GB2582892A

GB2582892A - Apparatus for propagating radio waves

Info

Publication number: GB2582892A
Application number: GB1901766.4A
Authority: GB
Inventors: Paul Watts James; Malcolm Hague Paul
Original assignee: Flann Microwave Ltd
Current assignee: Flann Microwave Ltd
Priority date: 2019-02-08
Filing date: 2019-02-08
Publication date: 2020-10-14
Also published as: GB201901766D0

Abstract

An apparatus for propagating radio waves has a tubular structure 10. The tubular structure 10 comprises a cross-section with a first pair of opposite internal sidewalls 11 substantially parallel to each other and a second pair of opposite internal sidewalls 12 extending between ends of the first pair of internal sidewalls 11. The second pair of opposite internal sidewalls 12 are longer than the first pair of opposite internal sidewalls 11 and the distance between the second pair of opposite internal sidewalls 12 in a direction parallel to the first pair of opposite internal sidewalls 11 at a midpoint 14 of the second pair of opposite internal sidewalls 12 is greater than the length of the first pair of internal sidewalls 11. Another aspect of the invention provides an apparatus for propagating radio waves which has a tubular structure and in which at least one curvilinear wall extends between a first pair of opposite internal sidewalls. The apparatus may be a waveguide and a cross-section of the tubular structure may be symmetrical about 180° rotation around the longitudinal access.

Description

APPARATUS FOR PROPAGATING RADIO WAVES

The invention relates to apparatus for propagating radio waves and especially, apparatus incorporating a transmission line structure, such as a waveguide.

Conventional waveguides for propagating or transmitting radio waves, such as microwaves and millimetre waves, generally have a rectangular cross-sectional profile and an internal surface that is a good electrical conductor. This is usually a metal, such as brass, copper or aluminium. In the case of aluminium, the internal surface may have a chromate passivation. In the case of brass, the internal surface may be coated with gold plate to further improve the electrical conduction.

Although waveguides are less flexible then cable, they are more efficient at transmitting or propagating the signal and are more reliable. In addition, they are able to cope with higher power signals that would cause cable to melt or arc.

A further advantage of using waveguides is that they act as a filter and prevent frequencies lower than half the wavelength, A, of the nominal operating frequency, f, of the waveguide from propagating. However, even with conventional waveguides there is a limit to the maximum power level that can propagate through them. This is because as the power within the waveguide increases, the electric field strength, E, within the waveguide also increases and the waveguide starts to heat up. Typically, with conventional waveguides, the waveguide will start to break down or arc when the electric field strength, E, at the internal surface of the waveguide is approximately 20-30 kVcm-1.

Attempts have been made to overcome this problem to enable waveguides to be used with higher powered signals. For example, conventional rectangular waveguides typically have a cross-sectional profile aspect ratio of 2:1 and one attempt to try and increase the power has been to increase the two opposite shorter sides of the waveguide so that the waveguide aspect ratio is 1:1 or closer to 1:1. In other words, so that the waveguide has a cross-sectional profile that is a square or closer to a square than a conventional waveguide with a 2:1 aspect ratio.

However, a problem with this approach is that the increase in height and the change in the aspect ratio results in the waveguide also supporting modes in the other plane and these modes may interfere with the intended modes to give unpredictable performance.

A second solution is to use pressurised air inside the waveguide. However this also has a number of disadvantages including that it requires extra seals within the waveguide network, the capability of the components within the waveguide network to withstand internal pressure and is expensive to implement due to the requirement for a pressurised system.

In addition, there is the further complication that antennas, such as horn antennas are normally open. They are therefore very difficult to seal and to use with a pressurised system.

A third solution is to fill the waveguide system with a gas that has a higher dielectric strength than air. A common gas that is used for this is sulphur hexafluoride (SF6). For example air typically breaks down at approximately 30 kVcm-1 but sulphur hexafluoride does not break down until approximately 150 kVcm-1.

While filling the waveguide network with sulphur hexafluoride can improve the power handling capabilities of the waveguide network, there are also significant disadvantages and problems with using this gas. In particular, if the gas breaks down it produces products that are extremely toxic, such as disulphur decafluoride (S2F1o). If sulphur hexafluoride leaks into the atmosphere in a non-ventilated or poorly ventilated area, it also carries the risk of asphyxia if too much is inhaled, by displacing oxygen in the lungs. This means that it is necessary to have appropriate gas detectors installed in all areas in the vicinity of the waveguide network to detect any gas leakage from the waveguide network and to minimise the risk of inhalation.

There are also the additional disadvantages that the gas is extremely costly and that the gas is a known greenhouse gas with an extremely high global warming potential of about 23,900 times that of carbon dioxide. According to the Intergovernmental Panel on Climate Change, sulphur hexafluoride is the most potent greenhouse gas that it has evaluated and it is named in the UN Framework Convention on Climate Change (Kyoto Protocol, 1997) as one of the gases for which emissions are to be reduced. Furthermore, since 1 January 2006, sulphur hexafluoride has been banned in Europe in all applications except high-voltage switchgear.

In accordance with a first aspect of the present invention, there is provided apparatus for propagating radio waves, the apparatus comprising a tubular structure having a longitudinal axis and comprising an electrically conducting internal surface; a cross-section of the tubular structure in a plane perpendicular to the longitudinal axis comprising at least two opposite linear internal sidewalls substantially parallel to each other and at least one curvilinear internal sidewall extending between the at least two opposite linear internal sidewalls; and the at least one curvilinear sidewall having a length greater than either of the at least two opposite linear sidewalls.

In accordance with a second aspect of the present invention, there is provided apparatus for propagating radio waves, the apparatus comprising a tubular structure, the tubular structure comprising a cross-section which comprises a first pair of opposite internal sidewalls substantially parallel to each other and a second pair of opposite internal sidewalls extending between ends of the first pair of internal sidewalls; and wherein the second pair of sidewalls are longer than the first pair of sidewalls and the distance between the second pair of opposite internal sidewalls in a direction parallel to the first pair of internal sidewalls at a midpoint of the second pair of internal sidewalls is greater than the length of the first pair of internal sidewalls.

The term "radio wave" or "radio waves" as used herein refers to electromagnetic radiation having a frequency from approximately 300 MHz to approximately 3 THz and includes the UHF, microwave, millimetre wave and sub-millimetre wave regions of the electromagnetic spectrum.

An advantage of the invention is by providing apparatus for propagating radio waves in which the separation of the longer sidewalls of the waveguide at their midpoint is greater than the length of the shorter sidewalls of the waveguide, it is possible to reduce the electric field strength in the vicinity of the midpoint of the longer sidewalls. This thereby enables signals to be propagated along the waveguide with higher powers than would be the case for a conventional waveguide of rectangular cross section with a 2:1 aspect ratio. In addition, the invention has the advantage of minimising the propagation of lower frequencies and propagation of modes in other planes along the apparatus.

Typically, at least one of the second pair of internal sidewalls is curvilinear, and preferably, comprises a concave portion.

Typically, the at least one curvilinear sidewall has symmetry of reflection about an axis perpendicular to the longitudinal axis.

Typically, the cross-section of the tubular structure has symmetry of reflection about at least one axis perpendicular to the longitudinal axis. Preferably, the waveguide has symmetry of reflection about two axes perpendicular to the longitudinal axis, which are typically mutually orthogonal.

The at least one curvilinear internal sidewall may comprise a first curved portion and at least one side portion. Preferably, the curvilinear sidewall comprises at least two side portions with the first curved portion between the at least two side portions. The first curved portion may be centrally located on the curvilinear side wall. More preferably, one side portion is located adjacent each of the first pair of sidewalls.

Typically, the at least one side portion may comprise at least one of: a linear portion and a second curved portion. Preferably, the at least one side portion comprises at least one of two linear portions and two second curved portions. More preferably, there are two linear portions and two second curved portions. One linear portion and one second curved portion may be located on one side of the first curved portion and the other linear portion and the other curved portion located on the other side of the first curved portion. Most preferably, the second curved portions are located between the first curved portion and respective linear portions.

The first curved portion is preferably concave with respect to the longitudinal axis. The at least one second curved portion, if present, may be of opposite curvature from the first portion. If there are two second curved portions, both may be of opposite curvature from the first curved portion. In other words, the at least one second curved portion may have a convex curvature with respect to the longitudinal axis.

Where an at least one side portion is located adjacent one of the first pair of sidewalls, the side portion may form substantially a right angle with the one of the first pair of side walls.

In other examples of the invention, the at least one curvilinear sidewall may comprise only a first curved portion, which is concave with respect to the longitudinal axis, which extends from one of the first pair of internal sidewalls to the other of the first pair of internal sidewalls.

Preferably, both of the second pair of internal sidewalls are curvilinear. More preferably, each of the curvilinear internal sidewalls are a mirror image of the other.

Typically, the first pair of internal sidewalls are linear. Preferably, the first pair of internal sidewalls are substantially parallel.

Typically, the first pair of internal sidewalls have substantially the same length.

Preferably, the cross-section of the tubular structure is symmetrical about a 180° rotation around the longitudinal axis.

Preferably, the tubular structure is formed from a conducting material, such as metal. Typical metals that may be used to fabricate the tubular structure include brass, copper and aluminium. The internal surface of the tubular structure may be coated with a conducting material to further improve its electrical conductivity for waveguide use. For example, the internal surface may be coated with gold or silver plate.

Typically, the tubular structure may have a ratio between a separation of the second pair of internal sidewalls, not adjacent to the first pair of internal sidewalls, and the length of the first pair of internal sidewalls of greater than 1:1 and less than or equal to 2:1. Preferably, the ratio is in the range of 1.25:1 to 1.75:1, even more preferably in the range of 1.4:1 to 1.6:1 and even more preferably 1.45:1 to 1.55:1. Most preferably the ratio is approximately 1.53:1. Typically, the separation is the separation at or adjacent to the midpoints of the second pair of internal sidewalls.

Preferably, the tubular structure may be an elongate tubular structure. Typically, the tubular structure comprises a waveguide. The apparatus may comprise radio wave transmitting components other than a waveguide or other radio wave transmitting components in addition to a waveguide.

Typically, the internal perimeter of the cross-section of the tubular structure for use with a particular radio wave frequency has a length equal to the length of the internal perimeter of a conventional waveguide with a 2:1 aspect ratio for the same radio wave frequency.

Preferably, the first curved portion of the longer side walls is 30% to 80% of the length of at least one longer wall and preferably between 40% to 70% of the length of the longer wall. Most preferably the length of the first curved portion is between 55% to 65% of the length of the longer walls.

Examples of apparatus for propagating radio waves in accordance with the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of a conventional rectangular waveguide for transmitting radio waves; Fig. 2 is a schematic cross-sectional view of a first example of a waveguide for transmitting radio waves in accordance with the invention; Fig. 3 shows the waveguide of Fig. 2 with a conventional rectangular waveguide for use with the same radio wave frequency superimposed on top; Fig. 4 is a cross-sectional view of the waveguide of Fig. 2 to scale and showing relative dimensions; Fig. 5 is a graph comparing electric field strengths for the waveguides shown in Fig. 3; Fig. 6 is a diagram illustrating the electric field strengths within the waveguide of Fig. 4; Fig. 7 is a cross-sectional view of a second example of a waveguide in accordance with the invention; Fig. 8 is a cross-sectional view of a the third example of a waveguide in accordance with the invention; and Fig. 9 is a cross-sectional view of a fourth example of a waveguide in accordance with the invention.

Fig. 1 shows a cross-sectional view of a conventional prior art waveguide 1. The waveguide 1 has a rectangular cross-sectional profile with two shorter parallel and opposite side walls 2 interconnected by two longer side walls 3 which are also opposite and parallel to each other. The rectangular waveguide 1 has 2:1 aspect ratio. This means that each of the side walls 3 is twice as long as each of the side walls 2. The waveguide 1 is typically manufactured from a metallic material, such that the internal surfaces of the waveguide 1 are electrically conducting.

The waveguide 1 can be used to transmit or propagate radio waves which have a frequency with a corresponding wavelength, A, less than or equal to twice the length of the longer sides 3. Frequencies below this minimum frequency will not propagate along the wavelength 1. Therefore, this minimum frequency at which radio wave propagation occurs within the waveguide 1 is defined as the cut-off wavelength, Ao for the waveguide 1. Any frequencies having a wavelength, A, greater than the cut-off wavelength, AD, will not propagate along the waveguide 1.

In conventional rectangular waveguides, such as the waveguide 1, the electric field strength, E, is perpendicular to internal surfaces 5 of the longer sides 3 and parallel to the shorter sides 2. It is found that the electric field strength, E, is weakest closest to the shorter sides 2 and is strongest at central axis 4 of the waveguide 1. If the power of the radio waves propagating along the waveguide 1 is too high, the air within the wave guide starts to break down and arcing occurs within the waveguide 1. Generally, breakdown occurs where the electric field, E, is highest and that is around the central axis 4. The breakdown typically initiates in the vicinity of imperfections in the internal surfaces 5 of the waveguide and the resulting plasma fills the whole cross-section.

Fig. 2 shows a first example of a waveguide 10 in accordance with the invention. The waveguide 10 has two linear side walls 11 which are opposite and parallel interconnected by two opposing curvilinear sidewalls 12, which extend between each of the sidewalls 11. Each of the linear sidewalls 12 comprises a linear portion 13 adjacent to each side wall 11 and a central curved concave portion 14 located centrally between the linear portions 13. Interconnecting the central curved concave portion 14 to each of the linear portions 13 is a connecting curved side portion 15. The curved side portions 15 each have a curvature opposite to that of the curvature of the concave curved section 14. It is noted that the curvilinear sidewalls 12 are both symmetrical about the axis 4 and are a mirror image of each other. In addition, the waveguide 10 has 180° rotational symmetry about longitudinal axis 16 of the waveguide 10.

Fig. 3 illustrates the cross-sectional profile of the waveguide 10 with the cross-sectional profile of the waveguide 1 superimposed on top in phantom. The cut-off wavelength, Ao, of the waveguide 10 is identical to the cut-off wavelength, Ao, waveguide 1. However, as shown in Fig. 3, the shorter sides 11 of the waveguide 10 are shorter in length than the shorter side walls 2 of the waveguide 1. In addition, the longer side walls 12 of the waveguide 10 are longer than the longer side walls 3 of the waveguide 1. However, the effective length of the internal perimeter of the waveguide 11 is identical to the length of the internal perimeter of the waveguide 1. Hence, the distance along the internal perimeter of the waveguide 11 from point A to point B in Fig. 3 is the same as the distance on the internal perimeter of the waveguide 2 from point C to point D. Fig. 4 is a cross-sectional view of the waveguide 10 drawn to scale and showing the relative dimensions of the waveguide 10. If the separation of the two shorter opposite sides 11 is defined as "C", then each of the shorter sides 11 has a length of approximately 0.317C and the separation of the longer opposite sides 12 at their midpoint on the axis 4 is approximately 0.571C, The central curved concave portions 14 extend between points F and G on Fig. 4 and has a radius of curvature of approximately 0.558C. The curved side portions 15 extend between the points E and F and between the points G and H. The curved side portions 15 have a radius of curvature of approximately 0.317C. The distance between the points E and H is approximately 0.908C.

These are an example of preferred relative dimensions for the waveguide 10. However, advantages of the cross-sectional profile of the waveguide 10 can be obtained without requiring the use of these exact relative dimensions.

For example, the relative separation of the longer sides 12 at their midpoint to dimension C could be in the range of greater than 0.5C to less than 2.0C, and preferably, less than 0.7C. More preferably, the relative separation is in the range of greater than 0.5C up to 0.6C and more preferably greater than or equal to 0.53C. Even more preferably, the relative separation is in the range 0.55C to 0.6C.

The relative length of the shorter sides 11 to dimension C may be in the range (120 to 0.5C, typically in the range (125C to 0.4C. Preferably, the relative length is in the range 0.27C to 0.37C.

The radii of curvature of the side curved portions and the central portions and the length of the linear portions may be selected as desired depending on the desired properties of the waveguide. For example, these could depend on the desired operating bandwidth and cut-off frequency.

A comparison of the electric field strengths at the internal surfaces for each of the waveguides 1, 10 are illustrated in the graph of Fig. 5 using a radio wave having a frequency of 3 GHz and equal power in each of the waveguides 1, 10. The graph of Fig. 5 shows a curve 21 of the electric field strength, E, versus distance from the central axis 4 to the point A and the point B (see Fig. 3) for the waveguide 10 and a curve 20 of the electric field strength versus distance from the central axis to the point C and the point D (see Fig. 3) for the waveguide 1.

These curves 20, 21 show that for the waveguide 11, the electric field strength at the internal surface of the waveguide 11 adjacent the central axis 4 is approximately 28% less than the electric field strength at the central axis 4 for the waveguide 1. Therefore, the waveguide 11 can transmit powers that are greater (typically, up to approximately 30% greater) than the maximum power that can be transmitted by the waveguide 1 before breakdown and arcing occurs.

Fig. 6 shows a graphical depiction of electric field strength within the waveguide 10. The lighter shading represents a lower electric field strength and the darker shading represents a higher electric field strength. It can be seen from Fig. 6 that by increasing the separation of the internal waveguide surfaces between the opposite sides 12 causes the electric field strength at the internal surfaces in the vicinity of the central axis 4 to be reduced and o be less than at the centre of the waveguide 10 in the vicinity of the longitudinal axis 16.

Figs. 7 to 9 show further examples of waveguides in accordance with the invention.

Fig. 7 shows a waveguide 30 in which two opposite curvilinear longer sidewalls 31 interconnect two shorter opposite parallel linear sidewalls 32. The curvilinear sides 31 each have a single curve that is concave with respect to longitudinal axis 33 and the curve extends from one shorter sidewall 32 to the other shorter sidewall 32. In the waveguide 30, the curvilinear sidewalls 31 do not have any linear sections.

Fig. 8 shows a waveguide 35 having one curvilinear sidewall 31 extending between two shorter opposite parallel linear sidewalls 36. A linear sidewall 37 extends between the shorter side walls 36 opposite the curvilinear sidewall 31.

Fig. 9 shows a waveguide 40 comprising two shorter opposite and parallel linear sidewalls 41. A linear sidewall 42 interconnects the sidewalls 41 on one side and a curvilinear sidewall 43 interconnects the sidewalls 41 on the opposite side. The curvilinear sidewall 43 is proportionally substantially the same as the curvilinear sidewalls 12 of the waveguide 10.

The waveguides 30, 35, 40 all have the advantage over the waveguide 1 of reducing the electric field strength at the internal surfaces of the sidewalls of the longer sides 31, 37, 42, 43 in the vicinity of the central axis 4 by increasing the separation of the longer sides 31, 37, 42, 43 in the vicinity of the central axis 4 relative to the separation of the side walls 31, 37, 42, 43 adjacent the shorter sidewalls 32, 36, 41. respectively. Although the waveguides 30, 35, 40 give improved power handling capabilities compared to conventional rectangular waveguides, such as the waveguide 1, it is expected that the waveguide 10 will provide further improvement.

An advantage of the invention is that it enables radio waves having higher powers to be transmitted using waveguides in accordance with the invention than would be possible with conventional rectangular waveguides.

Claims

CLAIMS1. Apparatus for propagating radio waves, the apparatus comprising a tubular structure having a longitudinal axis and comprising an electrically conducting internal surface; a cross-section of the tubular structure in a plane perpendicular to the longitudinal axis comprising a first pair of opposite internal sidewalls substantially parallel to each other and at least one curvilinear internal sidewall extending between the first pair of opposite internal sidewalls; and the at least one curvilinear sidewall having a length greater than either of the at least two opposite linear sidewalls.
2. Apparatus for propagating radio waves, the apparatus comprising a tubular structure, the tubular structure comprising a cross-section which comprises a first pair of opposite internal sidewalls substantially parallel to each other and a second pair of opposite internal sidewalls extending between ends of the first pair of internal sidewalls; and wherein the second pair of sidewalls are longer than the first pair of sidewalls and the distance between the second pair of opposite internal sidewalls in a direction parallel to the first pair of internal sidewalls at a midpoint of the second pair of internal sidewalls is greater than the length of the first pair of internal sidewalls.
3. Apparatus according to claim 2, wherein at least one of the second pair of internal sidewalls is curvilinear.
4. Apparatus according to any of claim 1 or claim 3, wherein the at least one curvilinear sidewall has symmetry of reflection about an axis perpendicular to the longitudinal axis.
5. Apparatus according to any of claims 1, 3 or 4, wherein the at least one curvilinear internal sidewall comprises a first curved portion.
6. Apparatus according to claim 5, wherein the at least one curvilinear internal sidewall further comprising at least one side portion, the at least one side portion comprising at least one of a linear portion and a second curved portion.
7. Apparatus according to claim 6, wherein the curvilinear sidewall comprises at least two side portions.
8. Apparatus according to claim 7, wherein the first curved portion is located between the at least two side portions.
9. Apparatus according to any of claim 7 or claim 8, wherein the at least two side portions comprise at least one of: (i) two linear portions and (ii) two second curved portions.
10. Apparatus according to claim 9, wherein there are two linear portions and two second curved portions.
11. Apparatus according to claim 10, wherein one linear portion and one second curved portion are located on one side of the first curved portion and the other linear portion and the other curved portion are located on the other side of the first curved portion.
12. Apparatus according to claim 11, wherein the second curved portions are located between the first curved portion and the respective linear portions.
13. Apparatus according to any of claims 5 to 12, wherein the first curved portion is concave with respect to the longitudinal axis.
14. Apparatus according to claim 13 when dependent on any of claims 6 to 12, wherein the at least one second curved portion is convex with respect to the longitudinal axis.
15. Apparatus according to any of claims 1, 3 or 4, wherein the at least one curvilinear sidewall comprises a single curved section which is concave with respect to the longitudinal axis.
16. Apparatus according to claim 15, wherein the single curved section may extend from one of the first pair of internal sidewalls to the other of the first pair of internal sidewalls.
17. Apparatus according to any of claims 1 or 3 to 16, wherein both of the second pair of internal sidewalls are curvilinear.
18. Apparatus according to any of the preceding claims, wherein the first pair of internal sidewalls are linear.
19. Apparatus according to any of the preceding claims, wherein the first pair of internal sidewalls are substantially the same length.
20. Apparatus according to any of the preceding claims, wherein the cross-section of the tubular structure is symmetrical about a 180° rotation around the longitudinal axis.
21. Apparatus according to any of the preceding claims, wherein the tubular structure has a ratio between the separation of the second pair of internal sidewalls and the length of the first pair of internal sidewalls of greater than 1:1 and less than or equal to 2:1.
22. Apparatus according to claim 21, wherein the ratio is in the range of 1.25:1 to 1.75:1.
23. Apparatus according to any of claims 5 to 14, wherein the first curved portion of the at least one curvilinear sidewall is 30% to 80% of the length of the curvilinear side wall.
24. Apparatus according to any of the preceding claims, wherein the tubular structure is an elongate tubular structure.
25. Apparatus according to any of the preceding claims, wherein the tubular structure is a waveguide.