Antenna devices

1. A communication device comprising:

a first antenna device for a signal having a frequency within an operation band, the first antenna device comprising:

a ground element;

a radiating element short-circuited to the ground element;

a positive feed connected to the radiating element;

a ground feed coupled to the ground element by a capacitive element, the capacitive element is a substantially open circuit for signals having a frequency lower than the operation band and the capacitive element is a substantially short circuit for signals having a frequency within or higher than the operation band; and

a proximity sensor coupled to the ground element such that the ground element forms an electrode to allow the proximity sensor to capacitively sense whether an object is in proximity of the electrode;

a second antenna device; and

an output power controller to selectively reduce output power of a signal applied to the second antenna device in response to the object being sensed by the proximity sensor of the first antenna device.

2. The communication device of claim 1 , wherein the output power controller is to selectively reduce an output power of a signal applied to the positive feed in response to the object being sensed by the proximity sensor.

3. The communication device of claim 2 , wherein the output power controller is to reduce the output power such that a specific absorption rate value is below a pre-defined threshold.

4. The communication device of claim 1 , wherein the operation band is a Wi-Fi band according to IEEE 802.11, a Bluetooth band, or a global positioning system band.

5. The communication device of claim 1 , wherein the radiating element is in a same plane as the ground element.

6. The communication device of claim 5 , wherein the radiating element and the ground element form a planar inverted-F antenna.

7. The communication device of claim 1 , wherein a capacitance of the capacitive element is less than 100 pF.

8. The communication device of claim 7 , wherein the capacitance of the capacitive element is 4 pF.

9. A communication device comprising:

a first antenna device for a signal having a frequency within an operation band, the first antenna device comprising:

a ground element;

a radiating element short-circuited to the ground element;

a feed input connected to the radiating element;

a feed ground;

a coupling between the feed ground and the ground element, wherein

signals having a frequency lower than the operation band of the first antenna device are suppressed from being transmitted between the feed ground and the ground element, and

signals having a frequency within or higher than the operation band are transmitted between the feed ground and the ground element; and

a proximity sensor coupled to the ground element such that the ground element forms an electrode to allow the proximity sensor to capacitively sense whether an object is in proximity of the electrode;

a second antenna device; and

an output power controller to selectively reduce output power of a signal applied to the second antenna device if the object is sensed by the proximity sensor of the first antenna device.

10. The communication device of claim 9 , wherein

the first antenna device is a Wi-Fi antenna device or a Bluetooth antenna device; and

the second antenna device is a an LTE antenna device.

11. A method comprising:

feeding a positive feed of an input signal to a radiating element short-circuited to a ground element, the radiating element and the ground element being part of a first antenna;

feeding a negative feed of the input signal to the ground element by:

allowing signals having a frequency within or higher than an operation band of the first antenna to be transmitted between the negative feed and the ground element, and

suppressing signals having a frequency lower than the operation band of the first antenna from being transmitted between the negative feed and the ground element;

sensing a capacitance of the ground element of the first antenna;

determining a presence of an object based on the sensed capacitance; and

selectively reducing an output power of a signal applied to a second antenna in response to the object sensed by the sensed capacitance of the ground element of the first antenna.

12. The method of claim 11 , further comprising selectively reducing an input signal power to the positive feed in response to the object sensed by the sensed capacitance of the ground element of the first antenna.

13. The communication device of claim 1 , wherein the capacitive element comprises a capacitor.

14. The communication device of claim 1 , wherein the ground element is floating at a frequency lower than the operation band.

15. The communication device of claim 1 , wherein the proximity sensor is coupled to the ground element through an inductor that is a substantially open circuit for signals having a frequency within or higher than the operation band, and is a substantially short circuit for signals having a frequency lower than the operation band.

16. The communication device of claim 9 , wherein the coupling comprises a capacitive element.

17. The method of claim 11 , wherein the negative feed is coupled to the ground element by a capacitive element that is a substantially open circuit for signals having a frequency lower than the operation band, and that is a substantially short circuit for signals having a frequency within or higher than the operation band.

18. The communication device of claim 1 , wherein the second antenna device has an operation band different from the operation band of the first antenna device.

19. The communication device of claim 9 , wherein the second antenna device has an operation band different from the operation band of the first antenna device.

20. The method of claim 11 , wherein the second antenna has an operation band different from the operation band of the first antenna.