KR20090009008A - Monopole antenna apparatus for receiving multiband broadcasting - Google Patents

Abstract

A monopole antenna apparatus for receiving multiband broadcasting is provided to maintain high antenna sensitivity and antenna gain by making the length of an extension antenna shorter than the length of a monopole antenna part. A monopole antenna apparatus for receiving multiband broadcasting comprises a receiver(120), a monopole antenna(130) and an extension antenna(140). The receiver is positioned on a substrate(110). The monopole antenna is lead-in and withdrew. The monopole antenna has a contacting portion(131) for electrical contact. The end part of the extension antenna is connected to the contacting portion and the receiver of the monopole antenna. The extension antenna is arranged with being adjacent to the monopole antenna. The length of the extension antenna becomes the length resonating in a prescribed band among a plurality of bands in the lead-in of the monopole antenna. The length of the extension antenna becomes the length resonating in the multiband including a prescribed band in the withdrawal of the monopole antenna.

Description

MONOPOLE ANTENNA APPARATUS FOR RECEIVING MULTIBAND BROADCASTING}

The present invention relates to a monopole antenna for multi-band broadcast reception, and more particularly, to a monopole antenna device for multi-band broadcast reception capable of receiving a relatively low VHF and UHF band simultaneously using a minimum area.

Due to the rapid spread of mobile communication systems and wireless data communication systems and the mixing of various existing wireless broadcasting and wireless technologies, the demand for wireless means capable of operating in multiple frequency bands or wide frequency bands has recently increased.

In particular, as the demand for digital convergence has soared, the user's taste to satisfy various needs by purchasing a single product has become common.

A representative example of such digital convergence is a combination of communication, broadcasting, and multimedia entertainment. The products that used to be individual products such as mobile terminals, broadcast receivers, and personal multimedia players have been combined to emerge as integrated products equipped with communication functions, broadcast reception functions, and multimedia playback functions. The road is constantly shrinking.

Examples of products with such integrated functions include various multimedia playback functions as well as terrestrial or satellite DMB (TDMB, SDMB, DVB-H, etc.) reception functions as shown in FIGS. 1 and 2. The digital broadcast receiver 10 and the digital broadcast receiver terminal 20 in which the DMB reception function is integrated into the mobile communication terminal function. In addition, products that have a broadcast reception function added to a game machine or a DMB reception function and a multimedia playback function added to a navigation using satellite GPS are also generalized.

However, convergence products that provide such functions still share configurations for a display unit, a user interface, a power supply, and the like, and configurations of a communication unit for performing a plurality of communication functions are combined in separate states. Accordingly, an antenna for each communication unit is separately used, and an external metal pipe antenna is generally applied to a broadcast reception antenna for receiving a relatively low frequency, in particular, the antennas 11 and 21 for digital broadcast reception.

Due to the use of such an external metal pipe antenna, a considerable area of the design area of the portable devices, which is becoming slimmer, must be allocated for the antenna, which causes a limitation in reducing the size of the portable devices. Even if the loss is reduced the length of the antenna. For example, recently used terrestrial multimedia broadcasting (TDMB) receiving antennas use 1/8 wavelength antennas instead of 1/4 wavelength antennas to reduce the size of the antennas exposed to the outside.

In particular, in recent years, various multimedia broadcasts are mixed and serviced in a single region or an adjacent region. Therefore, a method for receiving various multimedia broadcasts using a single receiver has been studied. A plurality of antennas may be mounted on the multimedia broadcasting receiver. In this case, however, the reception sensitivity is relatively good, but the size of the receiver is inevitably increased due to the space occupancy according to the individual antenna configuration. This is a phenomenon that is placed in front of the direction of the design of a portable device aimed at light weight and shorten the design, and there is an active research to configure an individual antenna having a limitation in physical placement and application as a multi-band antenna. .

This multi-band antenna supports handover between different broadcast services in a country or region where multiple broadcast services are simultaneously applied, with the basic purpose of supporting a plurality of different multimedia broadcast services using a single receiver. It can also be said to be essential.

3 illustrates a configuration of a monopole antenna for supporting a plurality of relatively low multimedia broadcasting bands. As shown in FIG. 3, a monopole antenna 40 having an electrical contact area at an end thereof and a position of the monopole antenna 40 are illustrated. It consists of a first antenna 45 and a second antenna 46 of different line lengths selectively connected according to. Each of the first antenna 45 and the second antenna 46 has feeding points 41 and 48 for connecting to a separate receiver, respectively.

That is, when the monopole antenna 40 is pulled upward, the antenna is connected to the first antenna 45 to receive the first frequency band, and when the monopole antenna 40 is pulled downward, the second antenna 46 is pulled out. ) Is an antenna for receiving a second frequency band.

Since the above configuration is such that broadcast reception for different bands is possible according to the position of the monopole antenna 40, the adjacent first antenna 45 and the second antenna 46 may be separated from each other by a substrate or separate line arrangement means (eg, For example, a variable substrate, a film, an electric wire, etc.) may be implemented to change characteristics due to adjacency, and broadcasts for different bands may not be received at the same time, and an area for layout may be difficult to reduce.

Therefore, there is an urgent need to develop an antenna with good reception characteristics while simultaneously supporting a plurality of broadcast bands distributed in a relatively low band such as digital broadcasting.

An object of the present invention, which is newly proposed in view of the necessity of the multi-band antenna, constitutes an extension line connected to the monopole antenna and a monopole antenna capable of pulling in and drawing out, and is predetermined when the monopole antenna is inserted. By receiving a broadcast signal of a band, and when the monopole antenna is drawn out, it also receives a broadcast signal of another band including the predetermined band, thereby operating as a multi-band antenna at the time of antenna extraction and a predetermined band at the time of antenna reception. It is to provide a monopole antenna device for multi-band broadcast reception to enable reception for.

Another object of the embodiments of the present invention is to use an extension antenna fixedly connected to one monopole antenna to minimize the space and to freely arrange the antenna, and to allow reception of two or more different broadcast bands. It is to provide a credit monopole antenna device.

Another object of the embodiments of the present invention is to design an extension line to receive a predetermined band in consideration of the length of the effective antenna caused by the change of the characteristics of the entire antenna line according to the introduction of the monopole antenna, and the extension line By providing a antenna structure that resonates for two or more kinds of bands when a monopole antenna is drawn out as a basis, a predetermined band provides a monopole antenna device for receiving a multi-band broadcast that allows reception when both antennas are pulled in and out. .

In order to achieve the above object, the monopole antenna device for multi-band broadcast reception according to an embodiment of the present invention includes a receiver implemented on the substrate; A monopole antenna capable of drawing in and drawing out and having a contact portion for electrical connection; The contact portion, the receiver portion and the distal portion of the monopole antenna are respectively connected, and the length of the monopole antenna is resonant with respect to a predetermined band among a plurality of bands to be received by the line extension effect according to the reception of the monopole antenna. And an extension antenna disposed adjacent to the monopole antenna to have a length that resonates in common for multiple bands including the band.

The extension antenna may be one of a pattern formed on the substrate, a pattern formed on a separate film or a flexible substrate, and a metal line including an electric wire.

The length of the extension antenna has an active state corresponding length longer than the theoretical resonance wavelength length for the predetermined band in consideration of the line extension effect generated when the monopole antenna is introduced, and the length of the monopole antenna is larger than the length of the extension antenna. Long is preferred.

The receiving unit may further include a matching circuit unit for the extension antenna, but the inductance value of the matching circuit unit may be utilized only by a fine adjustment degree using 10 nH or less, thereby ensuring maximum gain and sensitivity characteristics of the antenna. .

When the monopole antenna is drawn out, the monopole antenna and the extension antenna simultaneously receive the broadcast signal of the VHF band and the broadcast signal of the UHF band, thereby supporting multiple bands having a relatively long wavelength.

A monopole antenna for receiving a multi-band broadcast according to another embodiment of the present invention includes a monopole antenna unit capable of pulling in and out of an antenna and having a contact portion electrically connected to the antenna; An extended antenna line connected to the contact portion of the monopole antenna portion, the active state reception for the first band can be received according to the influence of the adjacent antenna due to the antenna introduction of the monopole antenna portion, the physical antenna length according to the antenna withdrawal of the monopole antenna portion And an extension antenna having a length and an arrangement to allow reception for the first band and the second band.

The monopole antenna unit and the extension antenna line may receive at least two different broadcast bands selected from VHF and UHF bands including digital multimedia broadcasting and analog broadcasting.

The length of the extension antenna is shorter than that of the monopole antenna to maintain the antenna gain and antenna sensitivity at a high level.

The monopole antenna device for multi-band broadcast reception according to the embodiment of the present invention constitutes a monopole antenna capable of retraction and retrieval and an extension line connected to the monopole antenna, and receives a broadcast signal of a predetermined band when the monopole antenna is retracted. When receiving the monopole antenna, and receiving the broadcast signal of the other band including the predetermined band, the antenna is not only operating as a multi-band antenna when the antenna is pulled out, but also receiving a predetermined band when the antenna is pulled in. By reducing the size of the antenna has the effect of promoting the user's convenience to use the antenna.

The monopole antenna device for multi-band broadcast reception according to an embodiment of the present invention uses an extension antenna fixedly connected to one monopole antenna to allow free antenna arrangement while minimizing space and to receive two or more different broadcast bands. This has a possible effect.

In the monopole antenna device for multi-band broadcasting reception according to an embodiment of the present invention, an extension line is designed to receive a predetermined band in consideration of the length of the effective antenna caused by the change of characteristics of the entire antenna according to the reception of the monopole antenna. In addition, when the monopole antenna is drawn out based on the extension line, the antenna structure resonates with respect to two or more kinds of bands, thereby minimizing the number and volume of antennas and applying the antenna to a portable device. Receiving at both the incoming and outgoing time of the can be received, there is an effect that can receive a broadcast of a predetermined band without drawing the antenna.

The present invention as described above will be described in detail with reference to the accompanying drawings and embodiments.

Figure 4 is an exemplary view showing a structure of an embodiment of the present invention, a monopole antenna having a contact portion 131 that is pulled up and out by operating up and down as shown in the drawing, and the terminal of the monopole antenna 130 is electrically connected The unit 130 and 131 and the contact portion 131 of the monopole antenna unit are connected to one side thereof, and the other side includes an extension antenna unit 140 connected to the receiving unit 120 on the substrate.

The extended antenna unit 140 may be a line formed on the substrate 110 as shown, but may be an antenna pattern formed on a separate film or a variable substrate, and may be made of an electric wire or a metal plate. That is, the extension antenna unit 140 may have various structures or arrangements according to the shape of the receiver structure 100 or the arrangement of the substrate 110 and the monopole antenna units 130 and 131.

When the monopole antenna 130 is drawn out, the length of the physical antenna connected to the receiver 120 is the length of the monopole antenna 130 plus the length of the extension antenna unit 140. Of course, the length may be offset according to the arrangement of the extension antenna unit 140, the extension antenna unit 140 may be installed in various ways in consideration of these characteristics, as a result of which the monopole antenna 130 In the case of integrated with the extension antenna unit 140, it is possible to receive broadcast signals having a relatively low frequency band through the antenna, in the case of bands having a difference of approximately 1/8 wavelength length by an integral multiple. Multiple bands can be simultaneously received via the antenna structure.

For example, in the case of a terrestrial digital multimedia broadcasting (TDMB) in the 200 MHz band or a digital video broadcasting- handheld (DVB-H) in the 450-740 MHz band, multiband reception is possible even with a single antenna. Therefore, antennas capable of receiving such multi-band broadcasts in China, where 200 MHz VHF band broadcasts or 400-800 MHz UHF band broadcasts will be simultaneously applied, or in regions where broadcasts of various bands are likely to overlap will be necessary. will be.

However, simply designing an antenna that supports multiple bands causes problems in size and volume and adds inconvenience to the user. Therefore, the antenna size should be minimized and the user's convenience should be considered. That is, the best way to reduce the mounting volume of the antenna is to configure a portion of the antenna inside the receiver structure 100 as shown, and to minimize the length of the monopole antenna 130 that is drawn out substantially .

In addition, since the monopole antenna 130 is always drawn out to the outside whenever a broadcast is received, the user may receive a broadcast for at least a portion of the band without drawing the monopole antenna 130 to the outside due to the inconvenience of the user. However, it is preferable that broadcast reception is possible near the transmitter.

For example, assume that the illustrated antenna configuration should support TDMB in the 200 MHz band and DVB-H band in the 450-740 MHz band. In this case, the antenna length for receiving TDMB at 1/4 wavelength length is theoretical and is about 37.5 cm in the laboratory environment, and the length of the antenna capable of receiving DVB-H at 1/2 wavelength length is 450 MHz. The reference value is approximately 33 cm. However, the length of the antenna in the active state in consideration of various external environments or structural conditions of the antenna should be slightly longer than this to obtain a practical sensitivity.

Therefore, in theory, when the length of the extension antenna unit 140 is adjusted in the range of about 30 to 40 cm, when the monopole antenna 130 is drawn out, the TDMB is received at the 1/4 wavelength length and the DVB is 1/2 the length. -H can be simultaneously received, and when the monopole antenna 130 is inserted (a configuration in which the monopole antenna and the extension antenna unit are electrically separated), the length of the extension antenna unit 140 is configured to be in a range of 15 to 20 cm, which is half the length. You can think of it as being able to receive TBMD at 1/8 wavelength and simultaneously receive DVB-H at 1/4 wavelength.

However, when the monopole antenna 130 is substantially introduced, reception characteristics through the extension antenna unit 140 are changed due to the influence of the monopole antenna 130, and thus, unlike the theoretical design, for all bands The reception performance is deteriorated, which causes a problem that cannot be used as a practical reception antenna.

If the monopole antenna 130 is drawn in, even when the monopole antenna 130 and the contact portion 131 maintain electrical connection characteristics (when the monopole antenna 130 is a conductor), the monopole antenna 130 is drawn out. The physical length difference between the one and the whole antenna may not seem to be large, but since the effective monopole antenna 130 length is reduced due to the influence of the adjacent extension antenna unit 140, in this case, the monopole antenna 130 In this case, the reception performance of all bands is deteriorated, and thus a problem that cannot be used as a practical reception antenna occurs.

Therefore, in the present embodiment, when the monopole antenna 130 is inserted, the reception is excellent for only one band among a plurality of bands in consideration of the characteristic change of the monopole antenna 130 on the extension antenna unit 140. While having the characteristic, when the monopole antenna 130 is drawn out, the monopole antenna 130 and the extension antenna unit 140 are designed based on a condition that enables reception of a plurality of bands.

In this case, since the monopole antenna 130 is made of a dedicated antenna material, the longer the length of the monopole antenna 130, the higher the antenna gain in the passive state and the higher the sensitivity in the active state. Accordingly, the length of the monopole antenna 130 is at least longer than the length of the extension antenna portion 140 while considering the limitations described above.

FIG. 5 illustrates the effective antenna length of the entire antenna when the monopole antenna 130 is electrically connected to the contact portion 131 and is connected to the physical length AL1 of the extension antenna 140. A virtual antenna length SL1 representing a state in which the introduced monopole antenna 130 is reduced due to an offset coupling or the like is added to operate with a substantial antenna length. Therefore, the length of the extension antenna unit 140 must be smaller than the target wavelength of the target band to be received.

For example, when the reception band through the extension antenna unit 140 is set to TDMB of the VHF band while the monopole antenna 130 is inserted in the above manner, the target wavelength is 1 of 150Cm which is the wavelength of the actual TDMB. Since 18.75 Cm becomes / 8, the physical line length AL1 of the extension antenna unit 140 is set smaller than this. This design parameter gives a limitation on the length of the extension antenna section.

In addition, as shown in FIG. 6, when the monopole antenna 130 is drawn out, the actual antenna length is the sum of the length AL1 of the extension antenna unit 140 and the length AL2 of the monopole antenna 130. It is possible to design an antenna capable of receiving two desired bands (TDMB and DVB-H in this embodiment). However, in this case, the length of the monopole antenna 130 should be as long as possible and the TDMB band should be received by the influence on the extension antenna unit 140 generated when the monopole antenna 130 is inserted. Therefore, the design parameters can be obtained here.

That is, AL1 must be shorter than a length for receiving a desired first band, and the length combining AL1 and AL2 must be capable of receiving two or more kinds of bands including the first band at the same time. In this case, it is possible to obtain a correlation that an effective antenna considering the influence on the extension antenna unit should be able to effectively receive the first band. In addition, in the above design parameters, the length of AL1 and AL2 can be obtained in accordance with the above conditions in consideration of the preferred design direction in which the longer the AL2 is, the better.

Meanwhile, in the case of the monopole antenna, the influence on the extension antenna varies depending on the material, length, and distance of the extension antenna, and the extension antenna is also received by the monopole antenna according to the width, arrangement, and material of the line. It should be taken into account that the effects may vary and the effects on the characteristics of the entire antenna may also vary.

Let's take an example of designing a monopole antenna device according to this embodiment. First select the desired bands and then determine the monopole antenna. In this embodiment, to set the monopole antenna as long as possible, about 195 mm can be used. Of course, this length is somewhat variable, but it is desirable to be somewhat longer than the length for receiving the desired maximum band of signals. In the illustrated embodiment, the length for receiving the highest band is 165-100 mm, which is 1/4 wavelength of DVB-H band (450-740 MHz), and the length for receiving the lowest band is 1/8 of TDMB. Since the wavelength is 187.5 mm, the length of the monopole antenna is set to about 195 mm, which is longer than this, for example.

In addition, while considering the structure of the extension antenna unit 140, the effects of the case where the monopole antenna 130 is introduced (a change in the length of the antenna, a characteristic change due to cancellation, coupling, etc.) of the extension antenna unit 140 Determine placement and length. In this case, it is preferable to design only for reception for one band selected from among desired bands.

Theoretically, if the design is neglected by the influence of the monopole antenna, it may be possible to receive all the multiband signals even when the monopole antenna is introduced by using half the antenna length (when the monopole antenna is electrically separated from the extension antenna when the antenna is inserted). Receive using the half length of the designed antenna, or when the monopole antenna is electrically connected to the extension antenna at the time of entry, or by using the same length as the designed antenna). It becomes below and becomes invalid. Therefore, the present embodiment uses a scheme of focusing only on reception of one of a plurality of bands in consideration of a change in antenna characteristics according to the introduction of a monopole antenna.

In this case, as shown in FIG. 7, one of the bands f1 selected for the desired bands f1 and f2 can be received even when the monopole antenna is inserted, and the user using the band dares to draw out the monopole antenna. Without being able to receive the broadcast of the corresponding band can greatly enhance the user's convenience.

6 illustrates a case in which the monopole antenna 130 is drawn out, and the extension antenna unit 140 and the monopole antenna 130 (substantially through the length and arrangement of the extension antenna unit 140 obtained in the above manner). However, the characteristics of the antenna coupled to the extension antenna unit 140, but is only shown vertically in the spatial arrangement of the drawing) is determined, in this case desired bands (f1, f2) as shown in FIG. Multiple reception of is possible.

5, it is preferable that the sum of the virtual extension antenna length SL1 and the length AL1 of the physical extension line is longer than a theoretical wavelength for receiving a desired band. Because the configuration is less efficient as an antenna than the material or construction of the monopole antenna, a little longer than the theoretical wavelength length indicates high sensitivity in an active environment that requires consideration of various propagation environments. This ensures some tolerances for the design and can facilitate the design of multiband monopole antennas using extension antennas, which must take into account various parameters. In addition, due to the characteristics of the extended antenna unit 140 designed to be smaller than the length of the substantially required wavelength, a matching unit may be configured in the receiving unit 120 to compensate for this length, but this is forcibly adjusted to a resonance frequency like a general matching unit. Note that the role of the matching part is only a fine adjustment because it is not intended. Therefore, while a general matching part applies inductances of about tens to hundreds of nH, in this embodiment, a matching part applying only an inductance of 10 nH or less can be used, so that the matching part configuration can be simplified. Depending on the band selection and placement design approach, matching may not be used.

In the example of the design method, the length of the monopole antenna is first selected, but the length of the extension antenna unit 140 is shorter than the wavelength length (for example, 187.5 mm in the case of TDMB) for receiving a desired selection band. After the length of the monopole antenna (for example, 160 ~ 187mm), the appropriate length of the monopole antenna can be selected while the multi-band reception and the specific band reception performance by the influence of the monopole antenna are considered.

Of course, a method of confirming an optimal combination by fine-adjusting the length and arrangement of the monopole antenna and the extension antenna unit while interworking the above-described methods may be applied, and a matching means may be further added if necessary.

Therefore, through the above configuration, a monopole antenna for outgoing two or more kinds of bands selected from TDMB, ISDB-T (Japanese terrestrial digital multimedia broadcasting standard), DVB-H (European digital multimedia standard), analog TV / radio broadcasting, etc. It can be received using, and it is possible to receive a broadcast of a predetermined band without drawing out the monopole antenna.

1 is an example of a digital multimedia broadcasting receiver.

2 is another example of a digital multimedia broadcasting receiver.

3 is an example of an antenna configuration supporting multiple bands

Figure 4 is an example showing the structure of an embodiment of the present invention.

5 is an exemplary view showing an effective antenna length at the time of monopole antenna insertion according to an embodiment of the present invention.

6 is an exemplary view showing the effective antenna length when the monopole antenna withdrawal according to an embodiment of the present invention.

7 is a graph showing the characteristics of the entire antenna when the monopole antenna in accordance with an embodiment of the present invention.

8 is a graph showing the characteristics of the entire antenna when the monopole antenna withdrawal according to an embodiment of the present invention.

** Description of symbols for the main parts of the drawing **

10: digital broadcast receiver 11: digital broadcast receiving antenna

20: digital broadcast receiving terminal 21: digital broadcast receiving antenna

30 receiver structure 40 monopole antenna

41: first antenna feeding point 45: first antenna track

46: 2nd antenna track 48: 2nd antenna feeding point

100 receiver structure 110 substrate

120: receiver 130: monopole antenna

131: monopole antenna contact portion 140: extension antenna

Claims (14)

A receiver implemented on the substrate; A monopole antenna capable of drawing in and drawing out and having a contact portion for electrical connection; The contact portion, the receiver portion and the distal portion of the monopole antenna are respectively connected, and the length of the monopole antenna is resonant with respect to a predetermined band among a plurality of bands to be received by the line extension effect according to the reception of the monopole antenna. And an extension antenna disposed adjacent to the monopole antenna so as to have a length resonating in common with respect to the multiple band including the band. The monopole antenna of claim 1, wherein the extension antenna is one of a pattern formed on the substrate, a pattern formed on a separate film or a flexible substrate, and a metal line including wires. Device. The multi-band broadcasting according to claim 1, wherein the length of the extension antenna has an active state corresponding length longer than a theoretical resonance wavelength for the predetermined band in consideration of a line extension effect generated when the monopole antenna is inserted. Reception monopole antenna device. The monopole antenna apparatus of claim 1, wherein a length of the monopole antenna is longer than a length of the extension antenna. The monopole antenna apparatus of claim 1, wherein the receiver further comprises a matching circuit unit for the extension antenna. 6. The monopole antenna device according to claim 5, wherein an inductance value of the matching circuit unit is 10 nH or less. The monopole antenna apparatus of claim 1, wherein the monopole antenna and the extension antenna simultaneously receive a broadcast signal of a VHF band and a broadcast signal of a UHF band when the monopole antenna is drawn out. 8. The monopole antenna apparatus according to claim 7, wherein the digital broadcast signal in the VHF band is a TDMB signal and the digital broadcast signal in the UHF band is DVB-H. A monopole antenna unit capable of retracting and withdrawing an antenna and having an electrical contact to the antenna; An extended antenna line connected to the contact portion of the monopole antenna portion, the active state reception for the first band can be received according to the influence of the adjacent antenna due to the antenna introduction of the monopole antenna portion, the physical antenna length according to the antenna withdrawal of the monopole antenna portion And an extension antenna having a length and an arrangement to enable reception for the first band and the second band by means of the antenna. 10. The monopole antenna apparatus of claim 9, wherein a matching circuit to which an inductance of 10 nH or less is further applied to an end of the extension antenna. 10. The monopole of claim 9, wherein the monopole antenna unit and the extension antenna line receive at least two different broadcast bands selected from VHF and UHF bands including digital multimedia broadcasting and analog broadcasting. Antenna device. The length of the extension antenna is shorter than 1/8 wavelength of the lowest band of the reception band, and the effective antenna length is 1/8 of the lowest band due to the influence of the antenna of the introduced monopole antenna unit. A monopole antenna device for reception of multi-band broadcast, characterized in that it is longer than the wavelength. 10. The method of claim 9, wherein the length of the extension antenna is shorter than 1/8 wavelength of the lowest band of the reception band, and the drawn monopole antenna unit is longer than 1/4 wavelength of the highest band of the reception band. A monopole antenna device for receiving multi-band broadcast. The monopole antenna apparatus of claim 9, wherein the length of the extension antenna is shorter than the antenna length of the monopole antenna unit.

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