Patent Description:
Point-to-point radio communication may use a parabolic reflector to create a focused beam of electromagnetic radiation. It is well understood that if a source of electromagnetic radiation is placed at a focal point of the parabolic reflector, then the parabolic reflector will create a beam of parallel rays of electromagnetic radiation.

Such an antenna can provide a high bandwidth as it can be operated simultaneously over many different frequency bands. However, it is bulky because of the distance of the focal point from the parabolic reflector and the size of the parabolic reflector.

It would be desirable to produce an antenna that is less bulky and operates over multiple frequency bands simultaneously.

Technical paper "Some studies of quasi-planar antennas", (XP055718844) discloses a dual frequency reflectarray comprising a repeating pattern of elements, a layout of which is Fig <NUM> (a).

<CIT> discloses a Cassegrain aerial comprising an auxiliary reflector which transmits of reflects according to the polarisation of energy incident on it, and a main, twist reflector which receives energy reflected from the auxiliary reflector, rotates it through <NUM>° and re-reflects into free-space through the intervening auxiliary reflector.

<CIT> discloses a reflective antenna. The antenna comprises a feed array, a secondary reflective surface and a primary reflective array. The feed array can emit an electromagnetic wave in a first polarization direction. The secondary reflective surface is used for reflecting the electromagnetic wave, emitted by the feed array, in the first polarization direction, and can make an electromagnetic wave in a second polarization direction. The primary reflective array is used for converting the electromagnetic wave, reflected by the secondary reflective surface, in the first polarization direction into the electromagnetic wave in the second polarization direction, and reflecting the same.

<CIT> discloses a radar, comprising a trans-reflector and a twist-reflector.

The invention is set out in the independent claim. According to the claimed invention, there is provided a multi- frequency folded lens antenna structure comprising: a stack comprising:.

In some, but not necessarily all examples, the multi-frequency twist-reflector is configured to selectively change the polarization for at least the first frequency band and for at least the second frequency band, non-contiguous to the first frequency band and is configured to not selectively change the polarization for at least a third frequency band between the first frequency band and the second frequency band.

In some, but not necessarily all examples, the multi-frequency twist-reflector is configured to have a multi-resonant impedance comprising a resonance at the first frequency band and a resonance at the second frequency band.

In some, but not necessarily all examples, the multi-frequency twist-reflector is configured to have a multi-resonant impedance that is non-resonant at a third frequency band between the first frequency band and the second frequency band, wherein the multi-frequency twist-reflector reflects electromagnetic radiation having a second polarization and a frequency within the first frequency band or the second frequency band as electromagnetic radiation having a first polarization in the same respective frequency bands and does not reflect electromagnetic radiation having a second polarization within the third frequency band as electromagnetic radiation having the first polarization.

In some, but not necessarily all examples, the multi-frequency twist-reflector comprises a periodic conductive surface that provides frequency selectivity, a dielectric layer and a reflective surface.

In some, but not necessarily all examples, a thickness of the dielectric layer of the multi-frequency twist-reflector is dependent upon both the first frequency band and the second frequency band.

In some, but not necessarily all examples, the multi-frequency twist-reflector comprises repeated parallel LC circuits each LC circuit providing a separate resonance.

In some, but not necessarily all examples, the multi-frequency twist-reflector comprises parallel, equally-spaced, discontinuous conductive strips, wherein conductive strip portions are separated in a first direction, parallel to the conductive strips, by first gaps and are separated in a second direction, orthogonal to the first direction, by second gaps.

In some, but not necessarily all examples, the first gaps have a constant size and wherein the second gaps have a constant size, the size of the first gaps being less than a size of the second gaps.

In some, but not necessarily all examples, the polarization-dependent trans-reflector is configured to have a single resonance impedance, wherein the first frequency band and the second frequency band are harmonic frequencies defined by the single resonance.

In some, but not necessarily all examples, the polarization-dependent trans-reflector comprises a polarization-selective reflective surface and a layer of dielectric, wherein the thickness of the dielectric depends on both the first frequency band and the second frequency band.

According to the claimed invention, the polarization-dependent trans-reflector comprises conductive strips on a dielectric, wherein a thickness of the dielectric is dependent upon both the first frequency band and the second frequency band such that the thickness of the dielectric corresponds to a first multiple number of half wavelengths for a resonant frequency of the first resonant frequency band and a multiple number of half wavelengths for a resonant frequency of the second frequency band.

In some, but not necessarily all examples, the multi-frequency folded lens antenna structure comprises a waveguide feed in the multi-frequency twist-reflector configured to provide electromagnetic radiation having the second polarization and having a frequency bandwidth covering at least the first frequency band and the second frequency band.

In some, but not necessarily all examples, the waveguide feed is configured to provide at one or more frequencies between <NUM> and <NUM> which lies within the second frequency band and at frequencies substantially one half of <NUM> to <NUM> which lie within the first frequency band.

In some, but not necessarily all examples, the multi-frequency folded lens antenna structure comprises a lens configured to receive electromagnetic radiation of the first polarization transmitted by the polarization-dependent trans-reflector.

In some, but not necessarily all examples, the lens is a Fresnel zone plate lens.

According to various, but not necessarily all, embodiments there is provided a base station comprising a backhaul radio frequency transceiver system comprising the multi-frequency folded lens antenna structure.

According to the claimed invention there is provided a
polarization-dependent trans-reflector comprising: parallel strips of conductor on a surface of a dielectric, wherein a thickness of the dielectric is dependent upon both the first frequency band and the second frequency band such that a thickness of the dielectric corresponds to a first multiple number of half wavelengths for a resonant frequency of the first resonant frequency band and a multiple number of half wavelengths for a resonant frequency of the second frequency band.

In some, but not necessarily all examples, a thickness of the dielectric corresponds to a wavelength for a resonant frequency of the second frequency band.

According to various, but not necessarily all, embodiments there is provided a multi-frequency twist-reflector comprising.

In some, but not necessarily all examples, the discontinuous conductive strips are configured to have a multi-resonant electrical impedance that is resonant at the first frequency band and at the second frequency band but not at the third frequency band, the third frequency band being between the first frequency band and the second frequency band,
wherein a thickness of the dielectric layer substantially corresponds to a whole number of quarter wavelengths of a resonant frequency of the first frequency band and a whole number of quarter wavelengths of a resonant frequency the second frequency band.

<FIG> illustrates an example of a multi-frequency folded lens antenna structure <NUM>. The multi-frequency folded lens antenna structure <NUM> comprises a stack <NUM> comprising: a polarization-dependent trans-reflector <NUM>, a dielectric gap <NUM> and a multi-frequency twist-reflector <NUM>.

The structure <NUM> is folded in that electromagnetic radiation <NUM> takes a zig-zag path through the stack <NUM> before it emerges from the stack <NUM>. The electromagnetic radiation <NUM> that emerges from the stack <NUM> has been reflected by the trans-reflector <NUM> and also by the twist-reflector <NUM>. The path length for the electromagnetic radiation <NUM> through the stack <NUM> is therefore significantly greater than the thickness of the stack <NUM> because of the two reflections. This means that a lens <NUM> may be placed adjacent the stack <NUM> that has a focal length F significantly greater than the height H of the stack <NUM> but substantially equal to the zig-zag path length L of the electromagnetic radiation <NUM> through the stack <NUM>, where F=L≈<NUM>. The multi-frequency folded lens antenna structure <NUM> is therefore a compact arrangement that enables the use of a lens that has a focal length greater than the height of the stack <NUM>.

The polarization-dependent trans-reflector <NUM> is configured to transmit electromagnetic radiation of a first polarization P1 incident, from within the stack <NUM>, out of the stack <NUM> and to reflect electromagnetic radiation of a second polarization P2 incident, within the stack <NUM>, towards the multi-frequency twist-reflector <NUM>.

The multi-frequency twist-reflector <NUM> is configured to selectively change the polarization of the reflected electromagnetic radiation, provided by the trans-reflector <NUM>, from the second polarization P2 to substantially the first polarization P1 and to direct the electromagnetic radiation of substantially the first polarization P1, within the stack <NUM>, towards the polarization-dependent trans-reflector <NUM> for at least partial transmission out of the stack <NUM>.

The multi-frequency twist-reflector <NUM> is configured to selectively change the polarization for at least a first frequency band F1 and for at least a second frequency band F2, non-contiguous to the first frequency band F1. The multi-frequency twist-reflector <NUM> is also configured to not change the polarization for at least a third frequency band F3 between the first frequency band F1 and the second frequency band F2.

The multi-frequency folded lens antenna structure <NUM> may comprise, within the multi-frequency twist-reflector <NUM>, an aperture <NUM> for receiving electromagnetic radiation <NUM> from a source <NUM>. The source <NUM> may, for example, be a waveguide feed <NUM> or another feed such as a printed microstrip based feed such as, for example, an Aperture Coupled Microstrip Patch antenna. The source <NUM> have a wide bandwidth that covers at least separated frequency bands F1 and F2 or may be a multi-frequency feed for frequency band F1 and for frequency band F2.

In this example, but not necessarily all example the dielectric <NUM> is a single layer dielectric substrate that has an upper and a lower surface or a single layer material. The dielectric <NUM> may be a solid, liquid or gas. It may for example be air. In this example, the upper surface is directly adjacent the polarization-dependent trans-reflector <NUM> and the lower surface is directly adjacent the multi-frequency twist-reflector <NUM>.

In this example, but not necessarily all examples, the waveguide feed <NUM> is configured to only provide electromagnetic radiation having the second polarization.

The bandwidth of the electromagnetic radiation <NUM> provided by the waveguide feed <NUM> has a bandwidth that covers at least some or all of the first frequency band F1 and some or all of the second frequency band F2.

The electromagnetic radiation <NUM> of the second polarization P2 provided by the source <NUM> is reflected by the polarization-dependent trans-reflector <NUM> towards the multi-frequency twist-reflector <NUM>. The reflected electromagnetic radiation <NUM> of the second polarization P2, that lies within the first frequency band F1 and the second frequency band F2, is reflected by the multi-frequency twist-reflector <NUM> as frequency limited electromagnetic radiation <NUM> of (substantially) the second polarization P2. The frequency-limited electromagnetic radiation <NUM> of (substantially) the second polarization P2 is substantially transmitted by the polarization-dependent trans-reflector <NUM>.

The first polarization P1 and the second polarization P2 are orthogonal linear polarizations, in this example.

The second frequency band F2 may lie within a desired communication band such as the V band for backhaul communication in a telecommunication system. The V band has a frequency range between <NUM> and <NUM>. The first frequency band may, for example, lie at a sub-harmonic of the second frequency band for example in the range <NUM> to <NUM>. In one particular example, the second frequency band F2 includes the frequency <NUM> and the first frequency band F1 includes the frequency <NUM>.

The lens <NUM> may be any suitable type of lens. For example, the lens may be a Fresnel lens, such as a folded Fresnel lens as illustrated in <FIG> or Fresnel zone plate lens. Alternatively, the lens <NUM> may be a hemispheric lens, for example as illustrated in <FIG>. Alternatively, the lens <NUM> may be a transmit array lens such as the folded transmit array lens illustrated in <FIG>.

The operation of the multi-frequency twist-reflector <NUM> can be understood with reference to <FIG>, <FIG>.

The multi-frequency twist-reflector <NUM> is configured to selectively change a polarization of incident electromagnetic radiation from the second polarization P2 to substantially the first polarization P1 and to reflect that electromagnetic radiation of substantially the first polarization P1 towards the polarization-dependent trans-reflector <NUM>. The multi-frequency twist-reflector <NUM> is configured to selectively change the polarization of the incident electromagnetic radiation for at least a first frequency band F1 and for at least a second frequency band F2 but not for a third frequency band F3.

The first frequency band and the second frequency band are non-contiguous and, in the examples shown in <FIG>, are separated by the third frequency band F3.

<FIG> illustrates <NUM> the return loss Snn (reflection coefficient) for transmission/reflection of the same polarizations. It can be seen from this FIG. that the loss is above a threshold value T (e.g. < -10dB) across the first frequency band F1 and across the second frequency band F2 but not across the third frequency band F3.

<FIG> illustrates <NUM> the return loss Snm (reflection coefficient) of the multi-frequency twist-reflector <NUM> for the transmission/reflection of different orthogonal polarizations. It indicates a very small loss (e.g. >-<NUM>. 5dB) across the first frequency band F1 and across the second frequency band F2. It indicates a greater loss (e.g. <-<NUM>. 5dB) across the third frequency band F3.

Consequently, the multi-frequency twist-reflector <NUM> accepts electromagnetic radiation <NUM> within the first frequency band F1 and the second frequency band F2 for polarization change but rejects electromagnetic radiation <NUM> within the third frequency band F3 for polarization change.

It can therefore be observed by comparison that the multi-frequency twist-reflector <NUM> is selective as regards frequency. The multi-frequency twist-reflector <NUM> accepts incident electromagnetic radiation <NUM> of the second polarization P2 for a polarization change to the first polarization P1 when that incident radiation lies within the first frequency band F1 or within the second frequency band F2.

The multi-frequency twist-reflector <NUM> reflects incident electromagnetic radiation of the first frequency band F1, when it has the second polarization P2, as electromagnetic radiation of the same frequency, the first frequency band F1, but with a first polarization P1 instead of a second polarization P2.

The multi-frequency twist-reflector <NUM> reflects incident electromagnetic radiation of the second frequency band F2, when it has the second polarization P2, as electromagnetic radiation of the same frequency, the second frequency band F2, but with a first polarization P1 instead of a second polarization P2.

The multi-frequency twist-reflector <NUM> does not reflect incident electromagnetic radiation of the third frequency band F3, when it has the second polarization P2, as electromagnetic radiation of the same frequency, the third frequency band F3, but with a first polarization P1 instead of a second polarization P2.

The reflection coefficients <NUM>, <NUM> illustrated in <FIG> are multi-resonant. This arises from a multi-resonant impedance of the multi-frequency twist-reflector <NUM>.

The multi-resonance of the impedance of the multi-frequency twist-reflector may, for example, be understood by reference to a simplified equivalent electrical circuit as illustrated in <FIG>. In this electrical circuit <NUM> a first arm <NUM> is in parallel with a second arm <NUM>. The first arm <NUM> is modelled as a series combination of a first inductance L1 and a first capacitance C1. The second arm <NUM> is modelled as a series combination of a second inductance L2 and a second capacitance C2.

The electrical impedance Z of the equivalent circuit <NUM> is zero when ω<NUM>. C1 = <NUM> and also when ω<NUM>. C2 = <NUM>, where ω= 2πf = 2πc/λ.

There is consequently a first resonance dependent on the first inductance L1 and the first capacitance C1 and a second resonance dependent upon the second inductance L2 and the second capacitance C2. It is therefore possible to independently control and vary the first resonance associated with the first inductance L1 and the first capacitance C1 by designing the multi-frequency twist-reflector <NUM> to have controlled values for the first inductance L1 and/or the first capacitance C1. It is also possible to vary the second resonance associated with the second inductance L2 and the second capacitance C2 by designing the multi-frequency twist-reflector <NUM> to have controlled values for the second inductance L2 and/or the second capacitance C2.

It will be understood that in the example of <FIG>, two zeroes have been created in the electrical impedance Z of the multi-frequency twist-reflector <NUM> by creating a cell comprising multiple parallel LC circuits. The cell is repeated over the surface of the multi-frequency twist-reflector <NUM> that receives the incident radiation <NUM>.

<FIG> illustrates an example of a periodic conductive surface <NUM> that may be used in the multi-frequency twist-reflector <NUM>. The periodic conductive surface <NUM> comprises islands of conductive patches <NUM> separated by gaps <NUM>, <NUM>.

The periodic conductive surface <NUM> provides frequency selectivity. In this example, the multi-frequency twist-reflector <NUM> comprises the periodic conductive surface <NUM>, a dielectric <NUM> and a reflector surface <NUM>. In this example, the dielectric <NUM> is a single layer dielectric substrate that has an upper and a lower surface. The upper surface comprises or is adjacent the periodic conductive surface <NUM> and the lower surface comprises or is adjacent the reflector surface <NUM>.

The periodic conductive surface <NUM> can be formed by discontinuous metallization of the upper dielectric surface. The reflector surface <NUM> can be formed by continuous metallization of the lower dielectric surface.

In this example, but not necessarily all examples, the periodic conductive surface <NUM> is formed from parallel, equally spaced, discontinuous metal strips <NUM>. The discontinuities in the metal strips create individual conductive portions <NUM>. The strip portions <NUM> are separated by first gaps <NUM> in a first direction d1 and by second gaps <NUM> in a second direction d2, orthogonal to the first direction d1.

In this example, the conductive portions <NUM>, each have a shape of a strip. They have a length in the first direction d1 than is multiple times greater than their width.

In this example, but not necessarily all examples, the strip portions <NUM> are in a single flat plane parallel to both the first direction d1 and the second direction d2 and parallel to the reflector surface <NUM>.

In this example, but not necessarily all examples, the strip portions <NUM> and first gaps <NUM> alternate to form a strip line <NUM> and the strip lines <NUM> thus formed are separated by the second gaps <NUM>.

In the example illustrated, the first gaps <NUM> have the same size, the second gaps <NUM> have the same size and the strip portions <NUM> have the same size. However, the first gap <NUM> is not equal in size to the second gap <NUM>. The first gap <NUM> is significantly smaller than the second gap <NUM>. The first gap <NUM> is significantly smaller than a width of the strip portion <NUM> in the second direction d2. The second gap <NUM> is greater than the width of the strip portion <NUM> in the second direction d2.

The strip portions <NUM> may be printed onto the upper surface of the dielectric <NUM>.

It will be appreciated that the first gaps <NUM> may be associated with first capacitances C1 and that the second gaps <NUM> may be associated with second capacitances C2. The first gap <NUM> may be modelled as a first capacitance C1 in series with an inductance L1 provided by an adjacent strip portion <NUM> in the same strip line <NUM> as the first gap <NUM>. The second gap <NUM> may be modelled as a second capacitance C2 in parallel to that inductance L1 and in series with an inductance L2 associated with a strip portion <NUM> in an adjacent strip line <NUM>. Thus the periodic conductive surface <NUM> may be modelled as parallel LC circuits each providing a separate, different resonance.

The strip lines <NUM> in <FIG> have an orientation at <NUM>° to the first polarization direction P1 and the second polarization direction P2, the first polarization direction and the second polarization direction being orthogonal.

The ability of the multi-frequency twist-reflector <NUM> to change the polarization of incident radiation <NUM> from the second polarization P2 to the first polarization P1 is dependent upon a thickness of the dielectric <NUM>. The thickness of the dielectric <NUM> depends on both the first frequency band F1 and the second frequency band F2.

The multi-frequency twist-reflector <NUM> rotates the incident electromagnetic radiation having the second polarization P2 so that it has the first polarization P1. The periodic conductive surface <NUM> is selective. It reflects incident electromagnetic radiation that has a polarization aligned with the first direction d1 and transmits electromagnetic radiation that has a polarization aligned with the second direction d2. The reflective surface <NUM> reflects the transmitted electromagnetic radiation that has a polarization aligned with the second direction d2. The distance between the periodic conductive surface <NUM> and the reflective surface <NUM> is defined by the height of the dielectric <NUM>. This distance needs to be such that it reverses the sign of the E-field of the electromagnetic radiation that has a polarization aligned with the second direction d2. This corresponds to the distance from the upper surface <NUM> to the lower surface <NUM> to the upper surface as being half a wavelength. This change in polarization changes the second polarization P2 to the first polarization P1.

The height of the dielectric <NUM> therefore needs to correspond to one quarter the wavelength of the incident radiation.

The incident radiation has two different frequency bands, the first frequency band F1 and the second frequency band F2.

The first frequency band F1 is associated with a first resonant frequency which defines a first resonant wavelength λ_1. The second frequency band F2 is associated with a second resonant frequency which defines a second resonant wavelength λ_2.

In some examples it may be desirable to select the height of the dielectric <NUM> in dependence upon the harmonics of the first resonant wavelength and the second resonant wavelength. For example, the dielectric height H may equal n x λ_1/<NUM> = m x λ_2/<NUM>, where n and m are the lowest valued integers for which the equation is true.

It will be appreciated from the foregoing that there are a number of different parameters that may be varied to change the performance of the multi-frequency twist-reflector <NUM>. For example, it may be possible to vary the width of the strip portions <NUM> in the second direction. It is desirable for these widths to be less than one half the resonant wavelength and preferably less than one tenth of the resonant wavelength. It is also possible to vary the length of the strip portions <NUM> by, for example, increasing the size of the first gap <NUM>. For example, the first gaps <NUM> may have a size of approximately <NUM> of the upper resonant wavelength. It is also possible to vary the size of the second gaps <NUM> between the strip lines. For example, the second gap may have a size of approximately <NUM> of a resonant wavelength. For example, the first gaps <NUM> may have a size of less than <NUM> the size of the second gaps <NUM> between the strip lines.

Other parameters that may be varied include the height H of the dielectric layer <NUM> and also the permittivity of the dielectric <NUM>. It will be appreciated that a change in the permittivity changes the wavelength of the electromagnetic radiation within the dielectric <NUM> and consequently changes the resonance wavelengths. It may be desirable for the dielectric <NUM> to be formed from a high permittivity material such as, for example, Arlon.

<FIG> illustrates an example of the polarization-dependent trans-reflector <NUM>. The trans-reflector <NUM> comprises a polarization selective surface <NUM> that overlies a dielectric layer <NUM>.

The polarization selective surface <NUM> comprises continuous conductive strips <NUM> on the surface of the dielectric <NUM>. Gaps <NUM> separate the strips <NUM>. The polarization selective surface <NUM> is configured to reflect incident electromagnetic radiation <NUM> that has the second polarization P2 and to transmit incident electromagnetic radiation <NUM> that has the first polarization P1. This occurs for the first frequency band F1 and the second frequency band F2.

In the example illustrated the second polarization P2 is parallel to the conductive strips <NUM> and the first polarization P1 is perpendicular to the conductive strips <NUM>. The conductive strips <NUM> are parallel to the polarization P2 of the source <NUM> of the electromagnetic radiation <NUM>.

The conductive strips <NUM> may be formed from metal.

<FIG> illustrates an example of return loss S11 (reflection coefficient) for the trans-reflector <NUM> illustrated in <FIG> for the case where the incident is P1 (perpendicular to the strips). In this example, the polarization selective surface <NUM> can be modelled as a single LC circuit and has a single resonance. The fundamental resonance is illustrated in <FIG> as f-<NUM>. There will be additional harmonic resonances at multiples of the fundamental resonant frequency f<NUM>. In this example, the first frequency band F1 corresponds to the fundamental resonant frequency f<NUM> and the second frequency band F2 corresponds to the first harmonic 2f<NUM> of the fundamental frequency f<NUM>.

The thickness (height) of the dielectric <NUM> depends on the first frequency band F1 and the second frequency band F2.

In some examples it may be desirable to select the height of the dielectric <NUM> in dependence upon the harmonics of the first resonant wavelength and the second resonant wavelength. For example, the dielectric height h= a x λ_1/<NUM> = b x λ_2/<NUM>, where a and b are the lowest valued integers for which the equation is true.

In the example where the first frequency band F1 corresponds to the fundamental resonant frequency f<NUM>, which has an associated fundamental resonant wavelength λo, and the second frequency band F2 corresponds to the first harmonic 2f<NUM> of the fundamental frequency f<NUM>, then λ_1= λo and λ_2= λo/<NUM> and h= λo/<NUM>. The height h is half the fundamental resonant wavelength λO (within the dielectric). This is one half the first resonant wavelength λ_1 and is the second resonant wavelength λ_2.

Referring back to the example in <FIG>, the width of the strips may be less than one half a resonant wavelength and may, for example, be less than one fortieth of a resonant wavelength. The gaps <NUM> between strips <NUM> may be less than one twentieth of a resonant wavelength.

<FIG> illustrates an example of the multi-frequency folded lens structure <NUM> comprising the stack <NUM> but not comprising a lens <NUM> or a source <NUM> of electromagnetic energy <NUM>. Such a multi-frequency folded lens structure <NUM> may be made and sold separately.

<FIG> illustrates an example of a polarization-dependent trans-reflector <NUM>. The trans- reflector <NUM> may be made and sold separately.

The polarization-dependent trans-reflector <NUM> comprises, as previously described and illustrated, parallel strips of conductor <NUM> on a surface of a dielectric <NUM>, wherein a thickness of the dielectric is dependent upon both the first frequency band F1 and the second frequency band F2 such that a thickness of the dielectric corresponds to a first multiple number of half wavelengths for a resonant frequency of the first resonant frequency band F1 and a multiple number of half wavelengths for a resonant frequency of the second frequency band F2. In some examples, the thickness of the dielectric <NUM> corresponds to a wavelength for a resonant frequency of the second frequency band.

<FIG> illustrates an example of multi-frequency twist-reflector <NUM>. The multi-frequency twist-reflector <NUM> may be made and sold separately.

The multi-frequency twist-reflector <NUM> comprises, as previously described and illustrated, a dielectric layer <NUM> supporting, on a first side, a reflective surface <NUM> and supporting, on a second side opposing the first side, parallel, equally-spaced, discontinuous conductive strips <NUM> defining conductive strip portions <NUM> that are separated in a first direction d1, parallel to the conductive strips <NUM>, by first gaps <NUM> and are separated in a second direction d2, orthogonal to the first direction, by second gaps <NUM>. The first gaps <NUM> have a constant size. The second gaps <NUM> have a constant size. The size of the first gaps <NUM> is smaller than the size of the second gaps <NUM>.

A thickness of the dielectric layer <NUM> causes the multi-frequency twist-reflector <NUM> to reflect electromagnetic radiation, having a second polarization P2 and a frequency within a first frequency band F1 or a second frequency band F2, as electromagnetic radiation having a first polarization P1 in the same respective frequency bands F1, F2. However, electromagnetic radiation having a second polarization P2 within a third frequency band F3 is not reflected as electromagnetic radiation having the first polarization P1.

The discontinuous conductive strips <NUM> are configured to have a multi-resonant electrical impedance that is resonant at the first frequency band F1 and at the second frequency band F2 but not at the third frequency band F3 (the third frequency band F3 being between the first frequency band F1 and the second frequency band F2). The thickness of the dielectric layer <NUM> substantially corresponds to a whole number of quarter wavelengths of a resonant frequency of the first frequency band F1 and a whole number of quarter wavelengths of a resonant frequency the second frequency band F2.

<FIG> illustrates an example of a base station <NUM> for a cell of cellular communication system. The base station <NUM> comprises a backhaul radio frequency transceiver system <NUM> comprising the multi-frequency folded lens antenna structure <NUM> for point-to-point communication, as described above.

Although embodiments have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the claims.

The term 'a' or 'the' is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising a/the Y indicates that X may comprise only one Y or may comprise more than one Y unless the context clearly indicates the contrary. If it is intended to use 'a' or 'the' with an exclusive meaning then it will be made clear in the context. In some circumstances the use of 'at least one' or 'one or more' may be used to emphasis an inclusive meaning but the absence of these terms should not be taken to infer and exclusive meaning.

Claim 1:
A multi-frequency folded lens antenna structure (<NUM>) comprising: a stack (<NUM>) comprising:
a polarization-dependent trans-reflector (<NUM>) a dielectric gap (<NUM>)
a multi-frequency twist-reflector (<NUM>), wherein the multi-frequency twist-reflector (<NUM>) is configured to selectively change the polarization for at least a first frequency band and for at least a second frequency band, non-contiguous to the first frequency band, and wherein
the polarization-dependent trans-reflector (<NUM>) is configured to transmit electromagnetic radiation of a first polarization incident from within the stack (<NUM>) out of the stack (<NUM>) and to reflect electromagnetic radiation of a second, different polarization incident within the stack (<NUM>) towards the multi-frequency twist-reflector (<NUM>), wherein the polarization-dependent trans-reflector (<NUM>) comprises conductive strips (<NUM>) on a dielectric layer (<NUM>), wherein a thickness of the dielectric layer (<NUM>) is dependent upon both the first frequency band and the second frequency band such that the thickness of the dielectric layer (<NUM>) corresponds to a first multiple
number of half wavelengths for a resonant frequency of the first frequency band and a multiple number of half wavelengths for a resonant frequency of the second frequency band, and
the multi-frequency twist-reflector (<NUM>) is configured to selectively change a polarization of the reflected electromagnetic radiation from the second polarization to substantially the first polarization and to direct the electromagnetic radiation of substantially the first polarization, within the stack (<NUM>), towards the polarization-dependent trans-reflector (<NUM>) for at least partial transmission out of the stack (<NUM>).