Multilayer f requency-selective surf aces for mi I I i metre and submillimetre wave applications

C.Antonopoulos R. Ca hill E.A.Parker I.M.Sturland

Indexing terms: Antennas, Millimetre wuves, Dichroics, Frequency-selective surfaces

Abstract: Frequency-selective surfaces provide efficient beamsplitting of energy in the millimetre and submillimetre wavebands and therefore are being employed as quasioptical components for front end channel demultiplexing and single sideband filtering in radiometers. The paper describes some of the current progress in developing this technology, and in particular shows how novel design approaches have been employed to tailor the transmission response of the filters and overcome some of the fabrication problems. Experimental and simulated results in the frequency range 5OOGHz validate the design technique and highlight the insertion loss mechanisms.

1 Introduction

Frequency-selective surfaces (FSS) are currently being developed to provide beamsplitting of channels at milli- metre and submillimetre wavelengths for radiometers which are being designed for both terrestrial and space- craft applications [I , 21. The frequency response of a printed resonant element FSS is largely determined by the element type and size, the lattice geometry, element periodicity and the electrical properties of the substrate material. For conductive (patch) element FSS, two classes may be described: lowpass single resonant arrays and highpass double resonant arrays. The former generate one main reflection barid which is sep- arated from the passband by a ratio of typically 2.5:l at the band edges (where the loss in each does not exceed ten per cent). Typical examples of resonant ele- ments in this class include rings 131 and tripoles [4]. Double resonant arrays, such as the double square loop [5 ] , have a usable transmission band situated between the two resonances. The location of the upper reso- 0 IEE, 1997 ZEE Proceedings online no. 19971406 Paper first received 2nd December 1996 and in revised form 22nd May 1997 C. Antonopoulos and E.A. Parker are with the Electronic Engineering Laboratory, The University, Canterbury, Kent CT2 7NT, UK R. Cahill is with Matrd Marconi Space, FPC 320, PO Box 16, Filton, Bristol, BS12 7YB, UK I.M. Sturland is with British Aerospace Sowerby Research Centre, FPC 267, PO Box 5, Filton, Bristol, BS12 7QW, UK

nance can be set by adjusting the inner element dimen- sions, effectively shifting this transmission band closer to the lower rejection band, and giving a spacing which can be as small as 1.2:l. In addition to the geometric parameters of the FSS, the frequency response can be tailored by cascading two or more screens where use is made of the interlayer interference effects to modify the width of the bands and the rate of roll off [6]. This technique, which introduces further design flexibility, requires precise control of the phase of the waves which are partially reflected from the surface of the arrays. In this paper we discuss the application of these structures to specific millimetre and submillimetre wave requirements, where the edge of band frequency sepa- ration ratio is in the region of 1.05 to satisfy stringent channel demultiplexing and single sideband filter (SSB) specifications in future submillimetre wave space borne sounding radiometers [2].

2 Design technique

The multiple scattering of the fields between cascaded arrays produces additional transmission peaks in the

f s

c

0 Fig. 1 Geometry of a two-layer FSS and array of single ring elements I is the raypath slant length, R is the reflection coefficient for rays incident in the dielectric

IEE P?nc.-Microw. Antennas Propug., Vol. 144, No. 6, DecenDer 1557 415

frequency response besides those intrinsic to the indi- vidual layers, and it is these which can be adjusted to broaden passbands or sharpen roll-off rates. Although altering array dimensions on a trial and error basis is one way of converging on a suitable design to meet particular specifications, a technique that is proving to be very useful in design and diagnosis is based on the Fabry Perot concept. The procedure is described in [7, 81, but a few analytic results are required for this paper. In Fig. 1, two plane FSS are separated by a dielectric of relative permittivity E,, characteristic admittance q and thickness s. Waves inside the dielectric are reflected at the FSS boundaries with reflectivities RI and R, and transmission amplitude T. The reflection phases are (PI(/) and $I,#). In general for FSS, these are frequency dependent. The power transmission coefficient for a wave incident on the two-layer structure at an angle 8 is

where

4 = (47rf/c)s&=i-- h( f ) - h(f) ("2) The transmission coefficient is maximised when RI = R, = R, and (P = 2 m , with n = 0, 1, 2 ... i.e. when

(2n~/c)s\IE,. - sin2 8 - m = [41( f )++2(f )1 /2 ( 3 ) When plotted as a function of frequency, the left hand side is a set of straight lines, the ray path length phase, with slope depending on 8, s and E,.. The right-hand side is the average phase curve of the boundary reflec- tivities and where it intersects the phase lines eqn. 3 is satisfied, i.e. transmission peaks occur in the frequency response of the structure. Guided by these plots, by an appropriate adjustment of the spacer parameters and $0, the frequency response can be systematically adjusted. An example of the use of this technique is given in Section 4. The Q factor of the peaks is

Q = (27rs/X)R/(l - E 2 ) (4) There are two overall sources of loss in FSS: imped- ancc mismatch to the incident signal and absorption in the FSS itself. The Fabry Perot concept can be used to separate them. When eqn. 3 is satisfied, i.e. at the transmission peaks, the mismatch is zero and absorp- tion effects can be isolated. This technique is used in Section 4.

3 Fabrication and Performance considerations

3.7 FSS structure The improvement in the predicted roll-off rate that can be achieved by cascading FSS screens must be traded off against the complexity in the fabrication of the dichroic surface (and consequent cost implications, par- ticularly at millimetre wavelengths) and the insertion loss which increases as the path length through the die- lectric increases (Section 4). The electromagnetic and mechanical properties of three types of multilayer FSS, with dimensions set by using the technique outlined in the previous Section, have been compared for an appli- cation which required the separation of the bands 330.0-362.OGHz (nine per cent) from 380.0-387.0GHz (two per cent). Lowpass single ring patch elements 131 were chosen for their geometrical simplicity and rela- tive insensitivity to angle of incidence.

416

3.7.7 Two-/ayer FSS: This structure is the simplest to fabricate (Fig. 2a) since the array patterns can be registered and printed on the opposite sides of the sub- strate material. However, to work well away from grat- ing responses and to obtain a broad lowpass response (Section 4.2), the lowest order transmission peak (cor- responding to n = 1 in eqn. 3 for patch arrays) must be excited. This implies that the optimum performance can be achieved when the substrate thickness is < 112, but since the dichroic surface is physically very thin at these submillimetric wavelengths, a trade off between electromagnetic performance and structural integrity must be established. Furthermore, experimental obser- vations to detect the surface profile distortion of fused silica, the preferred substrate material, suggest that for a 50" diameter wafer the material thickness must be greater than about 7 0 m . Other techniques, such as increasing the periodicity of the resonant elements, can be employed to increase the roll-off rate, but here a trade off must be made to avoid excitation of the grat- ing lobes in addition to minimisation of the levels of crosspolarisation and divergence of the rejection bands in the orthogonal planes of incidence [9]. The geometry and dimensions of the optimum design is shown in Figs. 1 and 2a, with 2v = 151 pn and conductor width l o p . This dichroic gives a predicted roll-off rate of 1.07: 1 with a passband width of 11.5% (where the loss is less than - 0.5dB) which peaks at 355GHz, and a reflection bandwidth of 6.5%.

----- 100ymthick a b-1 \I Fss fused silica rina ( E ~ =3.E)

b

i I ----- I ' -----

Fig. 2 Multilayer FSS structures a Two layer b Three layer

I airgap 650 pm I

Fig.3 Semi air spaced four-layer FSS structure

3.1.2 Three-layer FSS: In an earlier publication [ 101 it was shown that a significant improvement in the frequency response can be achieved with the addition of a third dichroic screen (Fig. 2b) having the same pat- tern geometry and interlayer separation as the two layer basic building block (Fig. 2a). Our computed results show that the transition rate from transmission to reflection is sharpened to 1.04:l and the passband and rejection bandwidths increase to 24.1% and 19.6%, respectively. A further advantage of this configuration is that the overall transmission response is relatively

IEE Pioc -Microw Antennas Pvopag , Vol 144, No 6, Decembev 1997

insensitive to the angle and plane of incidence. How- ever, the disadvantage of this configuration is the fabri- cation complexity of the three-layer sandwich structure. The bonding of the substrates requires the precise dep- osition of a thin uniform layer of glue with a permittiv- ity the same as or very similar to that of the wafer material. The importance of this is illustrated in Fig. 4 which shows the predicted percentage shift in the pass- band peak with the magnitude of the permittivity of a 5 p bonding layer. An alternative solution using chemical vapour deposition is currently being investi- gated for producing three-layer FSS, and will be reported elsewhere.

12 1 ,

L

+- -2

-L

-6

-8

I

1 2 3 L 5 6 relative permittivity

Fig.4 bonding layer permittivl@

Passband frequency shift of three-layer FSS; influence of 5,~nn

6

b

Fi 5 sukillimetric FSS

Scanning electron microscope image of the ring pattern on the

3.1.3 Four-layer FSS: Separating two buildiilg blocks with an narrow air gap as shown in Fig. 3 improves the design flexibility by permitting independ- ent control of the width of the dielectric layers and the air gap separating the two double layer dichroics. The Fabry Perot approach is used twice to tailor the fre- quency response of the FSS by independent design of the interlayer separation in the dielectric and air regions. The optimum design using the same array geometry as the two-layer block gives a roll off of 1.04:1, with a transmission bandwidth of 19.3% and a reflection bandwidth of around 31%. This novel config- uration offers relative fabrication simplicity since the two double layer FSS can be separated by a precision spacer glued to the outer regions of the inner surfaces of the two wafers to create the required 6 5 0 p air gap, and layer bonding is no longer required. Its perform- ance is, however, more sensitive to the plane of inci- dence than that of the three-layer FSS, and in TE the transmission bandwidth is reduced.

3.2 Measured and predicted frequency response The one, two and four layer semi air spaced FSS were fabricated by selectively wet etching a 2 p n thick alu- minium conductive layer which was sputtered onto 50" diameter optical quality fused silica (Spectrosil- B) wafers. This method of deposition provides good adhesion, uniformity and grain control which is impor- tant to provide sufficiently accurate linewidth uniform- ity (+/-0.5p) by virtue of the vertically etched walls, as shown in Fig. Sa. The interlayer separation of the two and four-layer arrays was achieved by controlled chemical thinning of the SiOz substrates, where by giv- ing special attention to the etch composition, tempera- ture and agitation, an accuracy of +l-l.Opn was achieved. A rapid scan Martin-Puplett interferometric spectrometer [ 111 with a spectral resolution of 1.5 GHz was employed to measure the transmission response of the FSS in the range 100 - l000GHz. Fig. 6-8 shows a comparison between measured and predicted coeffi- cients in the TM 45" plane in the frequency range 300 - SOOGWz. Here the frequency response of a single FSS array is shown to be strongly modified by cascading more screens. The band separation for the two-layer FSS is greatly reduced by exciting a passband centred at 3SSGHz, close to the lower edge of the reflection band. Interference effects (eqn. 3) between the two dou- ble layer FSS separated by an air gap are used to gen- erate a peak around 31SGHz (Fig. 8) to broaden the transmission band, while simultaneously suppressing the secondary lobe around 450GHz to below -15dB, thereby increasing the width of the rejection band.

The shape and position of the theoretical response curves, which were computed using a total of 441 Flo- quet modes and five current basis functions, correlate closely with the experimental results. For the single- layer FSS the centre of the reflection band is predicted to within two per cent, and for the multilayer dichroics within 0.5%. However, the measured loss in the region of the primary passband of the multilayer FSS is higher than predicted: e.g. the loss is less than 1dB in the range 300 ~ 350GHz for the four-layer semiair spaced FSS but increases to 1.5dB at the specified band edge at 362GHz. A value of 20 x given in [I21 for the loss tangent of the material at 300GHz was used in the design, although better agreement was obtained by

417 IEE Proc.-Microw. Antennas Puopag., Vol. 144, No. 6, Decembeu 1997

increasing this parameter to 157 x lo4 as shown by the open circles in Figs. 6-8. The effect of the magnitude of tan 6 is greatest in the passband region where the com- puter model predicts that the dichroic is impedance matched, thereby implying that the origin of the inser- tion loss is absorption through the material (Section 4).

n t . . . . . . . . . . . . . . . . . . 4

-5 m U

C 0 in . _ .: -10 vl

e U

-1 5

.......

i f... ... ................ .:.. .... ..... .: ....... .hJ ............... :

-20 ................ ........ ................ : ...................... 3 00 350 LOO L50 500

frequency,GHz

Fig. 6 strate, Jov T M 45' incidence __ measured 0 e o o o o

Transmission vesponse of single layer FSS on fused silica sub-

predicted (tan6 = 20 x IO4) predicted (tan6 = 157 x 10")

0

\ $1' : j :..,A . . . . . . . . . . . . . i ................ ;

. . . . . . . I , . . . . . . . . - . . . . . . . . . . . . . . . . .

300 350 LOO L 50 500 frequency, GHz

Fig.7 strate, for TM 45" incidence .. measured e e 0

o o o

Trunsmission response of two-luyer FSS on fused silica sub-

piedicted (tan6 = 20 x lo4) predicted (tan6 = 157 x 10")

0

- 5 l... . . . . . ....... 1 . . . . . . . . I ........ i ....... ; ......... m 9

?-IO --

e

0

E c

-1 5 --

-2 0 i . . . ......

3 00 350 LOO L50 500 frequency,GHz

Transmission response ofjouv-luyey semiair spaced FSS on fused ubstrate, for TM 45" incidence

predicted (tan6 = 20 x lo4) predicted (tan6 = 157 x

___ measured 0 e e o o o

418

Thus, the multilayer FSS loss is dependent on the reflectivity of the individual surface since this deter- mines the effective path length as a consequence of multiple internal reflections from the arrays. This observation is consistent with the results shown in Fig. 9 for the two-layer FSS orientated in the TE 45" plane, where the passband centred at 365GHz is nar- row because of the high reflectivity of the individual screens. Since the effective path length is large, the importance of the tan 6 parameter is very much greater than the example shown in Fig. 7, where in the TM 45" plane the individual screens are more transmissive in the passband frequency range. Other possible sources of absorption loss such as the element conductivity and the presence of surface contaminants have been investi- gated by high temperature sintering and eliminated, as have measurement inaccuracies.

4 Substrate loss

4. I Ray path attenuation The Fabry Perot view of a multiple-layer dichroic structure accounts for the transmissionifrequency response in terms of multiple internal reflections of ray paths at the layer boundaries. Higher boundary reflec- tivities R (Fig. 1) for waves travelling in the dielectric regions between the layers imply less energy transmit- ted across at each interaction. A greater proportion travels back in the dielectric, to be reflected again at the opposite boundary. For larger R, the total path length to the point where the amplitude has fallen to a particular level is larger and the path includes more reflections. There is a greater number of significant terms in the vector phasor sum of the component fields transmitted at each incidence on the boundary, and the total transmitted field is more sensitive to a change in wavelength. The phase condition for maximum trans- mission (eqn. 3) is more frequency dependent and the Q factor of the peaks in the transmission response is higher (eqn. 4). A second consequence is a greater absorption through dielectric loss along the ray path. This interpretation helps to separate the processes con- tributing to the observed losses in multilayer FSS and enables unbiassed comparisons of spacer materials to be made, as this Section shows.

0

-5

m U C z -10

?I

E

e L

-1 5

-2 0 .............. * ................................................

I I I 1

e

300 350 L 00 L50 500 frequency,GHz

Fi .9 Two-layer FSS transmission response, for TE 45" incidence: Cc%ulationsfor two values of tan 6 superimposed on measured results - measured e 0 0 o o o

predicted (tan6 = 20 x 10") predicted (tan6 = 157 x IO")

IEE Proc -Microw. Antennas Propag , Vol. 144, No 6, December 1997

Computer modelling and measurements to highlight the loss mechanisms have been carried out in the microwave region, where fabrication costs are much lower. Fig. 10 illustrates the effects of dielectric loss, including the fact that a material with a smaller tan 6 can give a higher loss. It refers to double layers of sin- gle-ring FSS arrays on a square lattice of side D = 6.3 mm and mean ring radius r = 2.03mm, with a conduc- tor width of 0.45". The computed plane wave trans- mission responses at 45" TM incidence is plotted for two cases (inset diagram), in which the two arrays are separated by (i) 3.27" (close to a typical thickness of commercially available dielectric materials) and (ii) 1.27". In the region between the layers, E,. has been set to 1 for the spacing material but the loss tangent is varied from 0 to 0.02. In (i) the wave interference gives a peak near 21 GHz and at higher frequencies there is a well defined reflection band. At lower frequencies the insertion loss remains at - 0.5dB or less. The effect of the loss tangent is clear, but is not so marked as in (ii), where there is a much more obvious peak, now at a higher frequency and closer to the reflection maximum in the transmission response of the individual FSS arrays, which resonate at 26GHz. The layer reflectivity, R, is therefore higher than in (i) and the peaks are therefore narrower (eqn. 4). In case (ii) the insertion loss exceeds 0.5dB between 17 and 23GHz, owing mainly to higher surface impedance mismatch to free space. Absorption loss is not significant over most of this frequency range since the curves are almost coinci- dent, but in the region of the transmission peak it is clearly very important for the two nonzero values of tan 6. For tan 6 = 0.02 the attenuation is about 1.4dB.

0

m u -1

I 3.27m;I.Z7mm 3.27" ~ ~

15 17 19 21 23 2 5 frequency, GH z

,;I: :I :< :I

i ' t -3

Fig.10 two-layer FSSfor structures ( i ) and (ii); for (iii), see text Spacer sr = 1 .O, tan6 =O.OO, 0.01 and 0.02

Influfnce of spacer loss tangent on trunsntission response of

Splitting the 1.27" structure (ii) at its centre plane and separating the two halves by 2mm ((iii) in Fig. 10 inset) gives the same array separation as in (i) and, since = 1, the same path length phase (eqn. 3). The transmission peak, therefore, returns to its location near 21GHz, but the attenuation at the peak is now less than in (i) (about 0.3dB instead of 10.5dB for tan 6 = 0.02), confirming that the attenuation is a distributed process throughout the dielectric spacer region. From the point of view of losses, thin supporting substrates are therefore obviously preferable if rigidity problems can be overcome.

IEE Proc -Microw. Antennas Propug., Vol. 144, No. 6, December 1997

4.2 Experimental results A three-layer FSS, designed to give a broad lowpass transmission response with a rapid roll off to the reflec- tion band, is described in [lo]. The device was required to operate at 45" TM incidence and was part of a project to develop FSS for quasioptical beamsplitters for millimetre and submillimetre wavelength radiome- ters. The FSS in [IO] was again a demonstrator model constructed at microwave frequencies to minimise fab- rication costs. The broad transmission band was achieved by placing the array elements (single ring patches) on a fairly widely spaced lattice and exciting a low Q transmission peak below the single-layer reflec- tion band, as described in Section 4.1. Ring arrays were etched on each side of an RT Duroid substrate 3.2" thick and a second dielectric layer with an array etched on one side only was bonded on top. The substrate material had a relative permittivity of 2.3 and a low loss tangent (0.001). An insertion loss of - O.1dB was measured at the transmission peak, with a -0.5dB band extending from about 8GHz to about 15GHz.

frequency, GHz Fig. 11 Boundury reflectivities for FSS ( i ) on Duroid, (ii) and (iiiJ on FR4 (see text). Bold curve shows reyectivity phuse for (iii), intersect- ing pathlength phase line ut A, where R is equalised in ( i) and (iii) (i) D = 6.3mm, Y = 2.03mm, E, = 2.33 (ii) D = 6.3mm, I' = 2.03mm, = 4.56 (iii) D = 5.Omm, r = 1.50mm, E, = 4.56

The two layer FSS measured before bonding had a loss of 0.2dB at its peak, near 14GHz. Confirmation that the observed losses can be attributed mainly to the substrate was obtained by a detailed comparison of the performance of a similar double layer FSS etched on a more lossy material. FR4, a glass-epoxy laminate, was chosen. It has a high loss tangent of 0.01 - 0.02, but also has a higher permittivity. The measured value was 4.6 at lOGHz, falling to 4.0 at 26GHz. As discussed in Section 4.1, simply etching ring arrays with the same dimensions as in [lo] onto this material would not give an unbiassed comparison of the losses. From the Fabry Perot point of view two parameters must first be adjusted. One is the boundary reflectivity, R. This must be equalised for each dielectric + FSS array, so that for every internal reflection in the dielectric the energy lost through transmission across the boundary is the same. The second is the ray path length between the bounda- ries, so that the total path in the dielectric for a set number of boundary reflections is the same in each case. Rut the original design specification required the transmission band edge to be near 15GHz, so the com- parison here is with the constraint that the transmission peaks with the two materials are now at the same fre- quency. In Fig. 11 the reflectivity for rays in the sub- strate is plotted (i) for the original array dimensions on

419

Duroid, i.e. D = 6.3mm, Y = 2.03mm, (ii) for the same array but on FR4 and (iii) on FR4 with the dimensions altered (D = 5.0mm, r = 1.5”) to give the same R (- -7dB) in the region of the transmission peak as in (i). The reflectivity phase curve for FR4 is also shown in Fig. 11 (bold curve), where it can be seen that the phase condition of eqn. 3 is satisfied by an intersection A near 15GHz with path length phase lines drawn for s = 2.4mm, a commercially available thickness. Fig. 12 shows the 45” TM transmission responses of the two double-layer FSS. The measured and computed loss for the Duroid spacer is about 0.ldB at the transmission peak. On FR4 it is 0.5dB, increasing to 1.5dB at lower frequencies.

-2 m U L

; -1 . 0 Q

a ? -6 - 4 F

-8 -

-1 0 10 12 1 1 16 18 20

frequency , G H z Fig. 12 (iii); cross and diamond symbols show measured values (i) Duroid: D = 6.3”. = 2.03”. w = 0.45. E. = 2.33. tan6 =

TM45” transmission responses for double layer FSS ( i ) and

0.0012, s = 3.175 (iii) FR4: D = 5.0mm, I = 1.50mm, w = 0.35mm, &r = 2.33, tan6 = 0.02, s = 2.43

5 Conclusions

The development of printed element FSS capable of providing quasioptical demultiplexing in millimetre and submillimetre wavelength multiband radiometers has been described in this paper. A set of multiple-layer FSS has been constructed to assess their performance and the associated fabrication problems at these wave- lengths. Excellent correlation between experimental and predicted transmission responses was found for two- layer and four-layer semi air spaced FSS designed to operate in the frequency range 300 ~ 500GHz. This type of beamsplitter generates a strong resonant response, thereby reducing the number of layers (com- pared with free standing cascaded inductive or capaci- tive grids typically used at these frequencies) required to split or combine closely spaced wavebands. Experi- mental studies, both direct and scale modelling at microwave frequencies, together with extensive compu-

ter modelling, gave consistent pictures of the critical influence of the spacer material not only on factors such as the bandwidths and roll-off rates, but also on the observed insertion losses at important practical regions of their transmission response. In addition to being used in the basic design process, the Fabry Perot concept has now also provided a method for separating mismatch loss from absorption, so that the latter can be studied in isolation. It showed that the absorption loss generally increases when the filter band spacing is reduced. This is a consequence of the increase in the ray pathlength, which is partially determined by the inherent layeriarray reflectivity. The loss can be reduced either by employing a suitable material with a very low tan 6 or by reducing the thickness of the sub- strate. In the latter case at submillimetre wavelengths the substrate would then provide minimal physical sup- port and the consequent fragility also implies unrelia- bility. In addition, the aiddielectric boundaries are important to the interference process which enables closely spaced bands to be separated (Section 2). The results of this work therefore emphasise the need for further effort at these frequencies on the electrical char- acterisation of space qualifiable materials.

6 References

1 CAHILL. R.. ROOKES. A.. BARTLETT. D.V.. PORTE. L.. and STURLAND, I.M.: “Millimetric FSS wave&ide beamsplit: ter’, Electron. Lett., 1995, 31, pp. 4748

2 MARTIN, R.J., and MARTIN, D.H.: Quasi ootical antennas for radiometric remote sensing’, Electron. Commun. Eng. J., 1996, 8, pp. 3748 PARKER, E.A., and HAMDY, S.M.A.: ‘Rings as elements for frequency selective surfaces’, Electron. Lett., 1981, 17, pp. 612- 614

4 PELTON, E.L., and MUNK, B.A.: ‘A streamlined metallic radome’, ZEEE Trans., 1974, A€’-& pp. 799-803

5 PARKER, E.A., LANGLEY, R.J., CAHILL, R., and VARDAXOGLOU, J.C.: ‘Frequency selective surfaces’. Proceed-

3

ings of 3rd international conference on Antennas and propagation (ICAP’83), 1983, pp. 459463 (IEE conference publication 219) MUNK, B.A., and LUEBBERS, R.J.: ‘Reflection properties of 6 two layer dipole arrays’, ZEEE Trans., 1974, A€’-22, bp.-766-173

7 PARKER, E.A., ANTONOPOULOS, C., and LIMA, A.C. de C.: ‘Multiple layer FSS design and diagnosis using the Fabry- Perot concept’. Proceedings of international conference on Elec- tromagnetics in advanced applications (ICEAA 95), Turin, 1995, pp. 441444

8 LIMA, A.C. de C., and PARKER, E.A.: ‘Fabry-Perot approach to the design of double layer FSS’, ZEE Proc., Microw., Antennas Propag., 1996, 143, pp. 157-162

9 CAHILL, R., and PARKER, E.A.: ‘Crosspolar levels of ring arrays in reflection at 45” incidence: influence of lattice spacing’, Electron. Lett., 1982, 18, pp. 1060-1061

10 CAHILL, R., PARKER, E.A., and ANTONOPOULOS, C.: ‘Design of multilayer frequency selective surface for diplexing two closely spaced channels’, Microw. Opt. Technol. Lett., 1995, 8, pp. 293-296

11 MARTIN, D.H.: ‘Polarising (Martin-Puplett) interferometric spectrometers’ in BUTTON, K. J. (Ed.): ‘Infrared and millimeter waves’ (Academic Press, New York, 1982)

12 ASFAR, M.N., and BUTTON, K.J.: ‘Millimeter wave dielectric measurement of materials’, Proc. ZEEE, 1985, 73, pp. 131-153

420 IEE Proc.-Microw. Antennas Propag., Vol. 144, No. 6, December 1997