EP2092605A1

EP2092605A1 - Flat antenna ground plane supporting body including quarter-wave traps

Info

Publication number: EP2092605A1
Application number: EP07822294A
Authority: EP
Inventors: Eduardo Motta Cruz; John Bartholomew
Original assignee: Bouygues Telecom SA
Current assignee: Bouygues Telecom SA
Priority date: 2006-11-08
Filing date: 2007-11-07
Publication date: 2009-08-26
Also published as: WO2008055920A1; FR2908237B1; FR2908237A1; JP2010509795A; US20100073248A1

Abstract

The invention relates to a flat antenna ground plane supporting body (10) including a longitudinal channel (20) defining a waveguide for the antenna and two rectangular grooves (31, 32) which extend along the length of the channel (20) and which are arranged on either side of the channel so as to define a double quarter-wave trap, characterised in that the part of the body between the channel and each groove is machined such that when the ground plane is mounted on the body, said part of the body is separated from the ground plane by an air cushion of a controlled dimension (b). The invention also relates to a flat antenna including a substrate, a substrate-supporting ground plane and a body as described above, to which the ground plane is applied or secured.

Description

FLAT ANTENNA MASS PLAN SUPPORT BODY COMPRISING QUARTER WAVE TRAPS

The field of the invention is that of telecommunication antennas, and more particularly that of flat antennas for radio-relay systems fed by waveguides.

The invention relates more particularly to a flat antenna supplied directly by a waveguide by implementing an electromagnetic coupling by slot between the waveguide and each of the supply lines of the radiating elements of the flat antenna.

As has been described for example in the French patent application of the Applicant filed November 14, 2005 under No. 0511527, such coupling can be achieved by providing slots in the ground plane of the antenna against each radiating element supply line, and a waveguide arranged with respect to the ground plane so as to achieve an electromagnetic coupling by slot between the waveguide and each of the supply lines.

According to a possible embodiment, the waveguide has a U-shaped section, and is arranged in such a way that the ground plane closes the space of the waveguide (the ground plane is then used as wall of the waveguide).

FIG. 1 shows schematically a sectional view of this embodiment. A channel made, for example by milling, in a metal body 1 defines a waveguide 2 having a U-shaped section.

The substrate 3 of the antenna rests on a ground plane 4, which ground plane 4 is then used as wall of the waveguide 2 to close the space of the waveguide.

Due to manufacturing imperfections and the small thickness of the substrate 3, the surface of the ground plane 4 used as wall of the waveguide 2 is likely to have a "gondola effect". An uneven electrical contact can then be established between the ground plane and the waveguide. And as shown in FIG. 1, an air layer 5 can then be interposed, at least locally, between the waveguide 2 and the ground plane 4.

The electromagnetic field then tends to propagate between the waveguide and the ground plane, which can cause significant losses.

Fig. 2 shows a map of the field E of the embodiment of Fig. 1 in a cross-sectional view. This figure illustrates the behavior of an electromagnetic wave passing through a waveguide section closed by a ground plane, with an air layer with a thickness e (see Figure 1) of approximately 0.05 mm, ie a thickness of the same order of magnitude as imperfections in the manufacture and assembly of mechanical parts.

There is the presence of a lateral progressive wave which generates significant transmission losses, of the order of 15 dB on a guide section of 16 cm.

It should be noted that the use of a double quarter-wave trap has been proposed for fixing the substrate of the antenna above the waveguide without welding, so as to avoid unpredictable electrical behavior.

The article by Van Der WiIt and Strijbos, entitled "A 40 Ghz planar array antenna using hybrid coupling" (in "Perspectives on Radio Astronomy - Technologies for Large Antenna Arrays, Proceedings of the Conference held at the ASTRON Institute in Dwingeloo on 12- 14 April 1999. "ISBN: 90-805434-2-X, 354 pages, 2000, p.129) thus provides (see Figure 6 and corresponding discussion) to associate with the waveguide a double quarter-wave trap ("Double quater-wave choke contruction" according to the terminology used in this article). The substrate can then be fixed to the waveguide by simple bonding. A flat antenna ground plane support metal body according to the preamble of claim 1 is known from the article by Kimura et al. entitled "Alternating-phase fed single-layer slotted waveguide arrays with choke dispensing with narrow wall contacts", IEE Proceedings H. Microwaves, Antennas & Propagation, Institution of Electrical Engineers; Stevenage, GB, vol. 148, No. 5.

This article proposes to study the respective influence of the depth c and the position w of the trap according to a model of analysis ("analysis model", see ^1st paragraph, right column, page 297 and corresponding figure 5) according to which, when the ground plane is attached to the body, a gap (gap) of 0.1 mm height is considered all along the body-ground plane junction. In addition, this article reports (see last paragraph, right-hand column, page 297 in relation to Figure 6), that the constant height space considered in the analysis model is artificial ("artificial, small gap of constant height "). It is understood that this separation of 0.1 mm corresponds to a simulation of leakage currents along the entire body-ground plane junction.

However, as already mentioned above, the ground plane actually presents, due to manufacturing imperfections, a gondola effect. An uneven electrical contact is then likely to be established between the ground plane and the body, causing transmission losses. The aforementioned article also recognizes this problem of irregular electrical contact: cf. left column, page 296, paragraph beginning with "Secondly, ..." in which it is stated that losses ("leakage") can be eliminated if there is regular electrical contact; as well as on page 297, left column, ^1st paragraph of the section "3. Loss from waveguide with choke".

We will thus recognize that in reality, the space ("gap") of this article is not of uniform thickness but is instead subject to the imperfections of the ground plane and its gondola effect. In other words, this space does not actually present a constant height; it's only in a model of analysis that this article artificially considers a space of constant height.

It will be further recognized that this space extends over the entire body-ground plane junction, and does not correspond to a machining of a single portion of this junction. In particular, this article shows no machining of the part of the body situated between the waveguide and each of the grooves of the double trap (see FIGS. 4 and 5a), and no means for practically controlling the thickness of the layer of air separating this part of the body from the ground plane.

The invention aims to reduce transmission losses to increase the efficiency of the antenna. It is more particularly intended to minimize the losses in the waveguide and in the operating frequency band of the flat antenna.

For this purpose, and according to a first aspect, the invention proposes a flat antenna ground plane support metal body comprising:

a longitudinal channel defining a waveguide for the antenna;

- two rectangular grooves extending along the channel, and arranged on both sides of the channel to define a double trap quarter wave; characterized in that the portion of the body located between the channel and each groove is machined so that when the ground plane is attached to the body, said body portion is spaced from the ground plane by a dimensionally controlled air layer .

Some preferred, but not limiting, aspects of this metal body are the following:

- The air layer has a thickness greater than the manufacturing accuracy of the ground plane;

the air layer has a thickness less than the wavelength λ associated with the frequency f at which the double trap is tuned;

the air layer has a thickness of between 0.05 mm and 1 mm; the air layer has a thickness of approximately λ / 10, where λ represents the wavelength associated with the frequency f at which the double trap is tuned;

the part of the body situated between the channel and each groove extends over a distance (a) equal to λ / 4, and each groove has a depth (c) equal to λ / 4, where λ represents the wavelength associated with the frequency f at which the double trap is granted;

the double trap is tuned to the center frequency f of the operating band of the antenna;

the body comprises a plurality of pairs of rectangular grooves extending along the channel, the grooves of each pair being arranged on either side of the channel to define a corresponding plurality of double trap quarter wave, each double trap being tuned to a different frequency;

the body comprises two pairs of grooves defining two double quarter-wave traps, the dimensions of the first trap tuned to a first frequency f1 (wavelength λi) are as follows: trap-guide distance a '= λi / 8, depth of the trap c '= 3λi / 8, thickness of the air layer b' = λi / 10, sum of the guide-trap distance and the trap width of = λ2 / 4; and the dimensions of the second trap tuned to a second frequency h (wavelength λi) are as follows: trap-guide distance e = 3 ^* λ2 / 8, trap depth g = λ2 / 8, thickness of the air f = λ2 / 10, sum h of the trap-guide distance and trap width h = λ2 / 2; and

the double traps are tuned to the duplex frequencies of the high and low channels of an operating band of the antenna.

According to a second aspect, the invention relates to an antenna comprising a substrate, a ground plane supporting the substrate and a body according to the first aspect of the invention against which the ground plane is plated or fixed. Other aspects, objects and advantages of the present invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and with reference to the appended drawings in which: , in addition to Figures 1 and 2 already commented:

FIGS. 3a and 3b show the introduction, on the equivalent circuit of a guide represented by its characteristic impedance Z _G , of a double quarter wave trap and two double quarter wave traps, respectively;

- Figure 4a shows a possible embodiment of a metal body according to the invention with a double quarter wave trap;

FIG. 4b shows a possible embodiment of a metal body according to the invention with two double quarter-wave traps;

- Figure 5 shows a map of the electric field in a cross sectional view of the coupling made by an embodiment of the invention.

According to a first aspect, the invention relates to a body, typically a metal body, for example made of aluminum, intended to serve as a flat antenna ground plane support. The ground plane supports the dielectric substrate of the antenna on which are arranged the radiating elements of the antenna.

As shown in the sectional views of Figures 4a and 4b, a longitudinal channel 20, 200 defining a waveguide for the antenna is made, for example by milling, in a metal body 10, 100.

The section of the channel is typically rectangular, and has a U-shaped section, the ground plane of the antenna being intended to serve as a wall to close the space of the waveguide.

The dimensions of the channel are a height (lateral branches of the U) of λc / 4 and a width (base of the U) of λc / 2, where λc represents the wavelength corresponding to the cutoff frequency of the guide (the guide acting as a high pass filter, for frequencies above the cutoff frequency).

Pairs of rectangular grooves 31, 32; 310, 320, 410, 420 extending along the channel are also arranged in the body 10 on either side of the channel 20, 200 to each define a double quarter wave trap. These grooves are for example made by milling the metal body.

In the context of the invention, the term "double quarter wave trap" should be understood as designating two quarter-wave traps arranged symmetrically on either side of the waveguide.

The term "quarter wave trap" refers to a groove formed in the body so as to form an electrical section of length equal to λ / 2 from the channel wall (where λ represents the wavelength in the guide of the signal propagated by the antenna, remember that λ = c / f, with c the speed and frequency). This section of length λ / 2 effectively makes it possible to reduce a short circuit (denoted by the reference CC in FIGS. 3a and 3b) at the level of the wall of the channel defining the waveguide (the limit condition of nullity of the tangential field at the wall here).

FIG. 3a shows the introduction of a double quarter-wave trap on the equivalent circuit of a guide represented by its characteristic impedance Z _G. We seek here to realize a trap tuned to the center frequency f (with λ = c / f) of the operating band of the antenna.

For example, for an antenna operating in the band 37.21 to 38.64 GHz; the center frequency f is 37.92 Hz.

In order to obtain a wider frequency range, additional traps can be designed by reducing other short circuits in addition to the first one.

In this respect, FIG. 3b shows the introduction of two double quarter-wave traps to obtain a wider frequency range, in using two near frequencies fi and h (with λi = c / fi and λ2 = c / fc). The traps are then tuned to the frequencies fi and h respectively

In the embodiment of FIG. 3b, a second short circuit CC is thus brought back in addition to the first. There are then two electrical paths in parallel for the two operating frequencies fi and h.

For example, for an antenna operating in the band 37.21 to 38.64 GHz, the two duplex frequencies corresponding to the central frequencies fi, h of the high and low channels are chosen, namely respectively 38.64 GHz and 37.21 GHz.

In the context of the invention, the section of length λ / 2 for bringing a DC short-circuit at the side wall of the guide comprises two portions which end-to-end represent λ / 2.

As indicated in FIGS. 4a and 4b, these portions are respectively:

- the distance (a); (at') ; (e) separating the channel 20, 200 (side wall of the channel with null condition of the electric field) and a groove 31, 32; 310; 320; 410, 420 (this distance thus representing the width of the "wall", ie the width of the part of the body situated between the waveguide and the trap); and

depth (c); (c ¹ ); (g) groove 31, 32; 310, 320; 410, 420 (the "gap" of the trap).

In other words, the following relationships are used to define quarter-wave traps:

- In Figure 4a showing the implementation of a single double trap tuned to the frequency f (with λ = c / f): λ / 2 = a + c;

FIG. 4b shows the implementation of two double traps tuned respectively to the frequency f1 and the frequency h (with λi = c / fi and λ2 = c / f2) λi / 2 = a '+ c' and λ2 / 2 = e + g. In the form of arrows Fλ, Fλi, and Fλ2 in FIGS. 4a and 4b, these segments of length λ / 2 which define the quarter-wave traps to reduce line loss in the guide are shown.

With these traps effectively, the lateral progressive wave is eliminated by reducing, by standing wave, a short circuit on the lateral wall of the guide.

Furthermore, the invention provides that the portion of the body located between the channel and each groove (the "wall") is machined so that when the ground plane is attached to the surface 11, 110 of the body 10, 100, said part of the body is spaced from the ground plane by a layer of air of controlled size.

The thickness of this air layer bears the reference b in Figure 4a, and the references b 'and f in Figure 4b.

Being able to control the thickness of the air layer prevents the ground plane making electrical contact with the body of the waveguide. This overcomes the disadvantages related to manufacturing imperfections parts, ensuring over the entire length of the waveguide the presence of a quarter-wave trap at a determined operating frequency.

Considering a manufacturing precision of the mechanical parts of the order of 0.01 mm, the part of the body between the channel and the groove is then machined so that the thickness b, b 'and f of the air layer is greater than this inaccuracy, and for example greater than 0.05mm.

Furthermore, the thickness of the air layer is also controlled by machining the body part between the channel and the groove so that this thickness remains sufficiently small relative to the wavelength of the operating frequency , as well as with respect to the small side of the guide (side wall of the U). It is here to privilege the propagation of the main mode TE10, and to avoid the creation of other undesirable modes by excessive deformation of the cross section of the waveguide. In the case of an operating frequency of 38Ghz, the machining is then performed for example so that the thickness of the air layer is less than 1 mm.

According to a preferred embodiment, this thickness is fixed at λ / 10 (ie at 0.78 mm for an operating frequency of 38 GHz).

Returning to the description of the embodiment of the body shown in FIG. 4a comprising a double trap, reference is made hereinafter to preferred dimensioning rules:

the distance (a) between the guide and the trap is λ / 4;

- the depth (c) of the trap is λ / 4 (we check well a + c = λ / 2);

the thickness of the air layer (b) is λ / 10;

the width (d) of the trap is λ / 8.

FIG. 5 shows a map of the field E in a cross-sectional view, for the embodiment of FIG. 4a in which an air layer (not shown in this figure) such as b = 0.05 has been considered. mm (minimum value of the range 0.05 - 1 mm presented above).

Comparing FIG. 5 to FIG. 2, the elimination of the lateral progressive wave is observed. The quantized transmission losses for a 16 cm section are less than 1 dB (compared with 15 dB loss on a non-trap device).

And for the embodiment shown in FIG. 4b of the body comprising two double traps, the preferred dimensioning rules are as follows (considering the frequencies f 1 and h as relatively close, as in the example here retained of the duplex frequencies of a 38Ghz antenna):

- for the first double trap given to λi (grooves 310 and 320):

- trap-guide distance a '= λi / 8;

depth of the trap c '= 3λi / 8 (we check well a' + c '= λi / 2);

- thickness of the air layer b '= λi / 10;

- sum of the trap-guide distance a 'and the trap width of = λ2 / 4. - for the second trap allocated to λ2 (grooves 410 and 420):

- trap-guide distance e = 3 ^* λ2 / 8;

depth of the trap g = λ2 / 8 (e + g = λ2 / 2 is well verified)

- thickness of the air layer f = λ2 / 10;

- sum h of the guide-trap distance e and the width of the trap h = λ _2/2.

Of course, it will be understood that the invention is not limited by the number of double traps. In particular, in order to obtain an even wider frequency range, it is possible to design additional traps, by bringing back other short circuits in addition to those existing. For example, it is thus possible to develop a metal body with a plurality of double traps, by providing a corresponding plurality of pairs of grooves, these grooves respecting the dimensioning rules presented above.

Furthermore, the invention is not limited to a metal body, but of course extends to a flat antenna comprising such a metal body.

In particular, the invention extends to a flat antenna comprising a substrate, a ground plane supporting the substrate and a body according to the first aspect of the invention against which the ground plane is plated or fixed, for example by gluing. .

The antenna comprises radiating elements disposed on the substrate, one or more supply lines of said radiating elements, and the ground plane may have one or more slots facing each supply line so as to ensure electromagnetic coupling by slot between the waveguide and each power line.

Claims

A flat antenna ground plane support body (10, 100) comprising:

- a longitudinal channel (20, 200) defining a waveguide for the antenna;

- two rectangular grooves (31, 32; 310, 320; 410, 420) extending along the channel (20, 200), and arranged on either side of the channel to define a double quarter wave trap; characterized in that the portion of the body located between the channel and each groove is machined such that when the ground plane is attached to the body, said body portion is spaced from the ground plane by a dimension air layer ( b; b '; f) controlled.

2. Body according to claim 1, characterized in that the air layer has a thickness greater than the manufacturing accuracy of the ground plane.

3. Body according to one of the preceding claims, characterized in that the air layer has a thickness less than the wavelength λ associated with the frequency f at which the double trap is tuned.

4. Body according to one of the preceding claims, characterized in that the air layer has a thickness between 0.05 mm and 1 mm.

5. Body according to one of the preceding claims, characterized in that the air layer has a thickness of about λ / 10, where λ represents the wavelength associated with the frequency f at which the double trap is granted .

6. Body according to one of claims 1 to 5, characterized in that the portion of the body located between the channel and each groove extends over a distance (a) equal to λ / 4, and each groove has a depth ( c) equal to λ / 4, where λ represents the wavelength associated with the frequency f at which the double trap is tuned.

7. Body according to the preceding claim, characterized in that the double trap is tuned to the center frequency f of the operating band of the antenna.

8. Body according to one of claims 1 to 5, characterized in that it comprises a plurality of pairs of rectangular grooves extending along the channel, the grooves of each pair being arranged on either side of the channel to define a corresponding plurality of dual quarter wave trap, each double trap being tuned to a different frequency (fi, f _2).

9. Body according to the preceding claim, characterized in that it comprises two pairs of grooves defining two double quarter wave traps, in that the dimensions of the first trap tuned to a first frequency f1 (wavelength λi) are the following: trap-guide distance a '= λi / 8, trap depth c' = 3λi / 8, thickness of the air layer b '= λi / 10, the sum of the guide-trap distance and the trap width of = λ2 / 4; and in that the dimensions of the second trap tuned to a second frequency h (wavelength λ2) are as follows: trap-guide distance e = 3 ^* λ2 / 8, trap depth g = λ2 / 8, thickness of the air layer f = λ2 / 10, sum h of the guide-trap distance and trap width h = λ2 / 2.

10. Body according to one of the two preceding claims, characterized in that the double traps are tuned to the duplex frequencies of the high and low channels of an operating band of the antenna.

11. Flat antenna comprising a substrate, a ground plane supporting the substrate and a body (10, 100) according to one of the preceding claims against which the ground plane is plated or fixed.

12. Antenna according to the preceding claim, characterized in that it comprises radiating elements disposed on the substrate, and one or more supply lines of said radiating elements, in that the ground plane has one or more slots facing each supply line, so as to provide electromagnetic coupling by slot between the waveguide and each supply line.