EP0711001A2 - Frequency selective surface devices - Google Patents
Frequency selective surface devices Download PDFInfo
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- EP0711001A2 EP0711001A2 EP95306717A EP95306717A EP0711001A2 EP 0711001 A2 EP0711001 A2 EP 0711001A2 EP 95306717 A EP95306717 A EP 95306717A EP 95306717 A EP95306717 A EP 95306717A EP 0711001 A2 EP0711001 A2 EP 0711001A2
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- frequency selective
- selective surface
- band
- transmission
- resonant
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q15/00—Devices for reflection, refraction, diffraction or polarisation of waves radiated from an antenna, e.g. quasi-optical devices
- H01Q15/0006—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices
- H01Q15/0013—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices said selective devices working as frequency-selective reflecting surfaces, e.g. FSS, dichroic plates, surfaces being partly transmissive and reflective
- H01Q15/0026—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices said selective devices working as frequency-selective reflecting surfaces, e.g. FSS, dichroic plates, surfaces being partly transmissive and reflective said selective devices having a stacked geometry or having multiple layers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q15/00—Devices for reflection, refraction, diffraction or polarisation of waves radiated from an antenna, e.g. quasi-optical devices
- H01Q15/0006—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices
- H01Q15/0013—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices said selective devices working as frequency-selective reflecting surfaces, e.g. FSS, dichroic plates, surfaces being partly transmissive and reflective
- H01Q15/0033—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices said selective devices working as frequency-selective reflecting surfaces, e.g. FSS, dichroic plates, surfaces being partly transmissive and reflective used for beam splitting or combining, e.g. acting as a quasi-optical multiplexer
Definitions
- This invention relates to frequency selective surface devices for separating or combining two channels of electromagnetic radiation.
- Each channel so separated or combined may in turn be sub-divided or sub-combined using another frequency selective surface device of the kind to which the invention relates, or using another type of separator or combiner.
- FIG. 1 One example of a frequency selective surface is shown in Figure 1.
- Incoming energy having spot frequencies f1 and f2 is separated at the frequency selective surface 1 into a reflected beam f2 and a transmitted beam f1.
- the frequency selective surface in Figure 1 separates the two frequencies f1 and f2.
- the device is reciprocal and can be used for combining frequencies f1 and f2 if the directions of incidence are reversed.
- a possible frequency response for such a frequency selective surface 1 is shown in Figure 2.
- the transmission band is defined as the band of frequencies over which in excess of 90% of the incident energy is transmitted
- the reflection band is defined as the band over which in excess of 90% of the incident energy is reflected.
- transmission and reflection bands are referred to in this text as for a 10% percentage loss in energy, it is possible to define the bands for other percentage transmission or reflection losses.
- the transmission band extends from a lower limit T L to an upper limit T U and the reflection band extends from a lower limit R L to an upper limit R U .
- frequency selective surface devices are for increasing channel capacity of reflector antennas, particularly in satellite communications, although also in terrestrial use.
- a single transmit reflector may be fed by two or more feed horns, or a single receive reflector may direct radiation into two or more feed horns.
- the frequency selective surface device transmits a large percentage of the energy incident on it in one frequency band and reflects a large percentage of the energy incident on it in another frequency band, and the physical separation or combination of the beams permits the use of one reflector with two feed horns.
- Each feed horn can then be optimised to the reflector for its particular frequency band.
- the frequency selective surface device may be mounted in a waveguide assembly to filter energy as a waveguide beamsplitter.
- frequency selective surface devices are also used as quasi-optical beamsplitters in multi-band radiometers (devices for detecting radiation, usually low-level and usually natural radiation). They are particularly applicable to high frequencies such as wavelengths in the region of centimetres, millimetres and in the sub-millimetre range and beyond into the infra-red region, but are of course generally applicable across the whole electromagnetic spectrum.
- Frequency selective surfaces may be used singly or in cascade. Each such frequency selective surface has a conductive pattern on a substrate.
- frequency selective surface consists of an array of conductive rings 5 which are printed onto a dielectric substrate 6.
- the individual rings are an integral multiple of the wavelength of the incident radiation in circumference and are therefore resonant, as well as being coupled to each other.
- the result of this is a sharper transition between transmission and reflection bands, as shown in full line in Figure 6. Nevertheless, the ratio between the lower edge of the reflection band and the upper edge of the transmission band is typically 2.5:1 to 3.01:1.
- double resonant elements on the substrate such as 7 or 8. While these are shown in cutaway regions, in practice the entire array would be uniformly made of each of these elements in place of the rings.
- the rings 5 are single resonant in the sense that they can resonate at only one series of related frequencies (which will be harmonically related in the case of normal incidence id assuming that the electrical properties of the dielectric do not vary with frequency, but in which the higher order resonances in particular shift with frequency for inclined angles of incidence on the frequency selective device).
- the double resonant elements have smaller additional sections which are separately resonant.
- the double ring 7 is resonant at integral multiples of the circumference of the outer ring and integral multiples of the circumference of the inner ring (for normal incidence).
- the Maltese cross (also called a Jerusalem cross) 8 is resonant at integral multiples of the length of its dipoles as well as the integral multiples of the length of its endcaps (again, for normal incidence).
- the effect of these additional resonances is to produce an additional reflection band, as shown by the broken line in Figure 6, so that the upper transmission band is pushed closer to the lower transmission band, and this reduces the ratio of the edge of the upper transmission band to the edge of the reflection band to around 1.3:1.
- the device is a high pass device.
- the printed resonant element array of Figure 5 is usually used singly, but proposals have been made to use an array of squares in cascade (R. Cahill, I.M. Sturland, J.W. Bowen, E.A. Parker, and A.C. de Lima, "Frequency selective surfaces for millimetre and sub-millimetre wave quasi optical demultiplexing", Int. J. of Infrared and Millimetre Waves, Vol. 14, No. 9, 1993 pp 1769-1788), and also an array of Jerusalem crosses in cascade (J.A. Arnaud and F.A. Pelow, "Resonant Grid Quasi-Optical Diplexers", Bell System Technical Journal, Feb. 1975 Vol. 54 No. 2 pp 263-283).
- radiometers which are designed to survey emissions over the sub-millimetre band in the earth's upper atmosphere.
- certain species which are of interest to atmospheric chemists emit energy over frequency bands which are very closely spaced, with edge band ratios of 1.03:1 or less.
- Such radiometers are normally fed by a single reflector antenna.
- the invention provides a frequency selective surface device for separating or combining two channels, which comprises at least two frequency selective surfaces, each defining a transmission bid and a reflection band of frequencies, each comprising an array of coupled resonant elements which elements are resonant at only one series of related frequencies, so that the transmission and reflection bands defined are relatively broad, and wherein the spacing of the surfaces is such that multiple reflections between the surfaces results in the reinforcement of these reflections on emergence, whereby the transmission and reflection bands have a relatively sharp transition, permitting combination or separation of closely spaced channels.
- interference effects between the layers to provide reinforcement of the reflections on emergence together with the use of an array of single resonant elements, permits frequency selective surface devices to be constructed which have channels spaced as closely as 1.03:1 ratios between the lower edge of the reflection bid and the upper edge of the transmission bid. While single resonant elements in the form of a square have been used before in cascade, the spacing has not been such as to take advantage of the reinforcement of the reflections on emergence to produce the closely spaced channels.
- the resonant elements are resonant loops, such as rings (not necessarily circular), or squares.
- tripoles consisting of three half-wavelength arms arranged at 120 ⁇ to each other may be used as the resonant elements.
- the array may be of such loops such as rings, squares, or tripoles, wherein the elements are slots in a continuous conductive surface. This would serve to provide a reciprocal of the characteristic provided by the elements themselves.
- Two layers may be used, but preferably three layers are used and, in each case, adjacent layers should be spaced by a maximum separation of one half a wavelength in the medium between the surfaces, so that the emerging waves reinforce on emergence, after taking into account the phase change that will occur on reflection at each array of resonant elements.
- the space-borne radiometer is illustrated in simplified form and is designed to survey emissions over the sub-millimetre band in the earth's upper atmosphere.
- Incoming radiation impinges on the reflector 9, and the radiation is split into transmitted and reflected beams at the frequency selective surface device of the invention 10.
- the frequency selective surface device can also be used to split beams propagating along waveguides, as shown in Figure 8, and the beam incident along the section of waveguide 11 is split into a transmitted frequency band propagating along section 12 and a reflected frequency band propagating along T-junction 13.
- the frequency selective surface device is illustrated in Figures 9 and 10.
- the device consists of rings 14 of conducting material e.g. copper photo-etched onto a dielectric substrate 15.
- rings 14 of conducting material e.g. copper photo-etched onto a dielectric substrate 15.
- the structure is manufactured by producing one screen with rings printed on both sides of the layer of dielectric and the other screen with the rings only on one side, and then sandwiching the two together.
- the laminate may be glass reinforced PTFE such as that sold under the trade name Duroid, a typical thickness is 3.1 mm and typical permittivity 2.33. Typical dimensions for the outside diameter of the ring are 4.5 mm and for the inside diameter 3.6 mm, and a typical spacing is about 6.7 mm. A typical thickness of copper is 10 ⁇ m.
- the spacing of adjacent layers of resonant rings is critical, and is chosen to be a maximum of one half of a wavelength in the substrate in the band for which the device is designed, typically a maximum of one half of the wavelength of the frequency at the upper edge of the transmission band. This typical value has been found to be a good compromise. Obviously, the reinforcement will be less than total for other wavelengths and differing angles of incidence, where the path length of the multiple reflections will be different. Referring to Figure 10a, the spacing is such that radiation incident on the top surface of the device and reflected back and forth between the first and second layer of rings, emerges from the second layer of rings in phase and therefore reinforces itself. Figure 10a does not show the second layer of dielectric and third layer of rings.
- ray b has undergone a phase change firstly at the lower layer of resonant rings 14 and secondly at the upper layer of resonant rings 14, before it emerges.
- the spacing between the layers is such that ray b emerges exactly one whole wavelength behind ray a.
- Ray c is a whole wavelength behind ray b.
- the thickness of substrate 15 must be less than one half of a wavelength in the substrate.
- the invention is applicable to any integral number of wavelengths between rays a, b, c but one wavelength difference is preferred.
- the reinforcement on emergence of course applies after the second layer 15 and rings 14 have been traversed.
- the thickness of the dielectric is not exactly one half of one wavelength of the radiation in the dielectric, as explained, because a phase change occurs on reflection at each layer of rings. This is because, on reflection, currents are induced in the rings, and the induced currents then re-radiate energy. The re-radiated energy is generally not in phase with the incoming energy which generated the currents.
- the phase difference between each successive multiple of reflection is one wavelength when these phase lags have been taken into account.
- a typical actual thickness may be one quarter of a wavelength of the radiation in the substrate taking into account effects of angle of incidence and reflection phase effects.
- the device may be manufactured by photolithographic etching of the pattern onto a thin conducting layer on both sides of a wafer and on a single side of a second wafer, so that the substrates may then be mated together and permanently fixed by applying a thin bonding layer between one of the conducting arrays and the blank face of the second substrate.
- the rings could also be printed using other techniques such as laser cutting or ion milling to remove the unwanted conducting film.
- the use of resonant elements permits design freedom in that the resonant frequency depends on the diameter of the ring, while the spacing can be varied independently.
- the geometry can be designed using a rigorous Floquet modal analysis program. This is described for example in "Rings As Elements For Frequency Selective Surfaces" by E.A. Parker and S.M.A. Hamdy, Electron. Lett vol. 17 no. 17 pp 612-614.
- FIG. 14 illustrates the trade-off between roll-off rate and transmission and reflection band widths for T.E. 15 ⁇ incidence.
- the widths of the transmission and reflection bands are defined as the frequencies at which the filter loss is less than 10% (-0.5dB).
- the percentage band width is defined over the range of frequencies where the loss does not exceed 10% i.e. (F U - F L )/F C x 100% .
- the resonant elements are illustrated as circular rings, but they could be rings of non-circular form such as squares or loops of any shape. Instead, they could be tripoles. As has been stated before the invention is also applicable to a double (as well as triple) layer of resonant elements.
- the invention is also applicable to the conducting surfaces forming the rings etc being replaced by slots in a conducting layer.
- a layer e.g. of resonant ring-shaped slots would give an inverse response to that of the respective conducting ring-shaped structure.
- the lower transmission band would be a reflection band
- the reflection band would be a transmission band
- the device would be high pass instead of low pass.
- the multiple internal reflections would be reinforced on emergence from the upper surface, instead of on being reinforced on emergence from the lower surface.
- the invention is applicable to radiometers for terrestrial use, and over any frequency in the electromagnetic spectrum, with or without a reflector antenna, and whether the frequency selective surface device is used in free space as in Figure 7, or is mounted in a waveguide as in Figure 8.
- the invention is also applicable to radio receivers whether used for space-borne or terrestrial applications, whether employing a waveguide or not, whether employing a reflector or not.
- the invention is applicable to the Cassegrain principle, where the feed horn which extends through the reflector antenna will reflect from the back of a convex frequency selective surface, and a feed horn at the focus of the antenna will transmit through the frequency selective surface, so that both frequency channels are combined in the output of the antenna or to dual offset reflector antennas.
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Abstract
Description
- This invention relates to frequency selective surface devices for separating or combining two channels of electromagnetic radiation.
- Each channel so separated or combined may in turn be sub-divided or sub-combined using another frequency selective surface device of the kind to which the invention relates, or using another type of separator or combiner.
- One example of a frequency selective surface is shown in Figure 1. Incoming energy having spot frequencies f₁ and f₂ is separated at the frequency
selective surface 1 into a reflected beam f₂ and a transmitted beam f₁. As shown, the frequency selective surface in Figure 1 separates the two frequencies f₁ and f₂. However, the device is reciprocal and can be used for combining frequencies f₁ and f₂ if the directions of incidence are reversed. A possible frequency response for such a frequencyselective surface 1 is shown in Figure 2. The transmission band is defined as the band of frequencies over which in excess of 90% of the incident energy is transmitted, and the reflection band is defined as the band over which in excess of 90% of the incident energy is reflected. While transmission and reflection bands are referred to in this text as for a 10% percentage loss in energy, it is possible to define the bands for other percentage transmission or reflection losses. In Figure 2, the transmission band extends from a lower limit TL to an upper limit TU and the reflection band extends from a lower limit RL to an upper limit RU. - One use of such frequency selective surface devices is for increasing channel capacity of reflector antennas, particularly in satellite communications, although also in terrestrial use. A single transmit reflector may be fed by two or more feed horns, or a single receive reflector may direct radiation into two or more feed horns. The frequency selective surface device transmits a large percentage of the energy incident on it in one frequency band and reflects a large percentage of the energy incident on it in another frequency band, and the physical separation or combination of the beams permits the use of one reflector with two feed horns. Each feed horn can then be optimised to the reflector for its particular frequency band. The frequency selective surface device may be mounted in a waveguide assembly to filter energy as a waveguide beamsplitter. However, such frequency selective surface devices are also used as quasi-optical beamsplitters in multi-band radiometers (devices for detecting radiation, usually low-level and usually natural radiation). They are particularly applicable to high frequencies such as wavelengths in the region of centimetres, millimetres and in the sub-millimetre range and beyond into the infra-red region, but are of course generally applicable across the whole electromagnetic spectrum.
- Frequency selective surfaces may be used singly or in cascade. Each such frequency selective surface has a conductive pattern on a substrate.
- One such pattern is a lattice grid. In one proposal (US-A-4 476 471), a three layer lattice grid has been proposed, the three
layers 2, 3, 4 (Figure 3) being used so that interactions between the layers generate a broad transmission band (Figure 4). Unlike the surface whose frequency response is illustrated in Figure 2, which is a low pass arrangement, the lattice grid provides a high pass response. The response of a single layer is shown by the dotted line and the full line shows the effect of the three layers together. Even after the sharpening effect of the three layers, the ratio between the lower edge of the transmission bid and the upper edge of the reflection band is still around 1:1.2. - Another proposed form of frequency selective surface consists of an array of
conductive rings 5 which are printed onto a dielectric substrate 6. (E.A. Parker and S.M.A. Hamdy, "Rings as elements for frequency selective surfaces", Electron. Lett., Vol. 17, No. 17, 1981, pp 612-614). The individual rings are an integral multiple of the wavelength of the incident radiation in circumference and are therefore resonant, as well as being coupled to each other. The result of this is a sharper transition between transmission and reflection bands, as shown in full line in Figure 6. Nevertheless, the ratio between the lower edge of the reflection band and the upper edge of the transmission band is typically 2.5:1 to 3.01:1. - It has also been proposed to use "double resonant" elements on the substrate such as 7 or 8. While these are shown in cutaway regions, in practice the entire array would be uniformly made of each of these elements in place of the rings. The
rings 5 are single resonant in the sense that they can resonate at only one series of related frequencies (which will be harmonically related in the case of normal incidence id assuming that the electrical properties of the dielectric do not vary with frequency, but in which the higher order resonances in particular shift with frequency for inclined angles of incidence on the frequency selective device). The double resonant elements have smaller additional sections which are separately resonant. Thus, thedouble ring 7 is resonant at integral multiples of the circumference of the outer ring and integral multiples of the circumference of the inner ring (for normal incidence). The Maltese cross (also called a Jerusalem cross) 8 is resonant at integral multiples of the length of its dipoles as well as the integral multiples of the length of its endcaps (again, for normal incidence). The effect of these additional resonances is to produce an additional reflection band, as shown by the broken line in Figure 6, so that the upper transmission band is pushed closer to the lower transmission band, and this reduces the ratio of the edge of the upper transmission band to the edge of the reflection band to around 1.3:1. The device is a high pass device. The printed resonant element array of Figure 5 is usually used singly, but proposals have been made to use an array of squares in cascade (R. Cahill, I.M. Sturland, J.W. Bowen, E.A. Parker, and A.C. de Lima, "Frequency selective surfaces for millimetre and sub-millimetre wave quasi optical demultiplexing", Int. J. of Infrared and Millimetre Waves, Vol. 14, No. 9, 1993 pp 1769-1788), and also an array of Jerusalem crosses in cascade (J.A. Arnaud and F.A. Pelow, "Resonant Grid Quasi-Optical Diplexers", Bell System Technical Journal, Feb. 1975 Vol. 54 No. 2 pp 263-283). - However, recently more stringent filtering requirements have been defined with the development of space-borne radiometers which are designed to survey emissions over the sub-millimetre band in the earth's upper atmosphere. Here certain species which are of interest to atmospheric chemists emit energy over frequency bands which are very closely spaced, with edge band ratios of 1.03:1 or less. Such radiometers are normally fed by a single reflector antenna.
- The invention provides a frequency selective surface device for separating or combining two channels, which comprises at least two frequency selective surfaces, each defining a transmission bid and a reflection band of frequencies, each comprising an array of coupled resonant elements which elements are resonant at only one series of related frequencies, so that the transmission and reflection bands defined are relatively broad, and wherein the spacing of the surfaces is such that multiple reflections between the surfaces results in the reinforcement of these reflections on emergence, whereby the transmission and reflection bands have a relatively sharp transition, permitting combination or separation of closely spaced channels.
- The use of interference effects between the layers to provide reinforcement of the reflections on emergence, together with the use of an array of single resonant elements, permits frequency selective surface devices to be constructed which have channels spaced as closely as 1.03:1 ratios between the lower edge of the reflection bid and the upper edge of the transmission bid. While single resonant elements in the form of a square have been used before in cascade, the spacing has not been such as to take advantage of the reinforcement of the reflections on emergence to produce the closely spaced channels.
- Advantageously the resonant elements are resonant loops, such as rings (not necessarily circular), or squares. Instead, however, tripoles consisting of three half-wavelength arms arranged at 120· to each other may be used as the resonant elements. Alternatively, the array may be of such loops such as rings, squares, or tripoles, wherein the elements are slots in a continuous conductive surface. This would serve to provide a reciprocal of the characteristic provided by the elements themselves.
- Two layers may be used, but preferably three layers are used and, in each case, adjacent layers should be spaced by a maximum separation of one half a wavelength in the medium between the surfaces, so that the emerging waves reinforce on emergence, after taking into account the phase change that will occur on reflection at each array of resonant elements.
- A frequency selective surface device constructed in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
- Figure 7 is a ray diagram of part of a spacecraft radiometer which uses the frequency selective surface device;
- Figure 8 is a plan view of a T-junction of a waveguide in which the frequency selective surface device is fitted as a beam splitter;
- Figure 9 is a plan view of the frequency selective surface device;
- Figure 10 is a side view of the frequency selective surface device, partly exploded for clarity;
- Figure 10a is an enlarged fragmentary view of the top two layers of rings only with the
substrate 15 sandwiched between; - Figures 11a to 11c illustrate the transmission response of the frequency selective surface device shown in Figures 9 and 10 (Figure 11c) and, for comparison, the frequency response of a two layer frequency selective surface device (Figure 11b), and that of a single layer frequency selective surface device (Figure 11a), all being illuminated in the T.E. mode at 15· incidence;
- Figures 12a to 12c illustrate the transmission response of the frequency selective surface device shown in Figures 9 and 10 when illuminated in the T.E. mode at angles of incidence of 0·, 15· and 45·, respectively;
- Figures 13a to 13c illustrate the transmission response of the frequency selective surface device shown in Figures 9 and 10 when illuminated in the T.M. mode at angles of incidence of 0·, 15· and 45·, respectively; and
- Figure 14 illustrates the relation between the band edge spacing and the bandwidth for the transmitted and reflected beams incident at 15· T.E. on the frequency selective surface device shown in Figures 9 and 10.
- Referring to Figure 7, the space-borne radiometer is illustrated in simplified form and is designed to survey emissions over the sub-millimetre band in the earth's upper atmosphere. Incoming radiation impinges on the
reflector 9, and the radiation is split into transmitted and reflected beams at the frequency selective surface device of theinvention 10. The frequency selective surface device can also be used to split beams propagating along waveguides, as shown in Figure 8, and the beam incident along the section ofwaveguide 11 is split into a transmitted frequency band propagating alongsection 12 and a reflected frequency band propagating along T-junction 13. - The frequency selective surface device is illustrated in Figures 9 and 10.
- The device consists of
rings 14 of conducting material e.g. copper photo-etched onto adielectric substrate 15. There are three layers of rings and two substrates, and the structure is manufactured by producing one screen with rings printed on both sides of the layer of dielectric and the other screen with the rings only on one side, and then sandwiching the two together. - Suitable dimensions and materials for the structure are as follows. The laminate may be glass reinforced PTFE such as that sold under the trade name Duroid, a typical thickness is 3.1 mm and typical permittivity 2.33. Typical dimensions for the outside diameter of the ring are 4.5 mm and for the inside diameter 3.6 mm, and a typical spacing is about 6.7 mm. A typical thickness of copper is 10 µm. Such a structure has been found suitable for radiation of the
frequency range 8 to 26 GHz. For operation in the range 300-400 GHz, the substrate could be fused silica (·=3.78), the conducting film thickness could be 2 µm, the substrate typical thickness could be 100 µm, the mean diameter of the rings could be 150 µm with a periodicity of 300 µm. - The spacing of adjacent layers of resonant rings is critical, and is chosen to be a maximum of one half of a wavelength in the substrate in the band for which the device is designed, typically a maximum of one half of the wavelength of the frequency at the upper edge of the transmission band. This typical value has been found to be a good compromise. Obviously, the reinforcement will be less than total for other wavelengths and differing angles of incidence, where the path length of the multiple reflections will be different. Referring to Figure 10a, the spacing is such that radiation incident on the top surface of the device and reflected back and forth between the first and second layer of rings, emerges from the second layer of rings in phase and therefore reinforces itself. Figure 10a does not show the second layer of dielectric and third layer of rings. Thus, for example, ray b has undergone a phase change firstly at the lower layer of
resonant rings 14 and secondly at the upper layer ofresonant rings 14, before it emerges. The spacing between the layers is such that ray b emerges exactly one whole wavelength behind ray a. Ray c is a whole wavelength behind ray b. Thus the thickness ofsubstrate 15 must be less than one half of a wavelength in the substrate. The invention is applicable to any integral number of wavelengths between rays a, b, c but one wavelength difference is preferred. The reinforcement on emergence of course applies after thesecond layer 15 and rings 14 have been traversed. - This technique is the well known Fabry-Perot etalon effect and, referring to Figure 11, it will be seen that the effect of the reinforcement of the emerging waves is to widen the reflection band from what it would have been had a single layer of rings only been provided as in Figure 11a. The dimensions are chosen so that the transmission band generated by the multiple reflections is at the upper edge of the lower transmission band, and therefore has the effect of increasing the roll-off at the transition (Figure 11b), as well as widening the reflection band. The centre frequency of the reflection band in Figure 11a is determined mainly by the mean diameter of the rings or more generally the physical size of the resonant elements.
- It should be mentioned that a two layer form of the frequency selective surface device, that is, as in Figures 9 and 10 but without the lower
dielectric layer 15 and the lowest array ofrings 14, is also within the scope of the invention. The addition of thesecond dielectric layer 15 and the third layer ofrings 14 has the effect, as can be seen from Figure 11c, of widening the lower transmission band and increasing still further the sharpness of the transition between the transmission and reflection bands. - The thickness of the dielectric is not exactly one half of one wavelength of the radiation in the dielectric, as explained, because a phase change occurs on reflection at each layer of rings. This is because, on reflection, currents are induced in the rings, and the induced currents then re-radiate energy. The re-radiated energy is generally not in phase with the incoming energy which generated the currents. The phase difference between each successive multiple of reflection is one wavelength when these phase lags have been taken into account. A typical actual thickness may be one quarter of a wavelength of the radiation in the substrate taking into account effects of angle of incidence and reflection phase effects.
- It should be added that the performance curves of Figures 11 are for illumination in the T.E. plane at 15· incidence. It will be noted that the centre resonant frequency of the single layer structure of Figure 11a remains almost unchanged with the addition of the second layer as in Figure 11b but the reflection band width increases substantially, and the ratio between the lower edge of the reflection band and the upper edge of the transmission band, both for 10% loss of energy, is 1.16:1. The addition of the third screen reduces the band spacing further to 1.07:1 while broadening the pass band width.
- The device may be manufactured by photolithographic etching of the pattern onto a thin conducting layer on both sides of a wafer and on a single side of a second wafer, so that the substrates may then be mated together and permanently fixed by applying a thin bonding layer between one of the conducting arrays and the blank face of the second substrate. The rings could also be printed using other techniques such as laser cutting or ion milling to remove the unwanted conducting film. The use of resonant elements permits design freedom in that the resonant frequency depends on the diameter of the ring, while the spacing can be varied independently. The geometry can be designed using a rigorous Floquet modal analysis program. This is described for example in "Rings As Elements For Frequency Selective Surfaces" by E.A. Parker and S.M.A. Hamdy, Electron. Lett vol. 17 no. 17 pp 612-614.
- The transmission response of the device of Figures 9 and 10 for different angles of incidence, for orthogonal T.E. and T.M. planes is illustrated in Figures 12 and 13. The performance of the invention is thus reasonably insensitive to the angle and plane of incidence. Figure 14 illustrates the trade-off between roll-off rate and transmission and reflection band widths for T.E. 15· incidence. The widths of the transmission and reflection bands are defined as the frequencies at which the filter loss is less than 10% (-0.5dB). Similarly the percentage band width is defined over the range of frequencies where the loss does not exceed 10% i.e.
- Of course variations may be made without departing from the scope of the invention. Thus, the resonant elements are illustrated as circular rings, but they could be rings of non-circular form such as squares or loops of any shape. Instead, they could be tripoles. As has been stated before the invention is also applicable to a double (as well as triple) layer of resonant elements.
- The invention is also applicable to the conducting surfaces forming the rings etc being replaced by slots in a conducting layer. Such a layer e.g. of resonant ring-shaped slots would give an inverse response to that of the respective conducting ring-shaped structure. For example, in Figures 11-13, the lower transmission band would be a reflection band, and the reflection band would be a transmission band, and the device would be high pass instead of low pass. In Figure 10a, the multiple internal reflections would be reinforced on emergence from the upper surface, instead of on being reinforced on emergence from the lower surface.
- The invention is applicable to radiometers for terrestrial use, and over any frequency in the electromagnetic spectrum, with or without a reflector antenna, and whether the frequency selective surface device is used in free space as in Figure 7, or is mounted in a waveguide as in Figure 8. The invention is also applicable to radio receivers whether used for space-borne or terrestrial applications, whether employing a waveguide or not, whether employing a reflector or not.
- Among alternative configurations for a reflector antenna, the invention is applicable to the Cassegrain principle, where the feed horn which extends through the reflector antenna will reflect from the back of a convex frequency selective surface, and a feed horn at the focus of the antenna will transmit through the frequency selective surface, so that both frequency channels are combined in the output of the antenna or to dual offset reflector antennas.
Claims (5)
- A frequency selective surface device for separating or combining two channels, which comprises at least two frequency selective surfaces, each defining a transmission band and a reflection band of frequencies, each comprising an array of coupled resonant elements which elements are resonant at only one series of related frequencies, so that the transmission and reflection bands defined are relatively broad, and wherein the spacing of the surfaces is such that multiple reflections between the surfaces results in the reinforcement of these reflections on emergence, whereby the transmission and reflection bands have a relatively sharp transition, permitting combination or separation of closely spaced channels.
- A frequency selective surface device as claimed in claim 1, in which the resonant elements are resonant loops.
- A frequency selective surface device as claimed in claim 1 or claim 2, in which the maximum separation between the frequency selective surfaces is one half of one wavelength in the medium between the surfaces in the range for which the device is operative.
- A frequency selective surface device as claimed in any one of claims 1 to 3, in which there are three frequency selective surfaces.
- A radiometer incorporating a frequency selective surface device as claimed in any one of the preceding claims.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB9422278 | 1994-11-04 | ||
GB9422278A GB2294813B (en) | 1994-11-04 | 1994-11-04 | Frequency selective surface devices |
Publications (3)
Publication Number | Publication Date |
---|---|
EP0711001A2 true EP0711001A2 (en) | 1996-05-08 |
EP0711001A3 EP0711001A3 (en) | 1997-07-16 |
EP0711001B1 EP0711001B1 (en) | 2001-12-12 |
Family
ID=10763902
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP95306717A Expired - Lifetime EP0711001B1 (en) | 1994-11-04 | 1995-09-22 | Frequency selective surface devices |
Country Status (3)
Country | Link |
---|---|
EP (1) | EP0711001B1 (en) |
DE (1) | DE69524546T2 (en) |
GB (1) | GB2294813B (en) |
Cited By (5)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2763177A1 (en) * | 1997-04-29 | 1998-11-13 | Trw Inc | FILTER EMPLOYING A FREQUENCY SELECTIVITY SURFACE AND ANTENNA USING THE SAME |
CN102760964A (en) * | 2012-07-03 | 2012-10-31 | 深圳光启创新技术有限公司 | Wave-transmitting meta-material and antenna cover and antenna system with material |
CN107852813A (en) * | 2015-07-30 | 2018-03-27 | 莱尔德技术股份有限公司 | The frequency-selective structure mitigated for EMI |
CN109560374A (en) * | 2018-11-22 | 2019-04-02 | 西安电子科技大学 | A kind of Fabry-Perot antenna of the low radar cross section of high-gain |
WO2021197277A1 (en) * | 2020-03-30 | 2021-10-07 | Telefonaktiebolaget Lm Ericsson (Publ) | Au and ru having cwg filters, and bs having the au or ru |
Families Citing this family (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2378820A (en) | 2001-08-17 | 2003-02-19 | Anafa Electromagnetic Solution | Electromagnetic filter |
GB2415093A (en) * | 2004-06-07 | 2005-12-14 | Qinetiq Nanomaterials Ltd | Method of producing composite materials |
US7794629B2 (en) | 2003-11-25 | 2010-09-14 | Qinetiq Limited | Composite materials |
GB2439975B (en) * | 2006-07-07 | 2010-02-24 | Iti Scotland Ltd | Antenna arrangement |
Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
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US4125841A (en) * | 1977-05-17 | 1978-11-14 | Ohio State University Research Foundation | Space filter |
Family Cites Families (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4476471A (en) * | 1981-02-09 | 1984-10-09 | Nippon Electric Co., Ltd. | Antenna apparatus including frequency separator having wide band transmission or reflection characteristics |
EP0096529A1 (en) * | 1982-06-01 | 1983-12-21 | Kent Scientific and Industrial Projects Limited | Dichroic plate |
-
1994
- 1994-11-04 GB GB9422278A patent/GB2294813B/en not_active Expired - Fee Related
-
1995
- 1995-09-22 DE DE1995624546 patent/DE69524546T2/en not_active Expired - Lifetime
- 1995-09-22 EP EP95306717A patent/EP0711001B1/en not_active Expired - Lifetime
Patent Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4125841A (en) * | 1977-05-17 | 1978-11-14 | Ohio State University Research Foundation | Space filter |
Non-Patent Citations (4)
Title |
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BELL SYSTEM TECHNICAL JOURNAL, vol. 54, no. 2, February 1975, USA, pages 263-283, XP002030965 ARNAUD, J.A. ET AL: "Resonant-Grid Quasi-Optical Diplexers" * |
CAHILL ET AL: 'Frequency selective surfaces for millimetre and submillimetre wave quasi optical demultiplexing' INTERNATIONAL JOURNAL OF INFRARED AND MILLIMETER WAVES vol. 14, no. 9, September 1993, NEW YORK US, pages 1769 - 1700 * |
ONDE ELECTRIQUE, vol. 71, no. 5, 1 September 1991, pages 54-61, XP000263123 ABBA P ET AL: "CONCEPTION ET REALISATION D'UN RADIOMETRE EN ONDES MILLIMETRIQUES POUR LE SONDAGE VERTICAL DE L'ATMOSPHERE" * |
WANNIER ET AL: 'Quasioptical band-rejection filter at 100 Ghz' REVIEW OF SCIENTIFIC INSTRUMENTS vol. 47, no. 1, January 1976, pages 56 - 58 * |
Cited By (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2763177A1 (en) * | 1997-04-29 | 1998-11-13 | Trw Inc | FILTER EMPLOYING A FREQUENCY SELECTIVITY SURFACE AND ANTENNA USING THE SAME |
CN102760964A (en) * | 2012-07-03 | 2012-10-31 | 深圳光启创新技术有限公司 | Wave-transmitting meta-material and antenna cover and antenna system with material |
CN107852813A (en) * | 2015-07-30 | 2018-03-27 | 莱尔德技术股份有限公司 | The frequency-selective structure mitigated for EMI |
EP3329750A4 (en) * | 2015-07-30 | 2018-08-22 | Laird Technologies, Inc. | Frequency selective structures for emi mitigation |
CN109560374A (en) * | 2018-11-22 | 2019-04-02 | 西安电子科技大学 | A kind of Fabry-Perot antenna of the low radar cross section of high-gain |
CN109560374B (en) * | 2018-11-22 | 2021-01-05 | 西安电子科技大学 | High-gain low-radar-section Fabry-Perot antenna |
WO2021197277A1 (en) * | 2020-03-30 | 2021-10-07 | Telefonaktiebolaget Lm Ericsson (Publ) | Au and ru having cwg filters, and bs having the au or ru |
Also Published As
Publication number | Publication date |
---|---|
GB9422278D0 (en) | 1994-12-21 |
EP0711001A3 (en) | 1997-07-16 |
GB2294813A (en) | 1996-05-08 |
EP0711001B1 (en) | 2001-12-12 |
DE69524546D1 (en) | 2002-01-24 |
GB2294813B (en) | 1998-04-15 |
DE69524546T2 (en) | 2002-05-08 |
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