Active reduction of electric field generated by a transmit antenna via an auxillary antenna structure
A wireless communication device and method includes an auxiliary antenna that can actively cancel at least a portion of a near-field component of an electric field generated by a main transmit antenna. The auxiliary antenna can help comply with specific absorption rate requirements and can reduce undesirable signal rectification in hearing aid components.
1. A method of operating a wireless communication device comprising a housing, an earpiece speaker integrated with the housing, a main antenna, and an a auxiliary antenna structure disposed inside the housing at a location in close proximity to the earpiece speaker, the method comprising the steps of:
reducing a first magnitude of an amplified RF signal to generate a drive signal having a second magnitude that is less than the first magnitude;
shifting a phase angle of the drive signal to generate a phase-shifted drive signal that is phase shifted with respect to the amplified RF signal; and
driving the main antenna with the amplified RF signal to radiate a first electric field having a first near-field component and a far-field component, wherein the first near-field component of the first electric field has a maximum intensity at a hot spot location and has a first x-component, a first y-component and a first z-component, and simultaneously driving the auxiliary antenna structure with the phase-shifted drive signal to radiate a second electric field having a second near-field component that reduces the magnitude of the first near-field component without causing destructive interference with respect to the far-field component of the first electric field, wherein the auxiliary antenna structure is oriented to radiate the second electric field in the same orientation as the first electric field generated by the main antenna, wherein the second near-field component of the second electric field has a second x-component that is substantially oriented with the first x-component, a second y-component that is substantially oriented with the first y-component, and a second z-component that is substantially oriented with the first z-component, and wherein the second near-field component destructively interferes with the first near-field component and reduces the magnitude of the first near-field component that is incident at the hot spot location.
2. A method according to claim 1 , wherein the auxiliary antenna structure is disposed underneath the hot spot location.
3. A method according to claim 1 , wherein the auxiliary antenna structure is a non-resonant auxiliary antenna structure that does not have natural frequencies of oscillation and does not have natural resonance on frequency bands that the main antenna operates over.
4. A method according to claim 1 , wherein the phase shifter comprises an adjustable phase shifter, and further comprising the step of:
adjusting a phase shift applied to the drive signal to control the phase of the second electric field.
5. A method according to claim 1 , wherein the step of shifting the phase of the drive signal to generate a phase-shifted drive signal having an angle that is phase shifted with respect to the amplified RF signal, comprises:
applying a discrete phase shift to the drive signal to generate a phase-shifted drive signal having a phase angle that is phase shifted with respect to the amplified RF signal such that the auxiliary antenna structure generates destructive interference to reduce the magnitude of the first near-field component at the hot spot location.
6. A method according to claim 1 , wherein the main antenna is disposed within the housing.
7. A wireless communication device, comprising:
a housing; an earpiece speaker integrated with the housing; a main antenna designed to radiate, in response to an amplified RF signal having a first magnitude, a first electric field having a far-field component and a first near-field component that has a maximum intensity at a hot spot location;
a coupler designed to receive the amplified RF signal and to reduce the first magnitude of the amplified RF signal to generate a drive signal having a phase angle and a second magnitude that is less than the first magnitude;
a phase shifter, coupled to the coupler and being designed to receive the drive signal, the phase shifter being designed to shift the phase angle of the drive signal to generate a phase-shifted drive signal that is phase shifted with respect to the amplified RF signal; and
an auxiliary antenna structure coupled to the phase shifter and being disposed inside the housing beneath the hot spot location and in close proximity to the earpiece speaker, the auxiliary antenna structure being designed to radiate, in response to the phase-shifted drive signal, a second electric field having a second near-field component that reduces the magnitude of the first near-field component at the hot spot location without causing destructive interference with respect to the far-field component of the first electric field, wherein the auxiliary antenna structure is oriented to radiate the second near-field component in the same orientation as the first near-field component, wherein the first near-field component of the first electric field has a first x-component, a first y-component and a first z-component, wherein the near-field component of the second electric field has a second x-component that is substantially oriented with the first x-component, a second y-component that is substantially oriented with the first y- component, and a second z- component that is substantially oriented with the first z-component, and wherein the second near-field component destructively interferes with the first near-field component to reduce the first near-field component at the hot spot location.
8. A wireless communication device according to claim 7 , wherein the second near-field component reduces the first near-field component that is incident at the hot spot location.
9. A wireless communication device according to claim 7 , wherein the auxiliary antenna structure is a non-resonant auxiliary antenna structure that does not have natural frequencies of oscillation and does not have natural resonance on frequency bands that the main antenna operates over.
10. A wireless communication device according to claim 7 , wherein the phase shifter comprises an adjustable phase shifter that allows a phase shift applied to the drive signal to be adjusted so that a phase of the second electric field is controllable.
11. A wireless communication device according to claim 7 , wherein the phase shifter applies a discrete phase shift to the drive signal to generate the phase-shifted drive signal, wherein the phase angle of the phase-shifted drive signal is phase shifted with respect to the amplified RF signal such that the auxiliary antenna structure generates destructive interference to reduce the magnitude of the first near-field component at the hot spot location.
12. A method according to claim 7 , wherein the main antenna is disposed within the housing.