WO2009121937A1

WO2009121937A1 - Coupler for a multiband radiofrequency system

Info

Publication number: WO2009121937A1
Application number: PCT/EP2009/053967
Authority: WO
Inventors: Nelson Fonseca
Original assignee: Centre National D'etudes Spatiales
Priority date: 2008-04-02
Filing date: 2009-04-02
Publication date: 2009-10-08
Also published as: EP2281320B1; EP2281320A1; FR2929796B1; FR2929796A1

Abstract

The invention relates to a coupler for a multiband radiofrequency system comprising: a central waveguide (10); a plurality of peripheral waveguides (20, 21); and coupling means (30, 31), characterized in that the peripheral waveguides are parallelepipedal and connected along a long side of the section of the peripheral waveguide to the central waveguide (10) via coupling means (30, 31) so as to form a symmetric coupler structure, the axis of symmetry of the structure being the longitudinal central axis (AA') of the central waveguide (10).

Description

COUPLER FOR MULTI-BAND FREQUENCY RADIO SYSTEM

GENERAL TECHNICAL FIELD The invention relates to the field of radio frequency (RF) multiband communications and finds application in satellite telecommunications systems combining, for example, the Ku and Ka bands.

The invention relates in particular to a coupler that can be integrated into the power supply system of a multiband antenna.

STATE OF THE ART

The proliferation of satellite telecommunications services in different frequency bands, from the C-band to the Ka-band, introduces the need for multi-band antennas to enable the best use of the reflectors (limited in number due to their size). on a satellite.

This association of applications can be envisaged on one and the same source as long as the considered frequency bands are disjoint.

For multimedia content broadcasting services via satellite, a need is identified associating the transmission frequency bands (Tx) and reception (Rx) of telecommunications signals in Ku and Ka bands:

- Ka / Rx: from 27.5 GHz to 30 GHz; - Ka / Tx: from 17.7 GHz to 20.2 GHz;

- Ku / Rx: from 13.75 GHz to 14.5 GHz;

- Ku / Tx: from 10.95 to 12.75 GHz.

Other combinations of frequency bands can also be considered (C / Ku, C / Ka ...). Sources responding to these applications require a power circuit to isolate each frequency band. This circuit generally comprises O dB coupler type components and / or ortho-mode junctions each allowing the extraction of one of the frequency bands.

Depending on the ratios and bandwidths between the frequency bands to be transmitted and extracted, the design of the power system components can be complex.

PRESENTATION OF THE INVENTION

According to a first aspect, the invention relates to a coupler for a multiband radio system ensuring a total coupling (this is called OdB coupler) of the radio frequency power of the highest frequency band of a multiband system while providing a sufficient isolation with the lower frequency bands.

Thus, the invention relates to a coupler for a multi-band radio frequency system comprising: a central waveguide; a plurality of peripheral waveguides; coupling means.

The coupler of the invention is characterized in that the peripheral waveguides are parallelepipedic and connected along a long side of the section of the peripheral waveguide to the central waveguide by means of the coupling means so forming a symmetrical coupler structure, the axis of symmetry of the structure being the central longitudinal axis of the central waveguide.

The central waveguide and the peripheral waveguides are sized to have the same propagation constant at the operating frequency of the peripheral guides, i.e., the high frequency band.

This dimensioning makes it possible to avoid the use of a dielectric material in the peripheral guides as used in couplers of known type. This simplifies the mechanical realization of the coupler of the invention.

In addition, the absence of dielectric material also makes it possible to reduce the insertion losses in the peripheral guides. It is in fact known to use a dielectric material in the peripheral guides to accentuate the frequency selectivity of the coupling.

In the case of the coupler of the invention, this selectivity is obtained in part by the height of the coupling means (which are preferably coupling slots), resulting in a natural rejection of the signals in the low bands.

This attenuation can then be accentuated by a recombination circuit of the peripheral guides sized with guides also under cut for the low bands.

In addition, the symmetry provided to the structure of the coupler of the invention makes it possible to minimize the generation of particularly disturbing higher order modes when the coupler is used with multiband antennas. Indeed, these modes constitute a loss of power by coupling with the fundamental mode in the band of interest and may also prove to be a source of interference for another frequency band.

With regard to the coupling means, these are rectangular waveguides (or slots) whose cutoff frequency allows the rejection of the low frequency bands in order to minimize the coupling of these frequency bands towards the peripheral guides. .

The coupling zone, characterized by a regular longitudinal positioning of several slots, must be long enough to ensure the coupling of almost all the energy present in the high frequency band.

According to a second aspect, the invention relates to a multiband antenna characterized in that it is powered by a radio frequency circuit comprising at least one coupler according to the first aspect of the invention. Preferably, the radiating element of the antenna is of horn type. PRESENTATION OF THE FIGURES Other features and advantages of the invention will become apparent from the description which follows, which is purely illustrative and nonlimiting, and should be read with reference to the appended figures in which: FIG. 1 illustrates a coupler comprising a central guide to square section and two peripheral waveguides; FIG. 2 illustrates a coupler comprising a central guide with square section and four peripheral waveguides;

FIG. 3 illustrates a coupler comprising a central guide with circular section and four peripheral waveguides;

FIGS. 4a and 4b respectively show a central waveguide with an octagonal section and a circular section with flats; Figure 5 is a side view of the coupler of Figure 2; Figure 6 is a detailed view of the coupling means of the coupler of Figure 2; Figure 7 is a block diagram of linear bi-polarization;

FIG. 8 is a block diagram of circular bi-polarization;

FIG. 9 is a block diagram illustrating isolated peripheral ports in Rx configuration; FIGS. 10a, 10b, 10c and 1 Od respectively illustrate the reflection coefficient, the insulation coefficient, the coupling coefficient and the transmission coefficient for a rectangular, circular, circular flat-flange waveguide coupler, octagonal and four peripheral waveguides based on targeted specifications; - Figure 1 1 illustrates a coupler according to the invention connected to a horn type antenna. DESCRIPTION OF ONE OR MORE MODES OF REALIZATION AND IMPLEMENTATION General structure of the coupler

FIG. 1 shows a coupler comprising a central waveguide 10 and two peripheral waveguides 20, 21 coupled to the central waveguide 10 via coupling means 30, 31.

The peripheral waveguides are parallelepipedic and connected along the long side of their section to the central waveguide 10 via the coupling means 30, 31.

It should be noted that, in the context of waveguide technology, the length L (understood as the guide) is always associated with the direction of propagation of the wave while the terminology "long side", of length noted usually has and "small side", of length usually noted b, is used to describe the section of the guide in a plane orthogonal to the direction of propagation.

The arrangement of the peripheral waveguides 20, 21 around the central waveguide 10 is such that the structure of the coupler is symmetrical.

The axis of symmetry of the structure is the central longitudinal axis AA 'of the central waveguide 10.

In addition, the longitudinal central axis BB ', CC of each peripheral waveguide 20, 21 is parallel to the central longitudinal axis AA' - the axis of symmetry of the structure - of the central waveguide 10.

On the coupler of Figure 1, one goes from one peripheral waveguide to the other by a rotation of 180 °.

The coupling means 30, 31 consist of sets of rectangular section waveguides also called "coupling slots". The waveguides are preferably all identical.

FIG. 6 shows two coupling slots 310, 314. The longitudinal central axis FF 'of each slot is perpendicular to the central longitudinal axis AA' of the central waveguide 10 (see FIG. 6).

The coupling slots are connected to the respective long sides of the central and peripheral waveguides by the rectangular section 312, orthogonal to the propagation direction of the wave in the slot along the axis FF '.

FIG. 2 shows a coupler comprising a central waveguide 10 and four peripheral waveguides 20, 21, 22, 23 coupled to the central waveguide by coupling means 30, 31, 32, 33 The constraints of symmetry and arrangement of the peripheral waveguides are identical to the coupler illustrated in FIG.

The peripheral waveguides 20-23 are parallelepipedic and connected along the long side of the section to the central waveguide 10 via the coupling means 30-33. The arrangement of the peripheral waveguides 20-23 around the central waveguide 10 is such that the structure of the coupler is symmetrical. The axis of symmetry of the structure being the central longitudinal axis AA 'of the central waveguide 10.

In addition, the longitudinal central axis BB ', CC, DD', EE ', of each peripheral waveguide 20-23 is parallel to the longitudinal central axis AA' - the axis of symmetry of the structure - of the central waveguide 10.

On the coupler of Figure 2, one goes from one peripheral waveguide to another by a rotation of 90 °.

The coupler of FIG. 2 can therefore be seen as the 90 ° superposition of two couplers of FIG.

It should be noted that preferably the peripheral waveguides are even, two or four.

FIG. 5 shows a side view of the coupler of FIG. 2 with square central waveguide 10 and four peripheral waveguides 20-23.

In Figure 1 and Figure 2, the central waveguide is a parallelepiped square section. The central waveguide may be of square section (10 in FIGS. 1 and 2), circular (100 in FIG. 3), octagonal (200 in FIG. 4a) circular with flats (300 in FIG. 4b), or rectangular section (not shown). It should be noted that from a geometrical point of view:

the octagonal section central waveguide 200 is parameterized by its small diameter D ₀ (see FIG. 4a);

the section of the central waveguide with circular section with flats 300 is designed as the intersection of a square of side I_CM and a circle of radius R _CM (see FIG. 4b).

Dimensioning of the central waveguide and peripheral waveguides

The coupler must be a total coupler, called OdB coupler, and must respect several sizing rules.

In particular, the guided wavelength must be the same in the central guide and the peripheral guides in order to ensure a coherent combination in phase of the signals coupled by the different coupling slots. The guided wavelength for the fundamental propagation mode, conventionally noted TE10 in a rectangular section guide, depends directly on the long side 22 of the peripheral waveguide (the side by which the coupling is effected) according to the following formula:

where D ₀ is the free propagation wavelength (D ₀ = c / f), D _c is the cutoff wavelength. This last parameter depends directly on the shape and dimensions of the section of the waveguide.

In the case of the TE10 mode, we have D _c = 2a according to the notations of FIG. The formula giving the guided wavelength reveals the high-pass filter behavior of a waveguide:

- below the cutoff frequency (fc = c / λ _c ), the propagation mode is strongly attenuated (it is said to be evanescent);

- above the cutoff frequency, the propagation mode is not attenuated, with insertion losses close (it is said that it is propagative).

To have a coherent combination in phase of the signals, it is therefore understood that the central guide 10 and the peripheral guides must be dimensioned with the same large side 22 when the central waveguide 10 is square-sectioned.

In the case of a central non-square section guide for which simple analytical formulas giving the wavelength guided according to the physical dimensions of the guide do not exist, the correspondence is obtained via optimization of the respective sections on a model. numerically (by the finite element method, for example). Furthermore, it is important to specify that this large guide side 22 is constrained by the lowest frequency of the frequency plane because the four frequency bands must be able to propagate in the central guide.

In a case where the sizing frequency is 10.95 GHz, there is the large side 22 of the peripheral waveguides greater than 15 mm taking a margin of about 10% to avoid the losses related to the proximity of the plane of frequency with the cutoff frequency.

Sizing of coupling means FIG. 6 illustrates two coupling slots 310, 314. As already mentioned, for the coupling slots, these are series of rectangular waveguides whose section 312 is attached to the central waveguide 10 and the peripheral waveguide (the latter is not shown in Figure 6). The dimensions of this rectangular section are such that the fundamental mode TE10 in the low frequency bands is evanescent.

In the case considered here, these are the Ka / Tx, Ku / Tx and Ku / Rx bands. Thus, the coupling is provided only in the high band Ka / Rx.

With, for example, a sizing frequency of 20.2 GHz, there is a large side a of the section of the slot less than 6.7 mm, again taking a margin of about 10%.

The optimization of the OdB coupler presented relates essentially to the spacing p between two successive slots, the total number of slots and their length.

We give below some pre-sizing rules for some of these parameters:

spacing p between two successive slots: λ _g (g _Ui of centrai) / 4 where λg (g _U i _dθ centrai) is the guided wavelength in the central waveguide calculated at the center of the band to be coupled ( in the case where the frequency is 28.75 GHz, this represents a spacing p of about 2.8 mm for a large side of the central or peripheral guides of the order of 15 mm); slot length: (2m + 1) λ _{g (fθntθ)} / 4 where λ _{g (fθntθ)} is the guided wavelength in the coupling slots also calculated at the center of the band to be coupled and m a positive integer or zero (in the case where the frequency is 28.75 GHz, this represents a length of 12.5 mm for a large side has slots of the order of 6.7 mm and m = 1). It should be noted that taking 3λ _{g (fθntθ)} / 4

(m = 1) makes it possible to ensure better rejection of the bands that one does not wish to couple via the slots. For some frequency combinations, slots of length _{λg (fθntθ)} / 4 (m = 0) could provide sufficient rejection. More generally, the slots must have a length which is an odd multiple of λ _{g (} f _θ nte) / 4; - The number of coupling slots is chosen so that the coupling area is between ten and twenty times λ _g (g _Ui of centrai) - This number must be optimized to ensure good coupling. If this number is below the optimum, the coupling is insufficient. If this number is above the optimum, there is a phenomenon of over-coupling which tends to degrade the performance of the component, a part of the coupled signal returning to the main guide.

An advantage to the structure presented with respect to the couplers as known is that it is not necessary to include dielectric material in the peripheral guides, this simplifies the mechanical realization and eliminates the associated radio frequency losses.

Coupling along the respective long sides of the central and peripheral guides 10 is "natural" in the case of guides of rectangular section and / or square. On the other hand, it is not easy to obtain good levels of coupling in the case of a section of the central guide of circular, octagonal or other shape.

In addition, the structure is symmetrical, which makes it possible to minimize the generation of higher order modes, which are particularly troublesome on multiband antennas because they constitute a loss of power in the band of interest but can in addition prove to be a source of interference for another frequency band.

Thus, all guided propagation modes that do not have the same symmetries as the structure can not appear. Obviously, this hypothesis is valid provided that the mechanical realization of this component ensures symmetry with good accuracy.

Operation

The structure shown in FIG. 1 is characterized by a linear mono-polarization operation, the electromagnetic field of which is parallel to the plane containing the axes AA ', BB' and CC. The structure shown in Figure 2, which can be seen as a superposition of two 90 ° linear single polarization structures, offers the possibility of polarization diversity operation.

Indeed, depending on the recombinations of the signals from the four peripheral guides 20-23, it is possible to separate two orthogonal linear polarizations E _v and E _H (see FIG. 7) or two orthogonal circular polarizations E _PCD and E _PCG (see FIG. 8).

Isolated peripheral ports are terminated with appropriate loads, as shown in Figure 9 (in Rx configuration). These adapted charges may consist of absorbent material disposed within the waveguide.

In the case of a high transmission band, power dividers in "tee" plane E or plane H are used.

Note that in order to limit the frequency dispersion related to a phase shift between the two combined accesses, the planar configuration E is preferable because the signals coming from two diametrically opposite peripheral guides are in phase opposition.

In the target application the high band is in reception, it is better to combine the signals via "magic tees" that have a good adaptation on all their ports. To do this, the component will be used as a standard tee power divider by terminating the port

'sum' or the 'difference' port by a suitable load.

Again, for phase considerations, it is best to load the sum port.

Results

Several couplers were tested in simulation using the finite element method in the time domain.

Here, results are presented with a 60-slot coupler comprising a central waveguide with a square, circular, octagonal, circular and flat-shaped sectional waveguide and four peripheral waveguides (see FIGS. 2, 3, 4a and 4b). The results presented relate to the fundamental propagation mode. However, the analyzes were performed considering all the propagative and degenerate modes in order to ensure representativeness and convergence of the results.

Results for several RF parameters are presented below.

Coefficient of Reflection Figure 10a illustrates the reflection coefficient in dB for each of the couplers presented.

The reflection coefficient characterizes the power reflected at the input of the component at the central waveguide. There is also talk of adaptation. The target value in the different frequency bands under consideration is -25 dB.

Due to the symmetries of the structure, the reflection coefficient presented may be indifferently that of the horn access or the low band excitation circuit access, in the case where the coupler is connected to a horn type antenna.

Coefficient of insulation

Figure 10b illustrates the isolation coefficient in dB for each of the couplers shown.

The insulation coefficient characterizes the power transmitted to the necessary ports in order to preserve the symmetries of the structure but towards which it is desired to minimize the power transfer. These ports are said to be isolated and are terminated by appropriate charges. The target values for this parameter in the low frequencies (Ka / Tx, Ku / Tx and Ku / Rx) are -45 dB and -3OdB for the high band. Coupling coefficient

Figure 10c illustrates the coupling coefficient in dB for each of the couplers shown.

The coupling coefficient characterizes the coupling between the central guide and the peripheral guides. Because of the symmetries of the structure, each peripheral guide must couple half of the energy of the corresponding linear polarization in the high frequency band, ie a theoretical coupling coefficient of -3.01 dB.

In the low frequency bands, it is desired that this coupling is as low as possible.

The coupling values obtained in low bands can be accentuated by exploiting the natural rejection of an undercut guide.

For this, it suffices to reduce the section of the peripheral guides beyond the coupler, in particular at the level of the recombination circuit as illustrated in FIGS. 7 and 8.

Coefficient of transmission

Figure 1 Od illustrates the coupling coefficient in dB for each of the couplers presented. The transmission coefficient characterizes the power level transmitted via the central guide. In the lower frequency bands, this parameter should be as close as possible to OdB, with the insertion losses (ohmic) close.

In the high band, the transmission level should be as low as possible, the target value being a level below -10 dB.

Summary of results

From the results illustrated in FIGS. 10a, 10, 10c and 10d, it can be seen that the coupler with a rectangular section central guide has the best performance in terms of coupling in the high frequency band. The difference in performance is about 0.2 dB worst case with the other forms of section. This also translates into a transmission coefficient in the center guide of the high frequency band at -14.2 dB. This value is degraded by 1 to 2 dB for other section shapes. This confirms that the desired coupling is more natural when the central guide is rectangular. Furthermore, it is also noted that the coupling with a square section central guide has a substantially wider bandwidth than the other solutions envisaged.

As far as the adaptation is concerned, the performances are very similar for the different section shapes in the low frequency band. By cons, in the case where the section is rectangular, the latter is less efficient than other forms of section in the high band.

This result is nonetheless nuanced because the dimensions of the section of the central guide have not been fully optimized, some parameters being imposed in order to arrive at standard values allowing a more generic connectivity between the different components. necessary for the realization of a multiband source.

As far as insulation is concerned, the performances are very similar for the different solutions.

Finally, the rejection of the low band towards the peripheral guides is better for the central guide with circular section. There is up to 5 dB difference with other solutions.

The performance of the octagonal solution is very close to the circular solution, except for the low band coupling coefficient to the peripheral guides for which the simulated performance approaches that of the square section guide.

Finally, for the circular section guide with flats an intermediate behavior between the square and circular central guide solution is observed, which is in line with what was expected since the flats were intended to improve the coupling of the solution with circular central guide. This solution appears to be a good compromise, but it is important to evaluate how it interfaces with components to circular or rectangular section (generic section forms for cornet type components, ortho-mode junction ...).

Application The presented coupler can be connected to a horn type antenna or to other components of the power supply circuit such as an ortho-mode junction, a filter and so on.

As several forms of section for the central guide have demonstrated good performance, it is possible to adapt the OdB coupler to other components to reduce or eliminate transitions between guides of different section.

The presented OdB coupler is relevant when a large ratio (greater than the octave) is observed between the lowest frequency and the highest frequency of the frequency plane. In the illustrative application, this ratio is close to three.

The multiband antenna thus obtained can, if necessary, be placed in front of a reflector, in particular for satellite telecommunications applications.

FIG. 11 shows a coupler OdB connected to a horn type antenna 1 10 and in front of a reflector 1 10 (the latter being optional).

Claims

A coupler for a multiband radio frequency system comprising: - a central waveguide (10);

a plurality of peripheral waveguides (20, 21);

coupling means (30, 31); characterized in that the peripheral waveguides are parallelepipedal and connected along a long side of the section of the peripheral waveguide to the central waveguide (10) via the coupling means (30, 31) of in order to form a symmetrical coupler structure, the axis of symmetry of the structure being the central longitudinal axis (AA ') of the central waveguide (10) and in that the coupling means are distributed over a so-called zone coupling whose length is between ten and twenty times the wavelength guided in the central waveguide calculated at the center of the band to be coupled.

2. Coupler according to claim 1, characterized in that the longitudinal central axis (BB ', CC) of each peripheral waveguide is parallel to the central longitudinal axis (AA') of the central waveguide (10). ).

3. Coupler according to one of the preceding claims, characterized in that the coupling means (30, 31) consist of series (30, 31) of waveguides (310, 31 1) of rectangular section, the longitudinal central axis (FF ') of each of them being perpendicular to the central longitudinal axis (AA') of the central waveguide (10).

4. Coupler according to the preceding claim, characterized in that the coupling means (30, 31) consist of series (30, 31) of waveguides (310, 31 1) of rectangular section connected to the long side of the section of the central and peripheral waveguides.

5. Coupler according to the preceding claim, characterized in that the rectangular waveguides forming coupling means are spaced from λ _g ( _{centric gU} ide) / 4, λg (g _U ide centrai) being the guided wavelength in the central waveguide calculated at the center of the band to be coupled.

6. Coupler according to one of the preceding claims, characterized in that the length of the coupling slots is equal to (2m + 1) λ _{g (fθntθ)} / 4, where λg (f _eri te) is the wavelength guided in the coupling slots calculated at the center of the band to be coupled and m is a positive integer or zero.

7. Coupler according to one of the preceding claims, characterized in that the peripheral waveguides are even in number, preferably two or four.

8. Coupler according to one of the preceding claims, characterized in that the central waveguide is section selected from the following group: square, circular, octagonal, circular flat or rectangular.

9. Antenna multiband, characterized in that it is powered by a radio frequency circuit comprising at least one coupler according to one of the preceding claims.

10. Antenna according to the preceding claim, characterized in that its radiating element is cornet type.