EP0424221A1 - Broadband power microwave window - Google Patents

Abstract

The present invention relates to a microwave window comprising a circular pane (31) mounted in a circular waveguide segment (32) connected on either side to a rectangular waveguide (34) containing a transformer for matching (35). The diameter of the circular waveguide is less than the diagonal of the rectangle. <??>A wall (36) ensures a tight junction, in relation to the outside, between the rectangular waveguide (34) and the circular waveguide segment (32). Each wall (36) is provided with an opening (37) included in a right cross-sectional portion common to the rectangular waveguide (34) and to the circular waveguide segment (32). The surface area of the opening (37) is less than the surface area of the right cross-sectional portion. <??>Applications to microwave power windows, with wide band and improved electrical and mechanical performance. <IMAGE>

Description

The present invention relates to a circular microwave window capable of operating at high power in a wide frequency band.

Microwave devices which operate at pressures different from atmospheric pressure require the use of microwave windows. They are intended to isolate these devices from atmospheric pressure but they allow the propagation of microwave waves without introducing reflection or internal resonance.

Microwave tubes generally operate at an extremely low pressure while certain components such as circulators, insulators, coaxial lines and waveguides may contain a gas brought to a pressure, higher than atmospheric pressure to increase their resistance in power.

Consequently, a microwave window must have sufficient strength to withstand pressures of the order of several kilograms per square centimeter without damage.

On the other hand, a microwave window must withstand brief anomalies such as electric flashes, mechanical stresses such as shock and vibration and temperature variations which can be significant, especially if the window is brazed on a frame or in a waveguide. If this is not the case, the window may break, the air will penetrate inside the microwave device and cause its deterioration.

It is moreover desirable that the microwave windows can be used in a wide frequency band. This frequency band will correspond to the band working frequencies of the microwave devices in which they are mounted.

In this frequency band they must not have parasitic internal resonance modes designated by the Anglo Saxon term of "ghost modes".

In addition, it is necessary that in this frequency band, the standing wave ratio is low and consequently the reflections not very important.

Among the microwave windows of the prior art, the “pill-box” type window is known. As shown in FIGS. 1a, 1b, this window is constituted by a thin blade 1 of a dielectric material brazed in a section of circular waveguide 2. The section of circular waveguide 2 is connected on either side other to a rectangular waveguide 3.

As shown more particularly in FIG. 1b, the diameter D of the circular guide 2 (which is also that of the dielectric strip 1) is substantially equal to the diagonal of the rectangular guide 3.

These dimensions make it possible to keep substantially the same electrical wavelengths guided in the rectangular guide 3 and the circular guide section 2.

In addition, the length L of the circular waveguide section 2 is chosen which is substantially equal to half of the guided wavelength l _g .

The pill-box window behaves like a half-wave matching transformer. As a result, the adaptation is correct at the central operating frequency but gradually degrades on each side.

This type of window has numerous parasitic resonance modes which reduces its useful operating bandwidth to around 12% compared to the central frequency.

In addition, due to its dimensions, the pill-box window is relatively fragile but, from a technological point of view it is easily achievable.

There are robust microwave windows. They are generally made of thick ceramic. They are mounted in a rectangular waveguide and obstruct the entire cross section of this guide. They have the shape of a rectangular parallelepiped and their thickness corresponds to a half wavelength of the central operating frequency in the rectangular waveguide for an alumina window.

These windows have an extremely reduced operating band due to the thickness of the ceramic; the operating bandwidth is approximately 5 to 7% of the central operating frequency in the rectangular waveguide for an alumina window.

Another type of window has been described in patent FR 2 558 306 filed on January 17, 1984. This window comes from the "pill-box" window but it has an enlarged operating band.

As shown in FIGS. 2a, 2b, the window comprises a thin circular blade 6 of dielectric material brazed in a section of circular guide 7. The diameter D ′ of the blade is equal to the diameter of the circular guide 7. The ends of this section of circular guide 7 are connected to a rectangular guide 5.

The window is equivalent to a resonant volume inside which unwanted parasitic resonance modes can develop. Inside the circular waveguide section 7 and at the frequencies used, the TM₀₁₀ and TE₁₁₁ modes can appear. So that these modes are not excited, it is necessary to choose judiciously the diameter D ′ of the blade 6, its thickness e ′ and the length L ′ of the circular guide section 7. Experience shows that one is led to decrease these three dimensions compared to those of conventional pill-box windows.

The diameter D ′ between the long side and the short side of the rectangular waveguide 5 is thus chosen. created at the level of the rectangular guide, inductive shutters 8, to obtain an adaptation of the window, on the central frequency of the operating band of the rectangular wave guide 5.

The thickness e ′ of the blade 6 is chosen to be as thin as possible, but sufficient to withstand the mechanical and electrical stresses to which it is subjected.

We added a half-wave matching transformer 10, made up of two elements of the same length, placed on either side of the circular waveguide section 7, inside the rectangular waveguide 5, so that they cover at least one of its long sides. This transformer enables sufficient adaptation to be made over the entire operating frequency band of the rectangular waveguide 5.

With this window, one obtains a bandwidth of use which corresponds approximately to 40% compared to the central frequency.

But this window is more complicated to make than the "pill-box" window. The diameter of the circular waveguide 7 is between the long side and the short side of the rectangular waveguide 5. Consequently, additional parts must be used to seal between the inside and the outside of the guides, these parts being placed at the junction between the rectangular guide 5 and the two ends of the circular guide section 7. Furthermore, the short length of the circular guide relative to its diameter does not promote its flexibility, necessary to allow differences in expansion between ceramic and guide.

The window according to the invention is more solid than the conventional "pill-box" window, it is also easy to produce and it operates in an enlarged frequency band without exhibiting parasitic resonance modes.

Its bandwidth performance will be close to that of the window described in patent FR 2,558,305, but it will be simpler to carry out. Its dimensions being larger than those of the window described in patent FR 2,558,306, it will be able to operate at higher peak power. In addition, the length of the circular guide having been increased compared to that of the circular guide of the window of patent FR 2 558 306, the flexibility of the guide is improved.

The invention proposes a microwave window comprising:
- a circular blade of dielectric material, mounted in a circular waveguide section having substantially the same diameter as the blade, the circular waveguide section being connected at its ends to a rectangular waveguide containing an adaptation transformer,
characterized in that:
- the diameter of the circular waveguide is less than the diagonal of the rectangle,
- at each end of the circular waveguide section, a wall provides a tight connection with respect to the outside, between the rectangular waveguide and the circular waveguide section, each wall being provided with an opening included in a straight section portion common to the rectangular waveguide and to the circular waveguide section, the area of the opening being less than the area of the straight section portion.

Each wall is transverse to the circular waveguide section.

The openings are identical and preferably have an oblong shape. The large dimension of each opening is parallel to the long sides of the rectangular waveguide.

When the openings are substantially rectangular, the matching transformer can be constituted by the extension of a large side of each opening on the corresponding side of the rectangular waveguide.

• - les figures 1a, 1b déjà décrites, sont respectivement une vue en coupe longitudinale et une vue en coupe, selon l'axe AA′ de la figure 1a, d'une fenêtre "pill-box" classique ;
• - les figures 2a, 2b déjà décrites, sont respectivement une vue en coupe longitudinale et une vue en coupe, selon l'axe BB′ de la figure 2a, d'une fenêtre décrite dans le brevet FR 2 558 306 ;
• - les figures 3a, 3b, 3c sont respectivement une vue en coupe longitudinale, selon le petit côté d'un guide rectangulaire, une vue en coupe selon l'axe CC′ de la figure 3a, une vue en coupe longitudinale, selon le grand côté du guide, d'une fenêtre conforme à l'invention ;
• - la figure 4 une coupe schématique, selon le petit côté du guide d'un mode de réalisation d'une fenêtre conforme à l'invention.

Other characteristics and advantages of the present invention will appear on reading the description given below with reference to the appended drawings in which:

• - Figures 1a, 1b already described, are respectively a longitudinal sectional view and a sectional view, along the axis AA ′ of Figure 1a, of a conventional "pill-box"window;
• - Figures 2a, 2b already described, are respectively a longitudinal sectional view and a sectional view, along the axis BB ′ of Figure 2a, of a window described in patent FR 2,558,306;
• - Figures 3a, 3b, 3c are respectively a longitudinal sectional view, along the short side of a rectangular guide, a sectional view along the axis CC ′ of Figure 3a, a longitudinal sectional view, along the large side of the guide, a window according to the invention;
• - Figure 4 a schematic section along the short side of the guide of an embodiment of a window according to the invention.

In the figures, the same references designate the same elements.

The window shown in FIGS. 3a, 3b, 3c comprises a thin blade 31 made of non-porous dielectric material such as alumina for example. This blade 31 is circular, has a diameter D1 and a thickness e1. It is mounted, in a sealed manner, in a section of circular waveguide 32 substantially of the same diameter. The circular waveguide section 32 has a length L1. The two ends 38, 39 of this circular waveguide section 32 are connected to a waveguide 34 which transmits microwave waves in an operating frequency band. Microwave waves enter the window at its first end 38 and exit from it at its second end 39. In the figure, the waveguide 34 is a rectangular waveguide, and the diameter of the circular waveguide section 32 is less than the diagonal of the rectangle.

The window according to the invention, like the window described in patent FR 2,558,306, must be able to work in a wide frequency band with good adaptation and without "ghost mode".

For this, a wall 36 is provided at each junction between the waveguide 34 and the circular waveguide section 32, provided with an opening 37. Each wall 36 provides a sealed junction with respect to the outside. between the waveguide 34 and the circular waveguide section 32. Each wall 36 extends in a plane transverse to the circular waveguide section 32.

The walls 36 are situated at each end 38, 39 of the circular guide section 32. Each wall 36 closes a section delimited by the union between the straight section of the circular wave guide section 32 and the straight section of the guide d wave 34.

At the junction between the waveguide 34 and the circular waveguide section 32, it is possible to define a portion 42 of straight section common to the two guides 32,34.

Each opening 37 is contained in this portion 42 of cross section. The area of each opening is less than that of the portion 42 of cross section.

The openings 37 will preferably be identical; they will be located in the central part of each wall 36.

Each opening 37 will preferably have an oblong shape. Its large dimension will be parallel to the long side of the waveguide 34.

Each wall 36, provided with its opening 37, brings in the waveguide 34 selfic and capacitive corrections.

The inductive corrections are given by portions 40 of the wall, included between each short side of the waveguide 34 and the edge of the opening 37 closest to this short side.

The capacitive corrections are given by wall portions 41, lying between each long side of the waveguide 34 and the edge of the opening 37 closest to this long side.

These corrections are intended to adapt the window to the transmission mode used in the waveguide 34.

Experience shows that we can increase and even double the length L1 of the circular guide section 32 with respect to the length of the window guide described in patent FR 2,558,306. It is also possible to increase the thickness e1 of the blade 31.

Next, inside the waveguide 34, on either side of the circular waveguide section 32, a half-wave adapter transformer 35. The wavelength considered corresponds to the central frequency of the operating band of the waveguide 34.

In FIGS. 3a, 3b, 3c it can be seen that each wall 36 comprises a single partition which closes both an end of the circular guide section 32 and the cross section of the wave guide 34. The wall 36 could have included a set of parts, for example a partition provided with an opening closing off the end of the circular waveguide section 32 and flanges of suitable shape for closing off the parts of the cross section of the waveguide 34 opening out of the circular waveguide section 32.

The openings 37 are substantially rectangular and their long sides 43 are parallel to the long sides of the waveguide 34.

The diagonal d of each opening 37 is then less than the diameter D1 of the circular waveguide section 32. The portion 42 of straight section is between the two long sides of the waveguide 34 and two arcs of circles of the section of circular waveguide 32.

One could have envisaged that the openings 37 are rectangular with rounded angles or have the shape of a rectangle with, attached to each of its short sides, a semicircle of diameter equal to the short side. Still other forms are possible.

In FIGS. 3a, 3b it can also be seen that the half-wave matching transformer 35 is constituted by the extension of one of the long sides 43 of each opening 37, in the waveguide 34, on either side of the waveguide section circular 32.

This extension is made on the same long side of the waveguide 34, over the same length. It could be spread over the two long sides.

The transformer 35 extends inside the waveguide 34, over the entire long side of the waveguide. Figure 3c shows this. The height of the transformer therefore on the side 43 is defined to obtain the adaptation of the window on a relative operating band of 30% relative to the central frequency in the waveguide 34.

Tests have shown that by operating a window according to the invention in S-band and by inserting it in a waveguide of RG48U type, very satisfactory band performance is obtained. The diameter D1 and the thickness e1 of the blade are:
D1 = 66 mm
e1 = 2.1 mm
the operating bandwidth of the window is 30% relative to the central operating frequency in the guide of type RG48U.

The mechanical strength of the dielectric strip is proportional to a coefficient K = (R / e) ²
R being the radius of the blade and e its thickness.

With the window according to the invention tested, a coefficient K = 247 is obtained.

A classic pill-box window tested under the same conditions gives the following results:
D = 85 mm e = 3 mm and the coefficient K is then K = 201
Its operating bandwidth is then 12% relative to the central operating frequency in the guide of the RG48U type.

A window as described in patent FR 2,558,305 tested under the same conditions gives the following results:
D ′ = 55 mm e ′ = 1.8 mm
K = 233
its operating bandwidth is 40% relative to the central operating frequency in the guide of type RG48U.

The window according to the invention, thanks to the openings 37 which are reduced relative to the sections of the waveguide 34 and of the circular waveguide 32, therefore makes it possible to work in an enlarged band with respect to the window "pill- box". It also has improved mechanical strength while retaining the extremely simple technology of the pill-box window. In addition, its dimensions are reduced.

Referring to Figure 4, we will now describe a practical embodiment of a window according to the invention.

We start by brazing the blade 31 of dielectric material such as ceramic, in a circular waveguide section. The walls 36 which can be produced by metallic plates of copper for example, are reported by brazing at the two ends 38, 39 of the circular waveguide section. Their shape is suitable for closing both the cross section of the circular guide section 32 and the cross section of the wave guide 34. The wave guide 34 is fixed on each side 36 by soldering other of the circular waveguide section 32. Each wall 36 has an opening 37 in its central part.

Two fixing flanges 50 can be used to ensure a tight connection between the waveguide 34 and the circular waveguide section 32.

The transformer 35 can be produced by two identical metal plates, made of copper for example. Each plate is for example brazed, flat inside the waveguide 34, on one of its long sides and on the wall 36.

The present invention is not limited to the examples described. Other variants may exist without departing from the scope of the invention.

Claims (8)

1. Microwave window comprising:
- a circular blade (31) of dielectric material, mounted in a circular waveguide section (32) having substantially the same diameter as the blade (31), the circular waveguide section (32) being connected to the level of its ends (38,39) at a rectangular waveguide (34) containing an adaptation transformer (35), characterized in that:
the diameter of the circular waveguide is less than the diagonal of the rectangle,
at each end of the circular waveguide section (32), a wall (36) provides a sealed connection with respect to the outside, between the rectangular waveguide (34) and the waveguide section circular (32), each wall (36) being provided with an opening (37) included in a portion (42) of cross section common to the rectangular waveguide (34) and to the circular waveguide section (32 ), the area of the opening (37) being less than the area of the portion (42) of cross section. 2. Microwave window according to claim 1 characterized in that each wall (36) is transverse to the circular waveguide section (32). 3. Microwave window according to one of claims 1 or 2 characterized in that the openings (37) are identical. 4. Microwave window according to one of claims 1 to 3 characterized in that each opening (37) is oblong. 5. Microwave window according to claim 4 characterized in that the large dimension of the opening is parallel to the long sides of the waveguide (34). 6. Microwave window according to one of claims 4 or 5 characterized in that each opening (37) is substantially rectangular. 7. Microwave window according to claim 6 characterized in that the adaptation transformer (35) is constituted by the extension of a large side (43) of each opening (37) on the corresponding side of the waveguide (34). 8. Microwave window according to claim 7 characterized in that each side (43) extends over the same length, so as to obtain a half-wave transformer in the rectangular waveguide (34).

Images