PORTABLE ELECTRONIC DEVICE
CROSS-REFERENCE TO RELATED
 This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 094131716 filed in Taiwan, R.O.C. on Sep. 14, 2005, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION  1. Field of Invention
 The present invention relates to a portable electronic device, and more particularly, to a portable electronic device having a signal transmitter/receiver to receive or transmit wireless signals.
 2. Description of Related Art
 Portable electronic devices such as mobile phones, personal digital assistants (PDA) or hand-held personal computers, pocket personal computers, are easy to carry and enable users to search, save, or process information. Moreover, they may be used as multimedia for playing games, showing images or films, or playing music. Such portable electric devices have recently become the most popular electronic products. However, they still have not met the demand of consumers. Therefore, global positioning system (GPS) navigators that enable users to receive geographic information at anytime and anywhere have received increasing attention from consumers. Examples of GPS navigators include the products of, eTrex Legend, iQue® M5 QuestTM 2 produced by Garmin International Ltd (PI://3w.garmin.com/). The GPS navigator has a display and digital geographic maps embedded therein. The antenna of the GPS navigator receives low-power, ultra high frequency (UHF) wireless electronic signals from 24 satellites and five ground stations to thereby obtain positional information. With such information and its embedded digital maps, the GPS navigator can utilize its trip planning function to thereby obtain accurate positional information and directional information.
 The display of the GPS navigator has two display modes: vertical and horizontal. For instance, the iQue® M5 produced by Garmin International Ltd. has a vertical-mode display, and QuestTM 2 has a horizontal-mode display. Vertical-mode GPS navigators may be easy to be held in hand, and horizontal-mode GPS navigators are more convenient for viewing broad scale of electronic maps. In order to provide more options for consumers, some manufacturers have introduced GPS navigators having displays that may change their display mode based on the position of the navigators. That is, when a GPS navigator is used in a vertical position, its display will be in the vertical mode, and when the GPS navigator is used in a horizontal position, its display mode will be switched to the horizontal mode. When a user holds a GPS navigator while walking, the display of the GPS navigator may be set to the vertical mode. When a user is driving, the display of the GPS navigator may be set to the horizontal mode to display broad scale of electronic maps.
 The switchable display mode of GPS navigators have made the GPS navigators easier to use. However, when a user changes the position of the GPS navigator, the change
may affect the transmission and reception of signals of the antenna of the GPS navigator. Taking the products of Garmin International Ltd., iQue® M5 and Quest® 2, as an example, both the products have an antenna connected to the body of the GPS navigator through a one-way pivot, so that the antenna can be received at the back of the body of the GPS navigator or can be rotated up to receive or transmit signals. However, if such a design is applied to GPS navigators with a switchable display mode, although the antenna could still be rotated up, the antenna's position after being rotated up will only match one type of display mode. For example, if the antenna is designed for a vertical display mode, when horizontal display mode is taken, the antenna cannot be totally rotated outward to sky to receive satellite signals. This problem may be solved by adopting a two-way hinge structure as is used in flip cell phones. However, in the two-way hinge structure, only one pivot is located at one end of the body of a GPS navigator. If the antenna is set to a position in light of the vertical display mode, then it may not be rotated outside the body of the GPS navigator totally when the navigator is in the horizontal mode.
SUMMARY OF THE INVENTION
 As described above, GPS navigators having a display mode switchable according to the position of the navigators may make it more convenient for users to view the display when the navigators are operated in different ways. However, at present, the design of GPS navigators with a switchable display mode has not taken into account the relationship between antenna position and display mode. If the antenna is positioned in light of the vertical display mode, then it may not be able to receive and transmit signals well when the navigator is in the horizontal display mode. If the antenna is positioned in light of the horizontal display mode, then it may not be able to receive and transmit signals well when the navigator is in the vertical display mode. In order to solve the problem, the present invention provides a portable electronic device having a signal transmitter/receiver whose position may be changed according to the way in which the portable electronic device is operated.
 A portable electronic device according to an exemplary embodiment of the present invention includes a body and a signal transmitter/receiver. A housing is formed on one end of the back of the body. A ball sleeve is formed on one corner of the housing. Also, a first rotation slot and a second rotation slot corresponding to the ball sleeve are formed on the housing. The signal transmitter/receiver has a ball spiral designed for being received in the ball sleeve, a reliable first ball corresponding to the first rotation slot, and a second rollable ball corresponding to the second rotation slot. The transmitter/receiver may be rotated around a first rotation axis defined by the first ball and the ball spiral, or may be rotated around a second rotation axis defined by the second ball and the ball spiral. Thus, the signal transmitter/receiver may have positions of an accommodating position that the signal transmitter/receiver is received in the housing, a first signal receiving position that the signal transmitter/receiver may be rotated outward around the first rotation axis, and a second signal receiving position that the signal transmitter/ receiver may be rotated outward around the second rotation axis. Therefore, the position of the transmitter/receiver may be adjusted to better transmit or receive signals according to the position of the body.