INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)

(51) International Patent Classification 3 (11) International Publication Number: WO 84/ 01

H01Q 15/00 Al (43) International Publication Date : 29 March 1984 (29.03

(21) International Application Number: PCT/GB83/00235 (81) Designated States: AT (European patent), BE (E pean patent), CH (European patent), DE (Euro

(22) International Filing Date: 22 September 1983 (22.09.83) patent), FR (European patent), GB (European tent), LU (European patent), NL (European pat SE (European patent), US.

(31) Priority Application Number : 8227074

(32) Priority Date: 22 September 1982 (22.09.82) Published

With international search report.

(33) Priority Country: GB Before the expiration of the time limit for amendin claims and to be republished in the event of the re of amendments.

(71) Applicant (for all designated States except US): KENT

SCIENTIFIC AND INDUSTRIAL PROJECTS LI¬ MITED [GB/GB]; Physics Laboratory,, The Universi¬ ty, Canterbury, Kent CT2 7NR (GB).

(72) Inventors; and

(75) Inventors/Applicants (for US only) : PARKER, Edward, Andrew [GB/GB]; 75 Whitstable Road, Canterbury, Kent (GB). LANGLEY, Richard, Jonathan [GB/GB]; 17 Richmond Gardens, Canterbury, Kent (GB).

(74) Agents: WARREN, Keith, Stanley et al.; Baron & War¬ ren, 18 South End, Kensington, London W8 5BU (GB).

(54) Title: FREQUENCY SELECTIVE SURFACES

(57) Abstract

A frequency selective surface or dichroic structure has an array of elements (1) each of which comprises at least t substantially concentric symmetrical closed figures (2, 3). For example, the element array may be a regular array of ide ical double-squares. Other polygons, or ellipses, may alternatively be used as the closed figures. The closed figures may formed as conductive paths on a dielectric substrate or, alternatively, the Babinet complement of such a structure may used.

FOR THE PURPOSES OFINFORMAπON ONLY

Codes used to identify States party to the PCT on the front pages of pamphlets publishing international ap- plications under the PCT.

AT Austria LI Liechtenstein

- AU Australia LK Sri Lanka

BE Belgium LU Luxembourg

BR Brazil MC Monaco

CF Central African Republic MG Madagascar

CG Congo MR Mauritania

CH Switzerland MW Malawi

CM Cameroon NL Netherlands

DE Germany, Federal Republic of NO Norway

DK Denmark RO Romania

FI Finland SE Sweden

FR France SN Senegal

GA Gabon SU Soviet Union

GB United Kingdom TD Chad

HU Hungary TG Togo

JP Japan S United States of America

KP Democratic People's Republic of Korea

FREQUENCY SELECTIVE SURFACES

The present invention relates to frequency selective or dichroic surfaces and members, more parti¬ cularly, although not exclusively, for reflector antennas and optical and infra red filters. A dichroic member is used to reflect electro¬ magnetic signals in one frequency band and to pass or transmit signals in another frequency band. One example of such a dichroic device is disclosed in US Patent 4l6θ254« The latter describes a microwave dichroic plate comprising an array of interlaced elements, each of which has first and second orthogonal arms of approxi¬ mately the same length and cro ssing at a point at the middle of the arms. The arms are arranged with their centre lines aligned parallel to the X and Y axes of the array and the arrangement is such that a line between the points of crossing of the arms of the closest adjacent elements has differing component, values relative to the X and Y axes. The elements may be formed as crossed slots in a metal plate, between which slots a metal lattice structure is formed. Alternatively, the dichroic plate may comprise the Babinet complement of the afore¬ mentioned structure in which the lattice structure and crossed slots are reversed so that the crossed slots become metallic segments separated by a dielectric. The purpose of the interlacing of the array elements is to reduce energy losses and to increase the bandwidth of signals which are transmitted by the plate. The design of most known dichroic plates or members is directed to alleviating these problems. In contrast, other operating parameters of dichroic members and the ease of design

of such members have not been significantly improved. Our copending European Patent Application No. 83303127.1 is directed to enhancing the operating parameters -of dichroic members, for example, the proximity and stability of reflection and transmission bands, and the mitigation of cross polarisation phenomena and also aims to facilitate the design of dichroic members. It describes a dichroic member comprising a regular grid having elements disposed within the interstices of the grid, these elements being identical and of sub¬ stantially symmetrical shape. For example, the elements may be in the form of hollow squares. The grid and elements may be formed as conductors or, alternatively, the Babinet complement of such an arrangement may be utilised.

It is also a general object of the present invention to enhance the operating parameters of frequency selective or dichroic surfaces or members and to facilitate the design of dichroic devices. The invention consists in a dichroic member having an array of elements, each of which comprises at least two substantially concentric symmetrical closed igures. Preferably, the array is a substantially regular array of similar or substantially identical elements. The closed figures forming the elements may be polygons, such as triangles, rectangles or hexagons, or may be ellipses. The sides of the figures forming each element may be of different widths and the spacing between adjacent elements of the array may be different from the spacing between the concentric figures of each element.

The smallest or innermost figure of each element may be of solid shape, instead of being hollow, as are the outer figures. The closed figures may comprise conductors or conductive paths disposed on a non-conductive substrate or, alternatively, may be the Babinet complements of such arrangements.

It has been found that the present invention enables the transition between reflection and trans¬ mission frequency bands to be made more rapid. Moreover, the widths of the bands are relatively insensitive to the angle of incidence of electromagnetic waves whilst the cross polarisation performance remains satisfactory and waves are not rotated by the dichroic member to any significant extent. Conversely to the structure described in our expending European application, the present invention provides a dichroic member which has a main reflection band lower than the main trans¬ mitting band.

Dichroic members are conventionally used with radio frequency signals and are widely used in microwave systems. Dichroic members constructed in accordance with the present invention have applications not only in reflector antennas but also in optical and infra red filters. In order that the present invention may be more readily understood, reference will now be made to the accompanying drawings in which:-

Figure 1 illustrates one array of elements for a dichroic member according to the invention, Figure 2 is a graph of transmission loss

versus frequency for a dichroic member having the element array shown in Figure 1,

Figure 3 i the equivalent circuit derived for the element array shown in Figure 1, Figure 4 s a schematic diagram of an antenna embodying a dichroic member of the construction illustrated in Figure 1, and

Figure 5 is a schematac diagram of a diplexer embodying such a dichroic member. Referring to the drawings, Figure 1 illustrates one form of a regular array of identical elements 1 for a dichroic member according to the invention. It is a periodic array in which each element 1 comprises two concentric hollow squares 2,3- These hollow squares may have sides formed by conductors disposed on a non- conductive substrate. For example, the conductive squares may be formed on a dielectric substrate sheet by printed circuit techniques. Alternatively, the array may be the Babinet complement of the aforementioned arrangement.

A dichroic structure of the type illustrated in Figure 1 provides a plane wave transmission coefficient plot of the form shown in Figure 2 and has two reflection bands f .f. which are relatively insensitive in location 1' 2 ^J and width to the angle of incidence of the electromagnetic waves impinging on the structure. Between the frequency bands f ,f„ is a band of high transmission f'_. In most applications, the latter would be used with bands f or f to give, respectively, the transmission and reflection bands required by a dual band system, as

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is hereinafter more fully described. Variation of the relative dimensions of the various conductors of the array elements 1 enable the widths and separation of the transmission and reflection bands to be adjusted to meet the operating requirements of a particular antenna or other device embodying the dichroic structure.

Dichroic arrays having a double resonant- frequency transmission characteristic, as described above, can be used as single-layer frequency-selective surfaces with transmission/reflection band centre ratios in the range 1.3:1 to about 2:1. They are typically designed using either a model analysis or, if one is available, an equivalent circuit model. The latter whilst not offering the comprehensive analysis capability of the former, permits extremely rapid computation of the array transmission characteristics. Array elements according to the present invention have the advantage of lending themselves to design by simple equivalent circuit models and an equivalent circuit model for the array of double squares illustrated in Figrue 1 is shown in Figure 3«

Referring firstly to Table I, this sets forth a comparison between the band centre frequencies predicted by the equivalent circuit model of Figure 3 and those measured for 13 different arrays of double square elements. The listed dimensions of the elemenets correspond to the dimensions indicated in Figure 1. The swept frequency transmission measurements were made for a plane-wave front incident on 20x20 cm arrays, printed on 0.027 mm polyester substrates (£. = 3 ) •

TABLE I "MEASURED AND PREDICTED BAND CENTRE FREQUENCIES"

Dimensions, mm Frequencies, GHz

Array ^f ! ^f τ ^f 2

No. P Meas ' d Model Meas ^■ d Model Meas ' d Mod ^w l ^d ι ^g l ^W 2 ^d 2 s ₂ el

1 5.0 0.17 4-83 0.17 0.17 3.50 0.50 11.5 11..5 17.6 17.5 24.1 i.24.5

2 5.0 0.17 4.83 0.17 0.17 3.00 0.75 U.5 11-5 21.0 20.0 29.4 29.8

3 5-0 0.17 4 ^" .83 0.17 0.17 2.50 1.00 11.5 11.5 25.0 24.0 36.4 35.7

4 5.0 0.17 4-57 0.43 0.17 3-35 0.44 14.5 14.6 19.8 20.2 26.6 26.8

5 5-0 0.20 4.60 0.40 0.20 2.40 0.90 14.9 14.7 26.5 26.0 41.0 39.4

6 5.0 0.20 4.60 0.40 0.20 2.90 0.66 14.6 14.6 23.0 22.8 32.3 32.4

7 5.0 0.06 4.76 0.24 0.42 3.61 0.52 12.5 12.9 18.5 19.4 29.5 30.1

8 5.2 0.40 4.85 0.35 0.30 3.15 0.45 15.1 14.8 22.2 22.0 31.8 30.6

9 5.2 0.39 4.82 0.38 0.28 2.82 0.61 15.3 15.0 24.6 24.0 35-7 34.3

10 5.2 0.17 4-78 0.42 0.30 3.40 0.52 14.1 14.4 20.4 21.0 29-9 29.7

11 5-2 0.20 4.82 0.38 0.34 2.65 0.88 14.3 14.3 26.0 25.5 41.5 38.8

12 5-5 0.25 5.12 0.38 0.25 2.62 1.00 13.3 13.0 24.8 24.0 38.0 36.4

13 5.5 0.20 5.10 0.40 0.34 2.63 1.04 13.3 13.4 25.8 25.0 41.0 38.5

It will be seen that the first three arrays listed in the Table have identical geometries apart from the inner square sides d_ which are 3 - 5 , 3 O and 2.5 mm long, respectively. The first reflection band centre on f- occurs at approximately the same frequency (11.5 GHz) for each of the three arrays. The empirical model shows this resonance to be independent of the inner-square side d„ but dependent on its width w_ .

The second resonant frequency f„ is determined by the inner-square dimensions only and f„ increases as drs decreases. The relative spacing of this second resonance with respect to f- controls the transmission- frequency band centred on f . Hence, the ratio f- _p /f is 1.5 for array 1, 1.8 for array 2 and 2.2 for array 3 - At oblique incidence, it has been found that the upper resonance tends to be sensitive to angle, in contrast to that at f-. For instance, array 3 at 45° has two high-frequency resonances within our frequency range, a narrow one at 27-5 GHz and a more prominent null at 34-5 GHz in the H-plane. They occur at 31.5 and 33 GHz, respectively, in the E-plane. In general, the reflection (f. ) and transmission bandwidths increase as the band separation increases. For example, the -0.5 dB bandwidths common to all angles of incidence up to 45° for arrays 1, 2 and 3 are respectively 14%, 17 and 20% in reflection. But in transmission the situation is less clear, since there appear to be losses of up to 0.5 dB at 45° incidence in the H-plane, reducing the common bnadwidths to below 10 . It is believed that it may be possible to optimise these widths for a given f / -, ratio by varying the dimensions of the element.

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Figure 3 shows the equivalent circuit derived for the array of double squares illustrated in Figure 1. The basLc equations for calculating the inductance and capacitance of strip gratings are found in "Waveguide Handbook" by N. Marcuvitz and published by Magraw-Hill in 1951. In general form they are given by:

inductance

capacitance: _£ _=4F(pjSj ) Y where

F(p,s, ) = 2 ^, (In cosec ^- + G(p,s,~X))

G is the correction term;p is the dimension illustrated on Figure 1: s, for inductance purposes, is the width of a conductor and, for capacitive purposes, is the spacing between conductors; and λ _» is the wavelength. The four circuit elements given in Figure and C

'fl, fl 'f2 f2 are calculated as follows:

, _fl = 2.0*(L ₁ /L ₂ )* where L _χ -=F(p,w ₁ , ^{^} λ)

P and L ₂ =F(p,w ₂ ^L)

C _fl = 0.75*C ₁ *^1 where C^F(p,g _χ , )

P

T T « ^{d L} f2 ⁼ — where L =F(p, 2w„,~ P ²

C„„=(C. in series with C,, ) 2

where C _£ =4F(ρ,g^

In each case the capacitance has been calculated using an effective dielectric constant of 1.12. It will be seen from the Table that close agreement is obtained at the first resonant frequency f-. At the upper resonance, the model is less satisfactory but nevertheless predicts f„ to within about 7%. The trans¬ mission frequency f,-, is adequately predicted (to within 5%) in all 13 cases by the equivalent circuit. As is usually the situation with equivalent circuit models at present, the results apply to plane wave normal incidence only, but are also good guides to the location of the bands at oblique incidence.

Figures 4 and 5 illustrate two applications of dichroic members having element arrays as shown in Figure 1. In Figure 4 _> which illustrates a Cassegrain antenna, the dichroic member is a curved secondary dichroic mirror 5 mounted in front of the prime focus feed 6 of the antenna and this permits the prime focus feed to be used at frequencies where the dichroic mirror is transmitting, whilst the Cassegrain feed

7 in the centre of the main reflector bowl 8 is used at frequencies where the dichroic mirror is reflective.

Figure 5 illustrates another arrangement enabling two feeds to be used simultaneously to give dual band capability. In this diplexer arrangement, the dichroic member is a flat dichroic plate 9 positioned between two feeds 10,11. The feed 10 operates at frequen¬ cies for which the plate 9 is reflective, whereas the feed 11 operates at frequencies for which the plate is transmissive. This diplexer arrangement can be used instead of the single band feed in a standard reflector antenna. Such operation

has been achieved hitherto by constructing the frequency selective surface from stacks of waveguides or arrays of elements such as resonant dipoles.

The applications shown in Figures 4 and 5 assist in highlighting further advantages of the element array illustrated in Figure 1. These advantages are concerned with applications inwhich multi-banding is ^• required. Such applications have been difficult to implement using known dichroic members. A major problem in implementing multi-banding applications is the cross polarisation which the known dichroic members exhibit when several are used together. The cross polarisation performance of the element array shown in Figure 1 has been found to be particularly improved in relation to known devices.

Whilst particular embodiments have been described, it will be understood that modifications can be made without departing from the scope of the invention, as defined by the appended claims. For example, each element of the array illustrated in Figure 1 may comprise more than two concentric squares. This increases the number of reflection and transmission bands available. The smallest or innermost square of the multi-square element may be solid in shape instead of hollow, as are the or each outer square.

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