GB2399690A

GB2399690A - Transmission line pressure window assembly

Info

Publication number: GB2399690A
Application number: GB0306082A
Authority: GB
Inventors: Barry George Morton Helme
Original assignee: Quasar Microwave Technology Ltd
Current assignee: Quasar Microwave Technology Ltd
Priority date: 2003-03-18
Filing date: 2003-03-18
Publication date: 2004-09-22
Anticipated expiration: 2023-03-18
Also published as: GB2399690B; GB0306082D0

Abstract

The assembly is intended for connection between two sections of a waveguide of predefined internal profile having end flanges. The assembly includes a body 1 having opposing coupling faces 3, 4 for contact with the end flanges and containing a passage 2 having an electrically conducting inner surface extending between the coupling faces. The passage contains a pressure-retaining electromagnetic window element 5 having a high pressure surface and a low pressure surface connected by a peripheral surface. The internal wall of the passage is configured to provide a window section 15 which surrounds and closely conforms to the peripheral surface of the window element, an inwardly-extending support section 16 which covers a peripheral portion of said low pressure surface of the window element, a first section 10 extending between the high pressure coupling face and the window section, and a second section extending between the low pressure coupling face and the support section. The first and second sections both include enlargements of the internal profile of the waveguide sections forming matching cavities which are tuned to minimise reflection of electromagnetic energy travelling along the waveguide within its intended range of operating frequencies.

Description

TRANSMISSION LINE PRESSURE WINDOW ASSEMBLY

TECHNICAL FIELD OF THE INVENTION

This invention relates to window assemblies which serve to isolate the environments within two lengths of electromagnetic waveguide from each other whilst allowing electromagnetic energy travelling along the waveguide to pass through the assembly.

BACKGROUND

In such window assemblies, the dielectric materials which are used to form the window element have a tendency heat up when they are subjected to electromagnetic energy. They may also exhibit increased absorption of energy with increasing temperature which, if not controlled, can lead to a thermal runaway effect. It is known, particularly in high power applications, to provide cooling of the assembly, and in order to ensure efficient heat transfer, the window element is usually fused into the assembly. Apart from the fact that this incurs a significant manufacturing cost, this technique also limits the materials which can be used, since differential expansion between materials must be avoided to reduce the possibility of stress fractures.

The current state of the art in such window assemblies may be exemplified by WO 02 33 776 A1, which describes a form of window assembly which is capable of providing very efficient cooling. The shape of the window may be chosen to minimise reflected power across the required operating bandwidth of the assembly. In a preferred embodiment the window is supported on the low pressure side by an inwardly-extending peripheral flange. Although this has significant advantages such as improving the pressure differential handling capability of the assembly, the provision of such a flange causes a significant increase in reflected power. It is suggested that this problem can be limited by providing the high pressure side of the flange with a step-like peripheral recess, but this causes a reduction in the pressure handling capabilities.

A practical consideration in the design of window assemblies is that the assembly should ideally connect with industry standard waveguide flanges.

The present invention seeks to provide a new and inventive form of window assembly which exhibits improved electromagnetic power handling properties and reduced loss across a wide range of operating frequencies combined with good differential pressure handling capability and low manufacturing cost. It is furthermore desirable that the window assembly should be capable of connection to industry standard waveguide flanges with minimum reflection of electromagnetic energy whilst at the same time maintaining a good pressure seal between the window assembly and the flange.

SUMMARY OF THE INVENTION

The present invention provides a transmission line pressure window assembly for connection between two sections of a waveguide of predefined internal profile having end flanges, the assembly including a body having opposing coupling faces for contact with the end flanges and containing a passage having an electrically conducting inner surface extending between the coupling faces, said passage containing a pressure-retaining electromagnetic window element having a high pressure surface and a low pressure surface connected by a peripheral surface, the internal wall of the passage being configured to provide: - a window section which surrounds and closely conforms to the peripheral surface of the window element, - an inwardly-extending support section which covers a peripheral portion of said low pressure surface of the window element - a first section extending between the high pressure coupling face and the window section, and - a second section extending between the low pressure coupling face and the support section, characterized in that the first and second sections both include enlargements of the internal profile of the waveguide sections forming matching cavities which are tuned to minimise reflection of electromagnetic energy travailing along the waveguide within its intended range of operating frequencies.

A further advantageous feature of the assembly is that the window element is less susceptible to stress fractures and need not be fused to the internal wall of the passage. The window element may be bonded to the window section and/or the support section using an interposed adhesive material, which significantly reduces manufacturing costs. The body of the window assembly can directly connect with any industry standard waveguide flange I type of appropriate internal waveguide dimensions, while achieving minimum reflection of electromagnetic energy and maintaining a good pressure seal between the opposing faces.

The internal enlargements which form the matching cavities may be contiguous with the coupling faces or they may be separated from the coupling faces by a linking section which forms a transition between the internal profile of the waveguide cavity and the matching cavity. The transition section may incorporate tuning elements which further help to reduce electromagnetic reflections.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a general view of a pressure window assembly in accordance with the invention as viewed from the high pressure side; Figure 2 is a similar general view of the assembly looking from the opposite low pressure side; Figures 3 and 4 are detailed views of the assembly looking from the high pressure side and low pressure side respectively; Figure 5 is a section through the assembly on line A-A'with the high pressure side on the left; Figure 6 is an elevation of the complete assembly looking from the high pressure side; Figure 7 is a similar sectional view to Fig. 5, but showing the complete assembly as connected between two lengths of waveguide; Figures 8 and 9 are general views of a second embodiment of pressure window assembly looking from the high and low pressure sides respectively; Figures 10 and 11 are detailed fragmentary views of the high and low pressure faces shown in Fig.s 8 and 9; Figure 12 is an E plane section A-A' through the window assembly as indicated in Fig.s 10 and 11; Figure 13 is a view of the high pressure face of the window assembly showing the position of a superimposed waveguide flange; Figure 14 is an E plane section through the window assembly as connected between two waveguide flanges; Figure 15 is a end elevation of a choke flange with which the second form of window assembly can be used; Figure 16 is an E plane section through the choke flange; Figure 17 is an elevational view of the high pressure face of a window assembly of the invention showing the position of the choke flange superimposed thereon; and Figure 18 is an E plane section through the window assembly connected between the choke flange and a plain low pressure flange.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to Fig.s 1 and 2, the pressure window assembly includes a platelike coupler body 1, which is preferably formed in one piece, with a central passage 2 extending between high and low pressure faces 3 and 4.

Although the body may be formed of material which is electrically conductive in the intended operating frequency range it may be formed of any suitable material with the passage 2 internally coated with suitable conductive material. A pressure-resisting window element 5 is fixed in the passage 2 as described below. The window element may be formed of a known dielectric material which is impermeable to any gas used on the high pressure side and which transmits electromagnetic energy at the required operating frequencies.

In this example the overall cross-sectional shape of the window element 5 and passage 2 are intended for use with waveguide having a standard rectangular internal cross section, but it will be appreciated that within the scope of the invention the shape can be modified for use with waveguide of any size and cross-section. Referring to Fig s 3 to 5, the window element has opposing high and low pressure surfaces 6 and 7 connected by a peripheral surface 8. Considering the E plane section AA'shown in Fig. 5, and starting at the high pressure face 3, the internal wall of the passage 2 includes a first matching section 10 which, in this example, is of uniform cross section, being generally oval with opposing straight sides 11 and 12 i joined by semi-circular ends 13 and 14 (Fig. 3) . The first matching section leads into a window section 15, which in this example is of the same internal profile as the section 10, immediately followed by an inwardly extending support section 16. As can also be seen in Fig. 3, the inner profile of the support section 16 is similar to that of the first matching section but of smaller dimensions, joining a second matching section 20 which continues to the low pressure coupling face 4. Unlike the matching section on the high pressure side, the second matching section 20 is not of uniform profile, being shaped to form a peripheral recess 21. Thus, after a; short link section 22, which continues the internal cross-sectional profile of the support section 16, the wall of the passage steps inwardly to the recess 21. As indicated by the dotted outline in Fig. 4, the cross-sectional shape of the recess 21 is similar to that of the tuning section 22 but the cross sectional area of the recess is even greater than that of the first matching section 10. At the outer end of the recess 21 the wall of the passage again steps outwardly to form a second linking section 23, the internal profile of which is substantially rectangular matching that of the specified waveguide.

The window element closely conforms to the peripheral cross-sectional shape of the window section 15, and a peripheral area of the window element on the low pressure side is covered and supported by the support section 16. The window element 5 may be sealed into the passage and bonded to the sections 15 and 16 by means of readily available adhesive bonding materials. Such bonding materials may be sufficiently flexible to permit the window element and the coupler body 1 to be formed of materials with significantly different coefficients of expansion without compromising the integrity of the seal. Furthermore, this allows the assembly to be formed of materials which will produce lower amplitudes of passive intermodulation interference signals.

Referring now to Fig.s 6 and 7, the window assembly is used between two lengths of waveguide 30 and 31 of the specified kind, which may terminate in industry-standard coupling flanges 32 and 33 respectively. The coupling flange 32 on the high pressure side is recessed to receive a sealing ring 34 which seals the interface between the flange and the high pressure face 3.

When the low pressure side is at atmospheric pressure the second flange 33 does not require a sealing ring, but if the low pressure side is above or below atmospheric pressure a similar sealing ring may be provided. The assembly may be clamped by bolts inserted through the flanges 30, 31 and fixing holes 36 provided in the body 1 in a standard pattern. It can clearly be seen in Fig. 6 that the peripheral profile of the first matching section 10 is wholly enclosed within the confines of the sealing ring 34 so that the necessary electrical continuity is maintained between the internal surface of the waveguide section 30 and the internal surface of the passage 2. It is also apparent in the E plane section of Fig. 7 that the matching section 10 is recessed relative to the internal profile of the waveguide, forming a tuned matching cavity between the waveguide section 30 and the window 5. The recess 21 forms a second tuning cavity on the low pressure side of the window 5. Under normal circumstances the presence of an interruption such as the support face 16 would give rise to standing waves within the waveguide and a resulting loss of power, but the tuning cavities minimise reflection of electromagnetic energy travailing along the waveguide within the intended range of operating frequencies. The precise configuration of the two tuning cavities can readily be determined using known design techniques.

The design of the window described in the first example above is such that the passage 2 has a high pressure matching section 10 which is uniform and has the same internal profile as the window section 15. There are some industry standard waveguide flange configurations where the sealing ring 34 and fixing holes 36 are significantly closer to the window aperture, relative to the window aperture size, than shown in the above example. In these circumstances the high pressure matching section 10 would be outside the confines of the sealing ring. This therefore means that the necessary contact between the flange surface within the sealing ring 34 and the face of the window body 3 is not complete, at least in part. This could degrade the electromagnetic and pressure seal performance. Therefore, there needs to be a narrowing of the high pressure part of the passage 2 in the areas where there is a gap between the high pressure surface 3 and the opposing flange face, so that full and sufficient pressure sealing contact is achievable between the window body surface 3 on the high pressure side and the flange face with the sealing ring 34.

By way of example, Fig.s 8 and 9 show the high and low pressure sides respectively of a window assembly which is designed for waveguide flange configurations where the sealing ring and the fixing holes 38 are significantly closer to the window aperture 39 and 40 relative to the window aperture size.

Fig. 10 shows more detail of the high pressure side of the window assembly.

The uniform dashed line 41 shows the tuned cavity profile on the high pressure side of the window. The short and long dashed lines 42 show the profile of the window element where it is hidden behind part of the front face on the high pressure side of the assembly. The fixing holes 38 are outside the confines of the tuned cavity 41.

Fig. 11 shows the detail of the low pressure side of the window assembly.

The uniform dashed lines show the extremity of the tuned cavity 43 on the low pressure side of the window. The faxing holes 38 are, again, outside the confines of the tuned cavity 43.

Fig. 12 shows the E plane (A-A) section of the window assembly. The window element is assembled into the assembly body by inserting it at an angle through the high pressure aperture 39 into the high pressure tuned cavity 45 and then seating it into the window recess 44 in the body of the window assembly.

Fig. 13 shows the high pressure side of the window, with the appropriate features of the high pressure flange face shown in dashed lines. The fixing holes 38 are outside the confines of the high pressure tuned cavity 41.

Fig. 14 shows, by way of example, the E plane cross section of the high power window assembly attached to industry standard plain flanges, with the high pressure flange having a sealing ring 46. It will be noted that in this case the internal extremity 41 of the high pressure tuned cavity is larger than the profile of the sealing ring 46. The electrical and pressure sealing contact between the flange and the window on the high pressure side are achieved by the reduced profile aperture 39. This provides sufficient surface contact between the window assembly and the flange 47 to meet the sealing requirements, even under the worst case dimensional tolerances. The electromagnetic requirements are achieved by using the appropriate dimensions of the matching cavity 45 and the aperture section 39 on the high pressure side, and on the low pressure side 49 and 50 respectively, and the window element 5.

In a third, important application of the invention the window assembly can be attached to an industry standard waveguide flange that has a sealing ring and a microwave choke. Such waveguide flanges are already known, but the arrangement will now be described to facilitate a greater understanding of the present invention.

By way of example, plan and E plane cross sectional views of an industry standard choke flange with sealing groove is shown in Fig.s 15 and 16. The flange has a central standard transmission line aperture 51 surrounded by a recessed face 53 and a choke groove 52 with an outer sealing groove 54 containing a sealing ring 55. The choke flange is connected to a second section of transmission line having a flange with a plain flat face 56, and the flanges are held together with bolts inserted through aligned holes 57in opposing flat faces 58 at the outer periphery of the flanges. A protruding ridge 59 separates the sealing groove 54 and ring 55 from the choke groove 52. When the choke flange is connected to the opposing plain flange the ridge 59 and the peripheral face 58 are the only areas of the choke flange which make direct mechanical contact with the plain flange. A pressure seal is achieved through the contact between the outer sealing groove 54, sealing ring 55, and the opposing surface 56 of the plain flange. This assembly is used to achieve good electromagnetic transmission within the waveguide transmission line 51 across the junction of the two flanges, especially when high currents and powers are involved. This reduces the sensitivity of the junction to contact resistance, surface imperfections and contamination, especially when the waveguide junctions are liable to be frequently dismantled or subject to movement.

This standard type of choke waveguide junction is based on a short circuited half wavelength transmission line, the short circuit 60 being transformed to and maintaining electrical continuity at the junction gap 61 at the inside wall of the waveguide, despite the presence of a mechanical clearance. The mechanical contact surface 59 is made at a point one quarter wavelength away from the junction gap 61 along the radial portion 62, and one quarter wavelength away from the short circuit 60 along the coaxial portion 63. The mechanical contact is therefore made where the electrical current is small and imperfections have little effect. The junction between the surfaces of radial portion 62 and coaxial portion 63 is a right angle which is formed with a radius 64 to increase the power handling capability of the junction. The flange is held to the opposing flange by bolts which pass through appropriate holes in the flange 57, which lie outside the extremity of the sealing groove 54. The profile of the choke on the standard flange is designed to be attached to a flange which has a plain flat face with a waveguide aperture having standard dimensions. The present window assembly enables the high power windows to be attached to choke flanges with minimal reduction in their electromagnetic and high pressure performances, without the need for additional parts.

By way of example, Fig. 17 shows a view of the high pressure side of the window assembly with the features of the sealing ring plus choke flange shown in dashed lines. This view shows the aperture in the window body 65 which is adjacent to the choke flange. The exposed face of the support structure 66 which holds the window element 5 is also visible. Between these two structures there is a further tuning structure 67. The extremity of the tuned cavity on the high pressure side is shown by the dashed lines 68.

Fig. 18 shows, by way of example, the E plane cross section of the high power window attached to an industry standard choke and plain flanges.

Starting from the left in the drawing, an industry standard choke flange 69 is attached to a high power window body 70 and a plain flange 71. This cross section shows the tuned cavity 72 on the high pressure side, with a linking section 73 between the cavity 72 and the physical gap 61 which forms the radial part of the choke. The dashed line 74 shows the position of the tuning structure 67. In this example, the profile of the waveguide aperture from the tuning structure 67 to the high pressure face 75 of the window body is the same as the profile 65. The two flanges are secured to the window body by a number of bolts (not shown) which pass through holes 57 in the flanges 69 and 71 and window body 70. The electromagnetic requirements are achieved by using the appropriate dimensions of, on the high pressure side, the matching cavity 72, tuning structure 67 and linking section 73, and on the low pressure side, the matching elements 76, linking section 77 and the window element 5. A pressure seal is achieved through the contact between the sealing groove 54, sealing ring 55, and the opposing surface 75 of the window body.

In the examples described, the bonding between the window element and the opposing surfaces of the passage 2 provide a good thermal path between the window element and the body 1 which provides efficient cooling of the window element. Additional cooling means may be provided, especially if the assembly is intended for high power use. The window element may be provided with internal cooling passages if desired, e.g. as disclosed in WO 02 33 776 A1.

The advantages of the window assembly may be summarised as follows: t Improved electromagnetic power handling capability.

- Increased maximum pressure differential.

- Improved electromagnetic performance, insertion loss, return loss and t passive intermodulation (pim) generation across a broad range of operating frequencies.

- Simplified manufacturing process and reduced manufacturing costs.

- A minimum number of component parts.

It will be appreciated that the features disclosed herein may be present in any feasible combination. Whilst the above description lays emphasis on those areas which, in combination, are believed to be new, protection is claimed for any inventive combination of the features disclosed herein.

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Claims

1. A transmission line pressure window assembly for connection between two sections of a waveguide of predefined internal profile having end flanges, the assembly including a body having opposing coupling faces for contact with the end flanges and containing a passage having an electrically conducting inner surface extending between the coupling faces, said passage containing a pressure-retaining electromagnetic window element having a high pressure surface and a low pressure surface connected by a peripheral surface, the internal wall of the passage being configured to provide: - a window section which surrounds and closely conforms to the peripheral surface of the window element, - an inwardly-extending support section which covers a peripheral portion of said low pressure surface of the window element - a first section extending between the high pressure coupling face and the window section, and - a second section extending between the low pressure coupling face and the support section, in which the first and second sections both include enlargements of the internal profile of the waveguide sections forming matching cavities which are tuned to minimise reflection of electromagnetic energy travailing along the waveguide within its intended range of operating frequencies.

2. A transmission line pressure window assembly according to Claim 1 in which the window element is bonded to the window section and/or l the support section using an interposed adhesive material.

3. A transmission line pressure window assembly according to Claim 1 or 2 in which at least one of the internal enlargements which form the matching cavities is contiguous with the coupling faces.

4. A transmission line pressure window assembly according to any preceding claim in which at least one of the internal enlargements which form the matching cavities is separated from the coupling faces by a linking section which forms a transition between the internal profile of the waveguide cavity and the matching cavity.

5. A transmission line pressure window assembly according to Claim 4 in which the linking section, or at least one of the linking sections, incorporates tuning elements.

6. A transmission line pressure window assembly substantially as described with reference to the drawings.

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