EP1589608A1 - Compact RF antenna - Google Patents

Abstract

The antenna has a counterpoise (14), and a radiant unit (12) presenting an open loop shape and including two branches (16, 18) coupled to an antenna cable. The branches respectively define a zigzag forming portion (22) and a rectangular portion (26). The portions are set to resonate at a preset cellular frequency band and extend along parallel directions causing radio-frequency disturbance and coupling phenomena between the portions.

Description

The present invention relates to a compact antenna of monopole type for the transmission and reception of signals, suitable for used in particular but not exclusively in a portable terminal operating at different frequency bands.

• une surface conductrice formant un élément radiant et comprenant une zone de connexion électrique,
• un contrepoids compact conducteur comportant une zone de connexion,
• une ligne d'alimentation comprenant un câble d'antenne relié aux zones de connexion dudit élément radiant et dudit contrepoids compact.

More specifically, the invention relates to a compact monopole type antenna comprising:

• a conductive surface forming a radiant element and comprising an electrical connection zone,
• a compact conductive counterweight comprising a connection zone,
• a power supply line comprising an antenna cable connected to the connection zones of said radiant element and said compact counterweight.

Currently, the specifications of an antenna must meet at least two requirements: on the one hand the antenna must be able to function at different frequency bands of the types of networks available and, on the other hand, the dimensions of the antenna must adapt to increasingly smaller dimensions of the terminals.

It is certain that the answer to these requirements should not be to the extent possible, nor to the detriment of the performance of the antenna, nor at the expense of cost.

Indeed, we already know antennas for portable terminals which are in the form of printed circuits.

However, most of these antennas not only have dimensions too big but still their mass plan depends on the plan mass of portable terminals, but they also have bandwidths too low.

An object of the present invention is to provide an antenna compact used in a portable terminal that has a very low volume and can be used at cellular frequency bands distinct.

This goal is achieved by the fact that the radiant element presents the form of an open loop with two branches connected together to the antenna cable and which define a first and a second part radiant, in that each of the radiating parts is able to enter into resonance at at least one cellular frequency and extends into a direction, and that these two main directions are substantially parallel so as to operate as a parasite and coupling between the two radiating parts.

Generally, the radiating parts and the counterweight present in the form of metal plates fixed on a support dielectric. It is however possible to realize the antenna by metallization a rigid or flexible dielectric substrate.

The compact antenna preferably extends longitudinally and the main directions of the radiating parts are parallel to the direction longitudinal axis of the compact antenna.

Preferably, the compact antenna is designed to work the main cellular frequencies of telephony: in particular GSM 850/900, DCS 1800, PCS 1900 and UMTS 2100 frequencies.

It is understood that the two radiating parts are arranged close to each other so that the phenomena of coupling or parasite can appear in order to widen the bandwidth of the antenna.

Preferably, the open loop has the shape of an element closing almost on itself and whose opening has a very small length compared to the total length of item.

Preferably the loop has only one opening, but we could very well make several openings in the loop.

Always preferably, but not necessarily, the counterweight and the conductive surface have a connection area unique.

Advantageously, the first radiant part is a surface substantially flat conductor, a portion of which is a juxtaposition of identical patterns extending in a substantially orthogonal direction at the main direction of the first radiant part.

Preferably, said pattern is substantially V-shaped so that the first radiant part has the shape of a zigzag extending in the main direction.

Preferably, said portion comprises at least four patterns identical.

Advantageously, the unfolded length of the first part radiant is substantially proportional to quarter of the wavelength of the wave having a central frequency substantially equal to the frequency the lowest cell.

Preferably, the lowest center frequency is a GSM frequency substantially equal to 900 MHz.

Obviously, the unfolded length of the first radiant part roughly equal to the length of the first part radiant if it was rectilinear.

Preferably, the unfolded length of the first part radiant substantially corresponds to the unfolded length of a pattern multiplied by the number of patterns composing the first radiant part.

Advantageously, the second radiant part is a conductor substantially rectilinear extending in the main direction of said second radiant part.

Preferably, the second radiant portion has a shape substantially rectangular. However, the second radiant part can also be composed of several rectilinear elements extending all in the main direction of the second radiant part.

Preferably, the length of the second radiant portion is substantially proportional to a quarter of the wavelength having a central frequency substantially equal to the highest cell frequency high.

Preferably, the highest cell frequency is a UMTS frequency substantially equal to 2100 MHz.

It is understood that the length of the radiant portion is substantially equal to the length of the rectilinear conductor.

Advantageously, the compact counterweight comprises an axis of symmetry parallel to the main direction of the first radiant part and in that it comprises at least one U-shaped portion comprising a first and a second branch and whose first branch is connected to the connection area of said compact counterweight.

We understand that a portion in the shape of a "U" has a much smaller footprint than a straight portion that would have the same unfolded length.

Advantageously, said at least one portion of conductor shaped "U" has two rectilinear branches each extending in a direction substantially parallel to the axis of symmetry of the compact counterweight.

As this portion extends in the main direction of the first radiant part, we understand that the length of the counterweight seen in this main direction is practically twice as small as if said portion of the counterweight was unfolded.

As a result, the compactness of the counterweight is improved.

Advantageously, the compact counterweight comprises two U-shaped portions, the second branches of which are arranged in vis-a-vis.

Preferably, the two U-shaped portions are symmetrical with respect to the axis of symmetry of the counterweight and the second branches of the U-shaped portions are parallel and arranged close to each other.

Advantageously, the compact counterweight furthermore comprises at least one rectilinear portion of conductor extending into a direction parallel to the axis of symmetry of the compact counterweight and a end of said rectilinear portion is connected to the connection area of the compact counterweight.

Preferably, said rectilinear portion of a conductor is parallel to the first and second branches of a shaped portion "U".

Advantageously, the rectilinear portion extends substantially between the two branches of a U-shaped portion.

We understand that this particular provision not only improves the overall compactness of the antenna but also improves the antenna performance

By "close", one must understand that the distance that separates two portions is much smaller than the wavelength of the wave having the highest frequency.

Advantageously, the lengths of the branches of a portion "U" and a rectilinear portion are substantially equal.

It is therefore understood that the unfolded length of a portion "U" is substantially equal to twice the length of a portion straight.

• la figure 1 représente l'élément radiant et le contrepoids selon un premier mode de réalisation de l'antenne compacte;
• la figure 2 représente l'élément radiant et le contrepoids selon un second mode de réalisation de l'antenne compacte;
• la figure 3 représente une vue en perspective en coupe partielle des parties de connexion de la partie radiante et du contrepoids ;
• la figure 4 représente la répartition des courants sur l'antenne compacte dans son premier mode de réalisation lorsqu'elle est utilisée à une fréquence GSM d'environ 900 MHz ;
• la figure 5 représente la répartition des courants sur l'antenne compacte dans son premier mode de réalisation lorsqu'elle est utilisée à une fréquence PCS d'environ 1900 MHz ; et
• la figure 6 est un diagramme représentant le gain de l'antenne en dBi en fonction de la fréquence en Gigahertz.

Other features and advantages of the invention will appear better on reading the following description of several embodiments of the invention given by way of non-limiting examples. The description refers to the appended figures in which:

• FIG. 1 represents the radiant element and the counterweight according to a first embodiment of the compact antenna;
• FIG. 2 represents the radiant element and the counterweight according to a second embodiment of the compact antenna;
• Figure 3 shows a perspective view in partial section of the connecting portions of the radiant portion and the counterweight;
• FIG. 4 shows the distribution of the currents on the compact antenna in its first embodiment when it is used at a GSM frequency of about 900 MHz;
• FIG. 5 shows the distribution of the currents on the compact antenna in its first embodiment when it is used at a PCS frequency of about 1900 MHz; and
• Figure 6 is a diagram showing antenna gain in dBi versus frequency in Gigahertz.

Referring to FIG. 1 , a compact antenna 10 will first be described in its first embodiment.

Preferably, the compact antenna 10 is designed to operate at the frequencies of the mobile telephony: GSM 850/900, DCS 1800, PCS 1900 and UMTS 2100 MHz.

To explain the operation of the antenna, we define a the lowest cell frequency and the highest cell frequency high.

The lowest cell frequency is approximately 850 MHz and the highest cell frequency is about 2100 MHz.

These two cellular frequencies do not constitute limits of frequency operation of the compact antenna, but allow to describe the architecture and explain the operation of the antenna compact. Indeed, it is possible to use the compact antenna at a frequency higher than the highest cell frequency, including UMTS frequencies which are of the order of 2100 MHz.

The compact antenna 10 is here in the form of an electrical circuit printed on an insulator not shown here. This insulator, commonly called dielectric can be made for example FR4 epoxy glass.

The compact antenna 10 comprises a first conductive surface forming a radiant element 12 and a second conductive surface forming a counterweight 14, which can also be called a ground plane.

The conductive surfaces are preferably substantially flat and made of copper.

As seen in Figure 1, the compact antenna 10 extends mainly in a longitudinal direction.

We will now describe more particularly the structure of the radiant element 12.

As seen in FIG. 1, the radiant element has substantially the shape of an open loop composed of two conductive branches 16, 18 extending substantially in principal directions parallel to the longitudinal direction of the compact antenna 10.

As can be seen in FIG. 1, the two branches are electrically connected together at one of their ends at an electrical connection zone 20.

More precisely, this connection area will be called in all the rest of the description: zone of connection of the radiant element.

The first branch 16 of the radiating element 12 comprises a first portion 22 forming a zigzag, that is to say a juxtaposition of patterns substantially "V" connected together by the ends of the bars of the "V".

As seen in Figure 1, the "V" patterns are oriented substantially in a direction orthogonal to the longitudinal direction of the radiant element.

Preferably, the portion 22 forming a zigzag comprises four "V" patterns and the end of the last "V" pattern opposite to the connection zone is connected to an end portion 24 terminating the first branch 16 .

This first branch 16 forms a first radiant portion capable of radiating preferably at frequencies belonging to the GSM frequency band, that is to say at frequencies between 850 and 1000 MHz.

The portion 22 forming a zigzag has a length substantially equal to one quarter of the wavelength having a central frequency a GSM frequency between 850 and 1000 MHz, and preferably substantially equal to 900 MHz.

The second branch 18 of the radiating element 12 comprises a substantially rectangular portion of conductor 26 extending in the longitudinal direction of the compact antenna 10 .

Preferably, the length of this rectangular portion 26 is substantially proportional to one-quarter of the wavelength for the highest central frequency between 1700 and 2100 MHz, preferably equal to 1900 MHz.

To further improve the performance of the antenna, as will be explained in more detail below, the unfolded length of the zigzag portion 22 is preferably substantially equal to twice the length of the rectangular portion 26 .

As already mentioned above and as can be seen in FIG. 1, the radiant element 12 forms an open loop. That is to say, there is an opening 28, forming a space between the ends of the branches 16, 18 opposite the ends connected to each other.

This opening 28 is preferably located between the end portion 24 terminating the branch 22 and the end of the rectangular portion 26 .

The end portion 24 has the shape of an "L" of which at least one of the arms has a rectangular shape 24 ' whose width is substantially equal to the width of the rectangular portion 26.

The unfolded length of the end portion 24 is preferably small relative to the unfolded length of the zigzag portion 22 .

It is therefore understood from FIG. 1 that said rectangularly shaped arm 24 ' is in the extension of the rectangular portion 26 .

Preferably, the width of the opening 28 , ie the distance separating the ends of the two branches is substantially equal to the width of the rectangular portion 26 . In any case, it is much less than one tenth of the wavelength for the highest frequency.

Another remarkable feature, of which an interest will be explained in more detail later, is that the rectangular portion 26 is relatively close to the ends of the bars of the "V" patterns. More precisely, the distance separating one end of a "V" pattern is much less than one-tenth of the wavelength for the highest cell frequency.

As can be seen in FIG. 3 , the connection zone 20 of the radiating element 12 is preferably electrically connected to a central conductor 30 of a connector 31 adapted to be connected to a supply line 32 .

Preferably, the supply line is a coaxial cable impedance 50 Ohms.

This supply line also comprises a shield 34 electrically connectable to a peripheral conductor 29 of the connector 31 , which is electrically connected to a connection zone 36 of the counterweight 14 and electrically isolated from the connection zone 20 of the radiating element 12 .

Referring again to FIG. 1, the structure of the counterweight 14 will now be described in greater detail.

This counterweight 14 has an axis of symmetry which extends parallel to the longitudinal direction of the compact antenna 14 .

More precisely, this axis of symmetry passes substantially through the center of the connection zone 36 of the counterweight 14 .

Referring to Figure 1 , we see that the counterweight 14 has two portions 38, 38 ' U-shaped and two straight portions 40, 40' . The two U-shaped portions and the two rectilinear portions being symmetrical to each other, only one U-shaped portion 38 and a single straight portion 40 will be described hereinafter.

The "U" shaped portion 38 has a first branch 42 whose end is electrically connected to the connection zone 36 of the counterweight 14 and a second branch 44 .

The two branches 42, 44 of the U-shaped portion are parallel to each other and extend each parallel to the longitudinal direction of the compact antenna.

The "U" shaped portion is arranged such that the second leg 44 is located between the first leg 42 and the axis of symmetry of the counterweight.

The rectilinear portion 40 of substantially rectangular shape is electrically connected to the connection zone 36 of the counterweight 14 at one of its ends and extends in a direction parallel to the longitudinal direction of the compact antenna 10.

The other end of the rectilinear portion extends between the two branches 42, 44 of the U-shaped portion so that said branches substantially border all of the lateral edges of the rectilinear portion 40 .

In addition, because of the symmetry, it is understood that the second leg 44 of the U-shaped portion 38 is disposed opposite the branch of the symmetrical portion shaped "U" 38 '.

Preferably, the distance between these two branches is very less than twentieth of the wavelength for cell frequency the highest, that is to say of the order of a millimeter.

In addition, the unfolded length of the "U" shaped portion 38 of the counterweight 14 is preferably substantially equal to or slightly greater than the unfolded length of the zigzag portion 22 .

Similarly, the length of the rectilinear portion 40 of the counterweight 14 is preferably substantially equal to or slightly greater than the length of the rectangular portion 26 of the radiating element 12 .

The principle of operation of the compact antenna will now be described with reference to FIGS. 4, 5 and 6 .

For the frequency substantially equal to the central frequency of GSM 900 MHz, that is to say close to the lowest cell frequency, the radiant portion whose length is substantially equal to a quarter of the wavelength enters into resonance . In other words, it is the zigzag portion 22 of the first branch 16 of the radiant element 12 that resonates. The distribution of the currents 46 is therefore more particularly located on the zigzag portion and on the connection zone 20 .

The electrical equilibrium involves in known manner a current distribution on a ground plane 14 in return for the distribution of the currents 46 on the radiating element 12 .

In the present invention, the counterweight 14 acts as a compact ground plane.

In other words, the distribution of currents on the counterweight is preferably localized on the U-shaped portions 38 , 38 ' as schematically shown in FIG. 4 , whereas the rectangular portions 40 are practically not covered. by the currents.

For the central frequency which is substantially equal to the GSM 1900 MHz frequency, that is to say close to the highest cellular frequency, the radiant portion whose length is substantially equal to one quarter of the wavelength of said wave goes into resonance. In other words, it is the rectangular portion 26 that comes into resonance.

However, because of the proximity of these two portions 26 and 16 , a coupling and parasite phenomenon occurs between these two portions so that the zigzag portion 22 also resonates.

The current distribution on the counterweight is such that they are spread over the portion of the counterweight whose length is close to quarter of the wavelength for the frequency band considered.

In other words, the distribution of the currents on the compact counterweight 14 is more particularly located on the connection zone 36 and on the rectilinear portions 40, 40 ' .

It is thus clear that despite the absence of an ideal theoretical mass plane, that is to say a very large plane, the compact counterweight 14 fulfills the function of double ground plane at the frequencies of use. 'antenna.

Referring to Figure 6 , the Gain / Frequency diagram of the compact antenna 10 in its first embodiment will now be described.

In this diagram, the abscissa represents the frequency applied to the supply line 32 of the antenna 10 . The frequency is expressed in Gigahertz and the spectrum varies between 0.8 GHz and 2.3 GHz.

The ordinate axis represents the gain in dBi of the antenna compact.

It can be seen that for a frequency close to 0.9 GHz, the gain of the antenna is high and is between 0 and 1 dBi. This frequency band 48 close to 0.9 GHz corresponds substantially to the frequency band GSM 850/900 MHz. The antenna is therefore adapted to this band of frequencies.

In addition, it is found that the gain of the antenna is particularly high for frequencies between 1700 MHz and 2200 MHz. This frequency band 50 is relatively wide and corresponds to the frequency bands DCS 1800, PCS 1900 and UMTS 2100. The antenna is therefore adapted to these frequency bands.

The width of this frequency band is notably due to the proximity of the rectangular portion 26 and the zigzag portion 22.

As already explained above, a coupling and an effect parasite appear at frequencies around 1900 MHz effect of broadening the bandwidth of the antenna at this frequency and therefore of allow the use of the antenna at higher frequencies, especially at UMTS 2100 frequencies.

We understand that the compact antenna of the present The invention is suitable for both GSM and UMTS frequency bands.

FIG. 2 represents a second embodiment of the compact antenna 100 in which the radiating element 120 is identical to that of the first embodiment of the compact antenna and in which the counterweight is identical except that it does not has no rectilinear portions as in the first embodiment.

Elements identical to those of the first embodiment bear the same reference multiplied by one hundred.

As seen in Figure 2 , the branches 440, 440 'are disposed close to each other and also close to the branches 420 so as to promote the phenomena of coupling and parasitism.

Claims (13)

Antenna type monopole (10) capable of operating at cellular frequency bands comprising: a conductive surface (12) forming a radiant element and comprising an electrical connection zone (20), a compact counterweight (14) having a conductive area; a supply line (32) comprising an antenna cable (30, 34) connected to the connection areas (20, 36) of said radiant element (12) and said compact counterweight (14), characterized in that said radiant element (12) is in the form of an open loop having two legs (16, 18) connected together to the antenna cable (30, 34) and defining a first (22) and a second portion radiant (26), in that each of the radiating portions (22, 26) is adapted to resonate at at least one frequency of the cellular frequency band and extends in a main direction, and in that these two directions main are substantially parallel so as to operate parasitically and in coupling between the two radiating parts (22, 26). Antenna according to claim 1, characterized in that the first radiant portion (22) is a substantially planar conductive surface, a portion of which is a juxtaposition of identical patterns extending in a direction substantially orthogonal to the main direction of the first radiant portion ( 22). Antenna according to claim 2, characterized in that said pattern is substantially V-shaped. Antenna according to claim 2 or 3, characterized in that said portion (22) comprises at least four identical patterns. Antenna according to any one of claims 1 to 4, characterized in that the unfolded length of the first radiant portion (22) is substantially proportional to a quarter of the wavelength of the wave having a central frequency substantially equal to the lowest cell frequency. Antenna according to any one of claims 1 to 5, characterized in that the second radiant portion (26) is a substantially straight conductor extending in the main direction of said second radiant portion (26). Antenna according to any one of claims 1 to 6, characterized in that the length of the second radiant portion (26) is substantially proportional to one quarter of the wavelength of the wave having a central frequency substantially equal to the frequency highest cell. Antenna according to any one of claims 1 to 7, characterized in that the compact counterweight (14) has an axis of symmetry parallel to the main direction of the first radiant portion (22) and in that it comprises at least one U-shaped portion (38, 38 ') comprising a first (42) and a second limb (44) and whose first limb (42) is connected to the connection zone (36) of said compact counterweight (14) . Antenna according to claim 8, characterized in that the U-shaped conductor portion (38, 38 ') has two rectilinear branches (42, 44) each extending in a direction substantially parallel to the axis of symmetry compact counterweight (14). Antenna according to claim 8 or 9, characterized in that the compact counterweight (14) has two U-shaped portions (38, 38 ') whose second legs are arranged facing each other. Antenna according to any one of claims 8 to 10, characterized in that the compact counterweight (14) further comprises at least one rectilinear conductor portion (40, 40 ') extending in a direction parallel to the axis of symmetry of the compact counterweight (14) and that one end of said at least one straight portion (40, 40 ') is connected to the connection area (36) of the compact counterweight (14). Antenna according to claim 11, characterized in that the rectilinear portion (40, 40 ') extends substantially between the two legs (42, 44) of a U-shaped portion (38, 38'). Antenna according to any one of claims 8 to 12, characterized in that the lengths of the legs (42, 44) of a portion "U" and a straight portion (40, 40 ') are substantially equal.

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