Antenna system and portable telephone set

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to an antenna device that prevents a decrease in gain of an inverted-F antenna and a mobile phone incorporating the antenna device.

【0002】

[Problems to be Solved by the Invention]

A mobile phone generally includes a whip antenna provided so as to protrude from the housing and a built-in antenna provided inside the housing, and the whip antenna is used as a main antenna for both transmission and reception for radiating and capturing radio waves, and is built-in. The antenna is used as a reception-only sub-antenna for capturing radio waves. The mobile phone is configured to perform diversity reception by the whip antenna and the built-in antenna.

【0003】

Since there is a great demand for miniaturization and weight reduction of mobile phones, the above-mentioned built-in antenna is also required to be small. As one of the built-in antennas that meet such demands, a plate-shaped inverted-F antenna has been conventionally adopted. In this inverted-F antenna, the radiation conductor plate is arranged so as to face the ground conductor plate at a predetermined distance, a short-circuit portion is formed at one end of the radiation conductor plate, and the short-circuit portion and the ground conductor plate are connected. It is composed of. Further, a feeding portion is formed at one end of the radiating conductor plate in a state of being separated from the short-circuited portion by a predetermined distance.

【0004】

However, this inverted-F antenna is easily affected by surrounding objects. For example, if you hold a mobile phone with a built-in inverted-F antenna in your hand, put it in your chest pocket, or place it on your desk, the inverted-F antenna will be affected. There is a problem that capacitive coupling occurs between the antenna and the human body or the desk, the resonance frequency of the inverted-F antenna is lowered, and the gain in the reception frequency band is lowered. In particular, when the mobile phone is placed on a desk, the polarization plane of the whip antenna has a relationship orthogonal to the main polarization (vertical polarization) component of the incoming wave from the base station, so that the gain of the whip antenna also decreases. There is a problem that a sufficient gain cannot be obtained even by diversity reception and the reception sensitivity is lowered.

【0005】

The present invention has been made in view of the above circumstances, and an object of the present invention is an antenna device capable of ensuring a desired gain in the reception frequency band even when a surrounding object approaches the inverted-F antenna, and the above-mentioned antenna device. The purpose is to provide a high-sensitivity mobile phone with a built-in antenna device.

【0006】

[Means for solving problems]

The means described in claim 1 can be adopted to achieve the above-mentioned object. According to this means, when the resonance frequency of the inverted-F antenna is lowered by being placed close to a human body or a desk, the detecting means detects that the resonance frequency is lowered and reduces the resonance frequency. To compensate, a switching signal is output to the switching means so that the resonance frequency of the inverted-F antenna rises.

【0007】

The radiation conductor plate of the inverted-F antenna is provided with a plurality of short-circuited portions corresponding to a plurality of different resonance frequencies, and the switching means receives the switching signal and selects one of the plurality of short-circuited portions. Connect to the ground conductor plate. As a result, the resonance frequency of the inverted-F antenna is controlled to be substantially constant in the reception frequency band or a frequency in the vicinity thereof regardless of the surrounding state in which the inverted-F antenna is placed, and the gain in the reception frequency band is reduced. Can be prevented.

【0008】

According to the means described in claim 2, as in the means described in claim 1, when the resonance frequency of the inverted-F antenna decreases, the detecting means detects the decrease and the resonance frequency of the inverted-F antenna increases. The switching signal is output to the switching means as described above. In this case, the radiation conductor plate of the inverted-F antenna is provided with a plurality of feeding portions corresponding to a plurality of different resonance frequencies, and the switching means receives the switching signal and is one of the plurality of feeding portions. Select. With this means as well, the resonance frequency of the inverted-F antenna can be controlled to be substantially constant, and a decrease in gain in the reception frequency band can be prevented.

【0009】

According to the means according to claim 3, the detection means includes a detection conductor plate, a transmission circuit, a level detection circuit, and a comparison circuit. Of these, the detection conductor plate is arranged so as to be substantially parallel to the radiation conductor plate. Therefore, when the antenna device is placed close to a surrounding object such as a human body or a desk, the detection conductor plate and these surrounding objects are arranged. In between, couplings occur according to the magnitude of the capacitive coupling that occurs between the inverted F antenna and these surrounding objects.

【0010】

Now, when the detection current is output from the transmission circuit to the detection conductor plate and the reflection level of the detection current is detected by the level detection circuit, the magnitude of the capacitive coupling that occurs between the detection conductor plate and the surrounding object described above The reflection level will be different accordingly. Therefore, when the detected reflection level is compared with the preset reference level using a comparison circuit, the capacitive coupling that occurs between the detected conductor plate and the surrounding object, that is, between the radiating conductor plate and the surrounding object. You can know the size of. Since the resonance frequency of the inverted-F antenna changes according to the magnitude of the coupling between the radiation conductor plate and the surrounding object, the resonance of the inverted-F antenna can be obtained by using the output signal of the comparison circuit as a switching signal for the switching means. The frequency can be controlled to be almost constant.

【0011】

According to the means described in claim 4, the antenna device is built in the mobile phone, and diversity reception is performed by the inverted F antenna and the main antenna. Regardless, the effect of diversity reception can be sufficiently obtained. As a result, the mobile phone can always be in a highly sensitive state.

【0012】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment in which the antenna device of the present invention is applied to a mobile phone will be described with reference to the drawings. FIG. 3 shows a rear view of the mobile phone. In FIG. 3, the housing 2 of the mobile phone 1 is configured by combining the front cover 3 and the back cover 4. An antenna case 5 is integrally provided in the upper corner of the housing 2 so as to project upward, and a whip antenna 6 (corresponding to the main antenna in the present invention) is inside the antenna case 5. Are arranged. In addition, the front cover 3 has a call start key, a call end key, a keypad with "0" to "9" number keys, a redial key, etc., a display for displaying the reception status of radio waves, a microphone, and the like. Speakers and the like (none of which are shown) are provided.

【0013】

Inside the housing 2, a multilayer-structured printed circuit board 7 on which electronic components and the like are mounted is stored and fixed in a state of being substantially parallel to the front cover 3 and the back cover 4. On the surface of the printed circuit board 7 facing the back cover 4 (hereinafter referred to as the back surface), the inverted F antenna 8 is located at a position biased to the side opposite to the arrangement position of the whip antenna 6 (left side in FIG. 3). It is installed. Further, on the inner side surface of the back cover 4 (the inner surface of the housing 2), the detection conductor plate 9 is attached at a position corresponding to the substantially center position of the printed circuit board 7 and not overlapping with the inverted F antenna 8. ing.

【0014】

FIG. 2 shows the mounting state of the inverted F antenna 8 and the detection conductor plate 9 as a perspective view with the covers 3 and 4 omitted. In FIG. 2, the inverted-F antenna 8 is a rectangular radiating conductor plate 10 supported by a support member (not shown) so as to be parallel to the printed circuit board 7 and separated by a predetermined distance, and the radiating conductor. From the narrow feeding portion 11 formed integrally with the radiation conductor plate 10 at the corner of the plate 10 so as to hang down from the end, and from the end of the same side as the side on which the feeding portion 11 is formed. It is composed of narrow stubs 12 and 13 (corresponding to a short-circuit portion in the present invention) formed integrally with the radial conductor plate 10 so as to hang down. In this case, one of the inner layers of the printed circuit board 7 is a ground layer having a ground potential, and the ground layer functions as a ground conductor plate for the inverted-F antenna 8.

【0015】

The peripheral length of the radiating conductor plate 10 is set to be approximately half of the wavelength in the reception frequency band of the mobile phone 1, for example, 1477MHz to 1501MHz. If the peripheral length is insufficient, the radiating conductor plate 10 is set. The structure may have a notch. The positions where the feeding portions 11 and the stubs 12 and 13 are formed on the radiating conductor plate 10 are set so as to obtain a predetermined resonance frequency, which will be described in detail later. Further, the power feeding unit 11 and the stubs 12 and 13 are in a state where their lower end surfaces are in contact with the pads 14, 15 and 16 formed on the back surface of the printed circuit board 7, respectively, and then soldered or the like. Conductively fixed to each of the pads 14, 15 and 16.

【0016】

On the other hand, the detection conductor plate 9 attached to the inner surface of the back cover 4 has a rectangular shape having an area smaller than that of the radiation conductor plate 10, and is substantially parallel to the radiation conductor plate 10. It is implemented. Further, a narrow contact piece portion 17 is integrally formed at a corner portion of the detection conductor plate 9 so as to hang down from the end portion thereof. Then, when the back cover 4 is closed to the front cover 3 and the printed circuit board 7 is housed in the housing 2, the lower end surface of the contact piece 17 can be made conductive to the pad 18 formed on the back surface of the printed circuit board 7. It is designed to come into contact. The radiating conductor plate 10 is also attached to the inner surface of the back cover 4 in the same manner as the detection conductor plate 9, and the lower end surfaces of the feeding portion 11 and the stubs 12 and 13 are attached to the pads 14, 15 and 16 in the mounted state, respectively. It may be configured to be in contact with each other.

【0017】

FIG. 1 schematically shows the electrical configuration of the antenna device 19. In FIG. 1, the feeding unit 11 of the inverted-F antenna 8 is connected to a receiving circuit (not shown). The stubs 12 and 13 are the usual electronic switches 20 (corresponding to the switching means in the present invention) configured as, for example, an IC of a GaAs process via pads 15 and 16 (not shown in FIG. 1), respectively. It is connected to the closed side contact 20b and the normally open side contact 20a. Further, the common contact 20c of the switch 20 is connected to the ground layer of the printed circuit board 7.

【0018】

Further, the detection conductor plate 9, the transmission circuit 21, the directional coupler 22, the level detection circuit 23, and the comparison circuit 24 constitute the detection means in the antenna device 19. That is, the transmission circuit 21 is configured to output a detection current having a frequency different from the reception frequency band described above, and the output end of the transmission circuit 21 is a directional coupler 22 and a pad 18 (FIG. 1 is a diagram). It is connected to the contact piece 17 of the detection conductor plate 9 via (not shown). Then, the coupling output of the directional coupler 21 is given to the level detection circuit 23.

【0019】

The level detection circuit 23 is configured as an envelope detection circuit using, for example, a diode and a capacitor (neither shown), and includes a buffer circuit such as an operational amplifier (not shown) connected as a voltage follower. It is connected to the comparison circuit 24 via.

【0020】

The comparison circuit 24 is designed to compare the output voltage of the level detection circuit 23 with a preset threshold voltage as a comparison reference, and is mainly composed of, for example, a comparator. The output (switching signal) of the comparison circuit 24 is given to the control terminal 20d of the switch 20 through a flip-flop circuit (not shown).

【0021】

Next, the operation of the above configuration will be described with reference to FIGS. 4 and 5. In the mobile phone 1, the transmitted wave output from the output circuit (not shown) is radiated as a radio wave through the whip antenna 6. On the other hand, the mobile phone 1 performs diversity reception, and of the radio waves captured by the whip antenna 6 and the radio waves captured by the inverted F antenna 8, the one with the higher reception intensity is given to the receiving circuit (not shown) as the received wave. .. In this diversity reception, the whip antenna 6 and the inverted F antenna 8 are arranged in the housing 2 so that good reception sensitivity can be obtained even when the direction in which the mobile phone 1 is placed changes. It has planes of polarization that are orthogonal to each other in the above-mentioned state.

【0022】

Regarding the reception of the inverted-F antenna 8, the case where the normally closed side contact 20b of the switch 20 shown in FIG. 1 is closed and the stub 12 is connected to the ground layer of the printed circuit board 7 via the switch 20 will be described. To do. In this case, FIG. 4 shows the case where the mobile phone 1 is placed in a free space, that is, a space where there are no objects around (A), and when the mobile phone 1 is placed on a resin desk with the back cover 4 facing down (B). The measurement result of VSWR (standing wave ratio) of the inverted F antenna 8 with) is shown. Here, the horizontal axis is the frequency, and the frequency range shown by the broken line indicates the reception frequency band of the mobile phone 1 described above. In FIG. 4, the frequency at which the characteristic curves A and B are minimized is the resonance frequency of the inverted-F antenna 8, that is, the frequency at which the impedance matching of the inverted-F antenna 8 is the best.

【0023】

When the mobile phone 1 is placed in free space, the inverted-F antenna 8 does not have a capacitive coupling with surrounding objects, so its resonance frequency is equal to the design value. Normally, the resonance frequency of the inverted-F antenna 8 is designed so as to fall within the reception frequency band. Therefore, the resonance frequency of the inverted-F antenna 8 at this time falls within the reception frequency band as shown in the characteristic curve A of FIG. 4, and the inverted-F antenna 8 can obtain the desired gain in this reception frequency band. it can.

【0024】

On the other hand, when the mobile phone 1 is placed on the desk, a capacitive coupling occurs between the inverted F antenna 8 and the desk, so that the resonance frequency drops as shown in the characteristic curve B in FIG. .. As a result, the VSWR in the reception frequency band of the mobile phone 1 increases, and the gain of the inverted F antenna 8 decreases. The amount of decrease in the resonance frequency varies depending on the material of the object placed around the mobile phone 1 and the distance between the mobile phone 1 and the object. For example, when the object is a metal plate, or when the housing portion on which the inverted F antenna 8 is arranged is held by hand, the resonance frequency is further lowered. On the other hand, when the mobile phone 1 is placed on Styrofoam, there is almost no change in the resonance frequency.

【0025】

Furthermore, FIG. 5 shows the measurement result of VSWR of the inverted F antenna 8 when the mobile phone 1 is placed in the free space and the stubs 12 and 13 (see FIGS. 1 and 2) are used. .. In FIG. 5, the characteristic curve A ́ shows VSWR when the normally closed side contact 20b of the switch 20 is closed and the stub 12 is connected to the ground layer of the printed circuit board 7, and the characteristic curve C is the characteristic curve C of the switch 20. It shows VSWR when the normally open side contact 20a is closed and the stub 13 is connected to the ground layer of the printed circuit board 7. The characteristic curve A ́ was measured under the same conditions as the characteristic curve A in Fig. 4, but there are some differences in the characteristics because the type of mobile phone 1 used for the measurement was different.

【0026】

As can be seen from FIG. 5, when the stub 13 is selected in the inverted F antenna 8, the resonance frequency is higher than when the stub 12 is selected. This is because, in the radiation conductor plate 10, the current from the stub 13 is reflected at the open end and reaches the power feeding portion 11 with the corner portion diagonal to the corner portion provided with the feeding portion 11 as the open end. This is because the current path is shorter than the current path where the current from the stub 12 is reflected at the open end and reaches the feeding unit 11, and the wavelength of resonance is shortened. Therefore, when the resonance frequency of the inverted-F antenna 8 decreases due to the usage mode of the mobile phone 1, the resonance frequency of the inverted-F antenna 8 is prevented from decreasing by switching from the stub 12 to the stub 13, and the resonance frequency is kept substantially constant. It becomes possible.

【0027】

Therefore, the means for detecting the decrease in the resonance frequency will be described below. The detection conductor plate 9 is formed small enough not to affect transmission / reception related to the whip antenna 6 and the inverted F antenna 8, and is provided at an appropriate distance from both antennas 6 and 8. Then, the detection conductor plate 9 constitutes a detection resonator together with the ground layer of the printed circuit board 7. In this case, since the detection conductor plate 9 is attached to the inner surface of the back cover 4, the distance to the object around the mobile phone 1 is smaller than that of the inverted-F antenna 8, and the detection conductor plate 9 is in a state of being easier to connect. .. As a result, it is possible to prevent a decrease in the bond between the detection conductor plate 9 having a small area and the surrounding object. Further, since the detection conductor plate 9 is provided substantially parallel to the radiation conductor plate 10 of the inverted F antenna 8, the change in the resonance frequency of the inverted F antenna 8 due to an object around the mobile phone 1 and the resonance for detection are caused. It tends to be almost equal to the change in the resonance frequency of the body.

【0028】

The low power detection current output from the transmission circuit 21 is supplied to the detection conductor plate 9 through the directional coupler 22. With the mobile phone 1 placed in the free space, the detection resonator is in a resonance state due to the detection current supplied from the transmission circuit 21. Then, in this resonance state, the reflection from the detection conductor plate 9 is small, and therefore the output level of the directional coupler 22 is small.

【0029】

On the other hand, when the mobile phone 1 is placed on the desk, a capacitive coupling occurs between the detection conductor plate 9 and the desk, so that the detection resonator deviates from the resonance state due to the detection current (see FIG. 4). .. As a result, the reflection from the detection conductor plate 9 increases, and the coupling output current of the directional coupler 22 increases. In this case, by appropriately setting the position, size, detection current, etc. of the detection conductor plate 9, the amount of change in the coupled output current and the amount of change in the resonance frequency of the inverted-F antenna 8 are quantitatively related. It becomes possible.

【0030】

The level detection circuit 23 converts this coupled output current into a voltage by passing it through, for example, a resistor, and then performs envelope detection by a diode and a capacitor. As a result, the level detection circuit 23 can obtain a detection signal as a voltage that is substantially proportional to the amount of reflected current from the detection conductor plate 9. Further, since the detection signal is output via an operational amplifier that functions as a voltage follower, the accuracy of envelope detection does not deteriorate even when the input impedance of the comparison circuit 24 is low.

【0031】

The level detection of the coupled output current and the switching of the switch circuit 20 by such a circuit are performed in a section (idle section) in which transmission / reception is not performed in a series of operations of the mobile phone 1 shown in FIG. First, the level is detected from the detection resonator in the idle section. At this time, if the mobile phone 1 is not placed on a desk or the like, a detection signal at a level lower than the threshold value is detected. In this case, the normally closed side contact 20b is selected in the switch circuit 20 by the switching signal from the comparison circuit 24, and the stub 12 is connected to the ground layer of the printed circuit board 7 to maintain the (initial) state. As a result, the resonance frequency of the inverted F antenna 8 is maintained in the normal state. Then, the level is detected periodically in the idle section, and the stub 12 is kept in the selected state while the detection level continues to be lower than the threshold value.

【0032】

On the other hand, when the mobile phone 1 is placed on a desk or the like and the detection level from the detection resonator becomes high, the switch circuit 20 is selected by the normally open side contact 20a by the switching signal from the comparison circuit 24. Then, the stub 13 is switched to the state of being connected to the ground layer of the printed circuit board 7. As a result, the resonance frequency of the inverted F antenna 8 becomes high. Then, the level is detected for each idle section, and the stub 13 is kept in the selected state as long as the state in which the detection level is higher than the threshold value continues.

【0033】

That is, the stub of the inverted F antenna 8 in the next reception section is selected according to the level detected in the idle section. Then, at the control terminal 20d of the switch circuit 20, a switching signal (for example, H level) for selecting the normally open side contact 20a is maintained when the detection resonator deviates from the resonance state due to the mobile phone 1 being placed on a desk or the like. When the mobile phone 1 is placed in the free space again and the detection resonator returns to the resonance state, a switching signal (for example, L level) for selecting the normally closed side contact 20b is continuously given.

【0034】

As described above, according to the present embodiment, when an object such as a metal plate, a human body, or a desk approaches the mobile phone 1 and the resonance frequency of the inverted-F antenna 8 decreases, the detection means decreases the resonance frequency. Is detected and the stubs 12 and 13 of the inverted-F antenna 8 are switched to set the resonance frequency high, so that the resonance frequency of the inverted-F antenna 8 can be prevented from decreasing, and the resonance frequency is always kept in or near the reception frequency band. Can be done. As a result, the gain of the inverted-F antenna 8 in the reception frequency band can be maintained high. As a result, even when the mobile phone 1 is placed close to another object, good reception sensitivity can be obtained by diversity reception.

【0035】

(Other Embodiments) The present invention is not limited to the embodiments described above and shown in the drawings, and may be configured as follows, for example. The number of stubs is not limited to two, and three or more stubs may be provided. In this case, the contact configuration of the switch 20 is changed according to the number of stubs. Then, the comparison circuit 24 compares the detection signal output from the level detection circuit 23 with a plurality of threshold values so as to output a multi-step switching signal according to the number of stubs to the switch 20. Configure. According to this configuration, the fluctuation of the resonance frequency of the inverted-F antenna 8 can be further reduced regardless of the ambient state of the mobile phone 1.

【0036】

One stub may be provided on the radiation conductor plate 10 of the inverted-F antenna 8, and a plurality of feeding portions corresponding to a plurality of different resonance frequencies may be provided. In this case, the switch 20 selects one from the plurality of power feeding units according to the switching signal output from the comparison circuit 24, and connects the selected power feeding unit to the receiving circuit. With this configuration, the same effect as that of the above-described embodiment can be obtained.

【0037】

The detection conductor plate 9 may be provided in a state of being mounted on the printed circuit board 7, and the shape is not limited to a rectangle. Further, the detection conductor plate 9 may be divided into a plurality of parts and arranged in consideration of the position where the mobile phone 1 is held and the mode when the mobile phone 1 is placed on the desk. In order to prevent the return current from the detection conductor plate 9 from returning to the transmission circuit 21, a circulator may be provided at the output end of the transmission circuit 21.

[Simple explanation of drawings]

[Figure 1]

Schematic electrical configuration diagram of an antenna device showing an embodiment of the present invention [Figure 2]

Perspective view showing the mounting state of the inverted F antenna and the detection conductor plate. [Fig. 3]

Rear view of mobile phone [Fig. 4]

Characteristic diagram showing VSWR when the inverted F antenna is placed in free space and on a desk [Fig. 5]

Characteristic diagram showing VSWR when switching the stub of the inverted F antenna [Fig. 6]

Diagram showing transmission / reception operation of a mobile phone [Explanation of symbols]

1 is a mobile phone, 6 is a whip antenna (main antenna), 8 is an inverted F antenna, 9 is a detection conductor plate (detection means), 10 is a radiation conductor plate, 11 is a feeding part, 12 and 13 are stubs (short circuit part). , 19 is an antenna device, 20 is a switch (switching means), 21 is a transmitting circuit (detecting means), 23 is a level detecting circuit (detecting means), and 24 is a comparison circuit (detecting means).