclosure. As such, body worn device  16  may be a bracelet, necklace, ring or other type of jewelry shaped device. Body worn device  16  may be a strap that is placed anywhere on the body of patient  12 , e.g., around an arm, leg, ankle, waist, head, or anywhere else on patient  12 . In some instances, body worn device may be integrated within a clothing article worn by patient  12 . 
       FIG. 5  is a schematic diagram illustrating another example multi-element antenna  30 ′. Antenna  30 ′ may correspond with antenna  30  of  FIG. 2 . Antenna  30 ′ conforms substantially to antenna  30  of  FIG. 3 , but both inner antenna element  32  and outer antenna element  34  are electrically coupled to telemetry module  22  via feed points  36 . Although inner antenna element  32  and outer antenna element  34  are electrically coupled the configuration still has the advantages described above, e.g., larger impedance and tunability. Inner antenna element  32  and outer antenna element  34  may be electrically connected to one another at other locations than feed points  36 . 
       FIG. 6  is a schematic diagram illustrating another example antenna  60 . Antenna  60  may correspond with antenna  23  of  FIG. 2 . Antenna  60  is a dual semi-loop antenna. Antenna  60  includes an inner semi-loop  62  and an outer semi-loop  64  located close to a ground plane  66 . Ground plane  66  mirrors inner semi-loop  62  and outers semi-loop  64  to form mirrored semi-loop  62 ′ and mirrored semi-loop  64 ′, respectively. In operation, antenna  60  conforms substantially to operation of antenna  30 . 
       FIG. 7  is a schematic diagram depicting an alternative embodiment of a multi-element antenna  70 . Antenna  70  may include a plurality of inner antenna elements  72 ′,  72 ,  72 ″ (collectively “ 72 ”) disposed proximate to a first outer antenna element  74 . The plurality of inner antenna elements  72  may have varying sizes in terms of diameter or width. In one example, the plurality of inner antenna elements  72  may be nested in a planar orientation within the outer antenna element  74 . According to this arrangement, one of the plurality of antennas may be selected and electrically coupled to telemetry module  22  via feed points  36 . The selective electrical coupling may be implemented via a switching via a switching network (not shown) such as a digital or analog switches or multiplexers. In so doing, the selected one of the plurality of inner antenna elements  72  may be used to tune the antenna or to vary the bandwidth of antenna  70 . 
     As further illustrated in  FIG. 7 , it should be readily understood that a plurality of outer antenna elements  74 ,  74 ′ (collectively, “ 74 ”) may be provided on antenna  70  and a suitable one of the plurality of outer antenna elements  74  selected to provide optimum impedance matching to the antenna, and for optimum antenna efficiency. In such an arrangement, a switching network may suitably be implemented to selective coupling of the desired outer antenna element. 
     As such, the plurality of inner antenna elements  72  and/or outer antenna elements  74  may be implemented in conjunction with the tuning network  38  described elsewhere in this disclosure. For ease of reference, the plurality of inner antenna elements  72  and/or outer antenna elements  74  will be referred to in this document as switchable tuning elements. One of the switchable tuning elements may be selectively electrically-coupled to define the inner antenna element and the outer antenna element. In doing so, the performance of antenna  70  is suitably optimized for impedance match and for the appropriate channels. 
     As with antenna  30 , the antenna elements  72 ,  74  of antenna  70  may be formed in a variety of geometrical shapes including without limitation square, rectangle, triangle, oval, as well as spiraling and helical configurations. Moreover, the plurality of inner antenna elements  72  may be formed in different shapes and those may also differ from that of outer antenna element  74 . 
     Additionally, the plurality of inner antenna elements  72  and outer antenna elements  74  may be coplanar or non-coplanar, coaxial or non-coaxial, collinear or non-collinear, or any combination thereof. Inner antenna elements  72  and outer antenna elements  74  may be located in parallel planes or located in non-parallel planes with an orientation that provides sufficient magnetic coupling between the selected inner antenna element  72  and outer antenna element  74 . 
     In order to optimize operation of antenna  70 , one or more tuning selection criteria may be monitored with the selection of the appropriate one of the inner antenna elements  72  or outer antenna elements  74  being based on the monitored criteria. Examples of such criteria have been discussed above with respect to the embodiments of antenna  30 . These criteria include, without limitation, sensed proximity to the body, RSSI, transmission loss, return loss, reflected power, standing wave ratio, and various quality of service indicators. 
       FIG. 8  is a schematic block diagram illustrating a system for real-time automatic self tuning of antennas of the present disclosure. According to this embodiment, a self-test module  82  may be incorporated into the IMD  14 , or body worn device  16 , or external device  18 . However, the illustration of  FIG. 8  solely depicts the self-test module  82  in conjunction with body worn device  16  for the sake of simplicity. Self-test module  82  may include a transceiver (not shown) and antenna  83  and is coupled to processor  26  for receiving control signals. Operably, self-test module  82  is controlled by processor  26  to generate a sequence of predefined test bytes that are received by antenna  83 . Antenna  83  may be embodied as any of the aforementioned embodiments of antenna  30  or  70 . The aforementioned tuning methodologies discussed in conjunction with the embodiments of antenna  30  and  70  may be utilized to tune the antenna  83  for optimized operation. The optimization may be accomplished by comparing the received test sequence with the transmitted test sequence with the optimal operation being determined as a function of the aforementioned criteria. The self tuning system of  FIG. 8  may suitably be employed to prevent loss of a portion of a given received signal. The self tuning may be performed periodically, on pre-determined intervals, or in response to a user input or detection of a change in the operating environment of the antenna. 
       FIG. 9  is a flow diagram illustrating example operation of antenna  30  or  70  in accordance with one aspect of this disclosure. Body worn device  16  obtains input indicating whether body worn device  16  is on patient  12  ( 170 ). Processor  26  may receive input from telemetry module  22  indicating an RSSI of the received signals. In another example, processor  26  or other component of body worn device  16  may receive input from sensor  25  or a combination of sensors indicating whether body worn device  16  is on patient  12 . For example, processor  26  may receive input from a proximity sensor having a capacitance that changes due to the proximity of sensor  25  to the skin of patient  12 . As another example, processor  26  may receive input from one or more electrodes that detect cardiac electrical signals of patient  12 . In other embodiments, processor  26  receives input from user interface  24  (e.g., button  46  of wrist worn device  40 ). 
     In the context of an implantable medical device, the antenna  30  or  70  may be tuned to optimize operation prior to implant and subsequent to implantation ( 170 ). IMD  14  may obtain input indicating whether IMD  14  has been implanted. The indication may be received directly from a user or automatically via one or more sensors that provide an indication of whether the IMD  14  has been implanted. 
     Processor  26  or other component of body worn device  16  or IMD  14  analyzes the input to determine the operating environment of the body worn device  16  or IMD  14  in relation to patient  12  ( 172 ). In other words, the determination is made as to whether the body worn device is on patient  12  or in the context of IMD  14 , whether the IMD has been implanted within patient  12 . If, for example, the antenna  30  or  70  is tuned for communication while body worn device  16  is worn on patient  12 , processor  26  may determine that body worn device  16  is no longer on patient  12  based on the above-discussed tuning criteria. Conversely, if antenna  30  or  70  is tuned for communication when body worn device  16  is not worn on patient  12 , processor  26  may determine that body worn device  16  is no longer on patient  12  in response to the tuning criteria. In the case of a proximity sensor, for example, the proximity sensor may output a “0” or “1” when an object is nearby or not, respectively, and processor  26  determines that body worn device  16  is on patient  12  when the output is a “0” and that body worn device  16  is not on patient  12  when the output is a “1”. As another example, processor  26  may analyze the input from the one or more electrodes and determine that body worn device  16  is on patient  12  when a cardiac electrical signal is detected. In the case of input from user interface  24 , processor  26  may determine that body worn device  16  is on patient  12  upon receiving the input signal from user interface  24 , e.g., upon actuation of button  46 . 
     With respect to IMD  14 , input from a sensor may be utilized to provide an indication of the depth of the implant within the body of patient  12 , in addition to determining whether or not the IMD  14  has been implanted. Alternatively, or in addition, processor  26  may determine that IMD  14  has been implanted in patient  12  based on one or more of the above-discussed metrics for the tuning criteria. 
     Similarly, the capacitance or inductance of tuning network  38  of antenna  30  or  70  may be adjusted to compensate for the impedance difference that arises in response to IMD  14  being implanted within patient  12  verses outside the body of patient  12 . Processor  26  will determine whether IMD  14  is implanted in the body of patient  12  (“YES” branch of block  172 ) and if so, the capacitance or inductance of tuning network  38  is adjusted to a first value ( 174 ). In response to determining that IMD  14  is not implanted in patient  12  (“NO” branch of block  172 ), the tuning module adjusts the capacitance or inductance of tuning network  38  to a second, higher value ( 176 ). 
     In response to processor  26  or other component of body worn device  16  determining that body worn device  16  is on patient  12  (“YES” branch of block  172 ), a tuning module adjusts the capacitance or inductance of tuning network  38  to a first value ( 174 ). In response to processor  26  or other component of body worn device  16  determining that body worn device  16  is not on patient  12  (“NO” branch of block  172 ), the tuning module adjusts the capacitance or inductance of tuning network  38  to a second, higher value ( 176 ). For example, the tuning module may apply a tuning signal (such as a DC bias voltage in the case of a varactor) when body worn device  16  is not on patient  12  and remove the tuning signal when body worn device  16  is on patient  12 . In another example, the tuning module may apply a first tuning signal (e.g., voltage signal at a first voltage level) when body worn device  16  is not on patient  12  and apply a second tuning signal (e.g., voltage signal at a second voltage level) when the body worn device is not on patient  12 . Any one of the first or second signals may be a default signal that is predetermined and stored in a memory of the device. In this manner, the capacitance or inductance of tuning network  38  is adjusted to tune the resonant frequency of the multi-element antenna based on whether body worn device  16  is on patient  12 . Moreover, the change in capacitance may compensate for the impedance difference that occurs when body worn device  16  is worn by patient  12  versus not worn by patient  12 . 
     In other embodiments, additional criteria may be utilized at block ( 170 ) in addition to or alternative to the determination of the operating environment of the body worn device  16  (worn or away from body of patient  12 ) and IMD  14  (implanted or away from body of patient  12 ). The additional criteria such as that discussed above may be utilized to further refine the tuning of the antenna for optimized operation. 
     The techniques described in this disclosure may be implemented, at least in part, in hardware, software, firmware or any combination thereof. For example, various aspects of the techniques may be implemented within one or more processors, including one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in programmers, such as physician or patient programmers, stimulators, or other devices. The term “processor” or “processing circuitry” may generally refer to any of the foregoing circuitry, alone or in combination with other circuitry, or any other equivalent circuitry. 
     Such hardware, software, or firmware may be implemented within the same device or within separate devices to support the various operations and functions described in this disclosure. In addition, any of the described units, modules or components may be implemented together or separately as discrete but interoperable logic devices. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. 
     When implemented in software, the functionality ascribed to the systems, devices and techniques described in this disclosure may be embodied as instructions on a computer-readable medium such as RAM, ROM, NVRAM, EEPROM, FLASH memory, magnetic data storage media, optical data storage media, or the like. The instructions may be executed to support one or more aspects of the functionality described in this disclosure. 
     Various examples have been described. It should be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the disclosure in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiments. It should also be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the disclosure as set forth in the appended claims and the legal equivalents thereof.