Annex 5: Survey of Active FSS PDF, 1.6 MB

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ANTENNA SYSTEMS

Application of FSS Structures to Selectively Control the Propagation of signals into and out of buildings Annex 5: Survey of Active FSS

E A Parker

S Massey

Consultant

ERA Report 2004-0072 A5 ERA Project 51-CC-12033 FINAL Report Annex 5

Client : Ofcom Client Reference : AY4464 Report edited and checked by: Approved by:

Martin Shelley Project Manager

Robert Pearson Head of Antenna Systems

March 04Ref. Z:\AS_Projects\Custom Antennas and Consultancy_SW\12033_RA_in_and_out_building_FSS\Reporting\FINAL REPORTING\Annex 5 Survey of Active FSS.doc

ERA Report 2004-0072 Annex 5

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Crown copyright 2004. Applications for reproduction should be made to HMSO.

This report has been prepared by ERA Technology Limited and its team for the Ofcom under Contract No. AY4464.

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The document may be distributed freely in whole, without alteration, subject to Copyright.

ERA Technology Ltd Cleeve Road Leatherhead Surrey KT22 7SA UK Tel : +44 (0) 1372 367000 Fax: +44 (0) 1372 367099 E-mail: [email protected]

Read more about ERA Technology on our Internet page at: http://www.era.co.uk/

Prof E A Parker


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Contents

Page No.

1. Introduction 5

2. Active configurations 6

2.1 Incorporation of active devices 6

2.2 Variable substrates 15

2.3 Variable coupling modes 18

2.4 Conclusions 20

3. Manufacturing costs 20

3.1 Cost elements 20

4. Regulatory issues 22

4.1 Diode grids 23

4.2 Ferro-electric arrays 25

5. Overall conclusions 26

6. Acknowledgement 26

7. References 26

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Figure list

Page No.

Figure 1: Return loss curves of an experimental adaptive radar absorber 6

Figure 2: Quasi-optical mixer array 7

Figure 3: Circuit equivalent of Quasi-optical mixer array 8

Figure 4: Dipole FSS with capacitive end-caps 8

Figure 5: Measured performance of “active wallpaper” 9

Figure 6: Measured performance of “active wallpaper” 9

Figure 7: Active FSS with interleaved strips and dipoles 10

Figure 8: Active FSS using loops 10

Figure 9: Performance of active FSS using loops 11

Figure 10: Performance of active FSS using loops and varactor diodes 11

Figure 11: Phase switched screen 12

Figure 12: Performance of phase switched screen 13

Figure 13: Performance of optically illuminated dipole FSS 14

Figure 14: Variable FSS using localised heating 14

Figure 15: Ferroelectric FSS 15

Figure 16: Performance of ferroelectric FSS using ring slots 16

Figure 17: Performance of FSS with ferrite substrate 17

Figure 18: FSS with liquid substrate 17

Figure 19: Performance of FSS with liquid substrate 18

Figure 20: Close coupled FSS 19

Figure 21: Performance of close coupled FSS 19

Table list

Page No.

Table 1: Impact of building regulations on diode grids 24

Table 2: Impact of building regulations on ferro-electric arrays 25

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1. Summary

This is Annex 5 to the Final Report provided under Ofcom Contract AY4464, Application of FSS Structures to Selectively Control the Propagation of signals into and out of buildings. It gives a detailed description of the work carried out to assess whether Active FSS structures offer potential to compensate for walls which have time-varying properties due to environmental and ageing factors.

After a short introduction, Section 3 described different Active FSS structures that have been proposed and identifies their key characteristics. Section 4 assesses the cost of implementation of the most promising technologies and, in Section 5, a brief assessment of the regulatory issues that would need to be addressed should the technology be adopted is provided.

2. Introduction

In the majority of cases, FSS structures consist of an array of elements arranged on or embedded in one or more dielectric layers, which not only provide mechanical support but also influence the frequency dependence of the transparency of the structure. Other types exist; for example, perforated metal plates which behave like arrays of waveguides with a high pass frequency characteristic have been used where weight is not an important issue, and in high power environments. Diodes have been located in the guides to control the transmission properties. Structures consisting solely of dielectric material with spatial variation of the permittivity have also been considered. Perforated metal plates might find application as FSS in buildings, and dielectric waveguide plugs are being proposed as a means of transmission through concrete floors. However, this discussion of active FSS configurations is confined to what are loosely called printed element arrays, where the conductor is etched, deposited or laid down onto a dielectric surface.

The arrays are usually, but not always, periodic. Their finite electrical size may also be significant, particularly in long wavelength applications. But here, except for one case which may be relevant to the question of the cost of active FSS, the elements are identical. The FSS structures are “active” in the sense that their frequency/transmission responses can be modified in response to an applied stimulus of some kind. Most of the work carried out in this subject area has been undertaken in defence related projects, and consequently it is difficult to deduce from the open literature the level of development that has been achieved.

Figure 1 shows the return loss curves of an experimental adaptive radar absorber based on gratings with integrated electronic components. The diagram shows some sample curves which can be measured at different settings of the electronic components. Note that return loss values were not shown due to the sensitive nature of the application.

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Figure 1: Return loss curves of an experimental adaptive radar absorber

Comparatively few papers have been published on active FSS structures. Brief details of a representative selection of them are given in this report. The technology of active FSS is relevant to RCS reduction/modification, false Doppler, and similar issues. It is not a new concept. A few reports began to appear in papers published in the early 1990’s, presumably dealing with less sensitive topics arising from work carried out in the previous decade. Figure 1 appears in a paper dating from 1991 on smart skins from Deutsche Aerospace [Ref 1]. The structure behaved as a variable absorber and was based on two cascaded gratings. The lack of performance details shows the sensitivities involved in this subject area.

3. Active configurations

The mechanisms that have been proposed for rendering FSS active or reconfigurable can be divided into three categories, ie those in which:

• active devices are embedded in the FSS, • substrates have variable electrical properties, • mode coupling between cascaded arrays which can be altered.

3.1 Use of active devices

This class appears to have been investigated over a longer time than the others, and the simpler configurations have been shown to be practical and in principle to be suitable for covering areas of several square metres.

By introducing active components into an FSS array the surface reactance, the transparency, reflectivity or signal absorption become variable characteristics, enabling the user of the structures to vary the characteristics as and when needed. The active devices can be placed between the array elements, within the elements themselves, or between layers and ground planes. The structure relating to Figure 1 was of this type. Three mechanisms have been investigated for adjusting the state of the devices, and hence the state of the FSS: an applied DC voltage or current bias, the intensity of optical illumination, and localised temperature changes. At present, switching by means of an applied bias appears to be the most feasible in applications to buildings.

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Note that all the work described in this report refers to the use of PIN diodes. Varactor diodes can also be used, and offer the potential for variable performance between two “end states”, rather than just an “ON” and an “OFF” state, as is the case with a PIN device.

3.1.1 Applied voltage bias

Active FSS and phased array antennas share some similar concepts. A paper published in 1981 [Ref 2] with the title ‘Radant: new method of electronic scanning’, by Chekroun et al. discusses the concept of an artificial dielectric with variable refractive index. The dielectric consisted of ‘grids of wires containing many (PIN) diodes connected together’, and was used to construct lens systems for beam scanning at microwave frequencies. A subsequent article [Ref 3] in the same year briefly considered the issue of diode failure.

Developments at millimetre wavelengths led to the incorporation of Schottky barrier varactor diodes into cascaded grids on gallium arsenide substrates about 2cm square, to give a quasi-optical frequency multiplier at 66GHz [Ref 4] and a beam steering array at 93GHz [Ref 5]. Changing the DC bias changed the device reactance and the reflectivity phase. There was a measured reflection loss of 7dB. A later quasi-optical mixer for operation near 10GHz [Ref 6] used the array sketched in Figure 2, which consists of Schottky diodes linking dipole elements. Other than the shape of the dipoles, it closely resembles a typical array used as an active FSS. A circuit analysis based on the geometry in Figure 3 for quasi optical applications at 99GHz was presented in 1991 [Ref 7].

Figure 2: Quasi-optical mixer array

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Figure 3: Circuit equivalent of Quasi-optical mixer array

Descriptions of specifically active FSS configurations emerged at about this time. In the late 1980’s the group at the University of Kent worked on active FSS in a project funded initially by British Aerospace. Early versions consisted of dipole arrays in which the ends of the elements were connected by PIN diodes, as in Figure 3, or modified dipoles again connected together by diodes but with the additional capacitive end caps shown in Figure 4 [Ref 4].

Figure 4: Dipole FSS with capacitive end-caps

A demonstrator was manufactured to operate at microwave frequencies above 10GHz. It functioned very well; the transparency could be switched very rapidly by about 20dB. An analysis of similar structures was also carried out in Australia [Ref 9]. In a later project, a switchable dipole array was fabricated specifically to act as active wallpaper [Ref 10]. It was hung on a block wall 12cm thick. The transmission response measured between the two adjacent rooms separated by the wall is shown in Figure 5. At 2.4GHz, the transparency could be switched by about 25dB. Since the array elements were linear dipoles, the switching occurred in one plane of polarisation only. A series of experiments with two superimposed active arrays with the dipole axes set orthogonal to each other showed that the same performance could be obtained in two perpendicular planes of polarisation.

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The cost of fabricating active FSS would be reduced if the number of active devices required could be reduced. The array shown inset in Figure 7 contains dipoles in only one in three columns of elements, the other two being conducting strips. The basic structure is the sequence ABC reflected in conducting walls. Its transmission response was measured in a waveguide simulator [Ref 11], and is

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1 2 3 4 5 6

Frequency, GHz

-30

-20

-10

0Tr

ansm

issio

n, d

B

Reflecting state at 2.4 GHzTransmitting state

Figure 5: Measured performance of “active wallpaper”

The active wallpaper was also used to screen a small cubic enclosure with walls measuring 54cm x 54cm. The internal fields were sampled at intervals of about 2cm to establish the spatial distribution of the switching level at frequencies near 2.4GHz. The distribution was not uniform and except in a few locations did not attain the 25dB level seen in Figure 5. It was influenced by the configuration of the edge regions of the FSS, and by the surface of the internal walls; in one case absorber lined and in the second free of absorber. The performance is summarised in Figure 6.

Figure 6: Measured performance of “active wallpaper”


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plotted in the diagram. There is a deep reflection null near 13.5GHz. An array consisting entirely of conducting strips would be reasonably transparent at this frequency, suggesting that an effective active FSS could be realised by inserting active elements in columns B only.

Figure 7: Active FSS with interleaved strips and dipoles

Other array elements

Dipoles are the amongst the simplest array elements available and most work on active FSS appears to have focussed on them: hwoever, some investigations using the rectangular structures sketched in Figure 8 were also described in [Ref 11]. Figure 9 illustrates results from a waveguide simulator containing a sparsely populated array in which diodes were inserted in alternate columns of elements. The FSS transparency could be varied by about 20dB near 12 GHz.

Figure 8: Active FSS using loops

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Much work has been carried out at Sheffield University on the design of absorbers mainly for radar applications. In recent years, active FSS have been used to construct absorbers using phase switched screens (PSS) consisting of an active impedance layer separated from a conducting back plane by a

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Figure 9: Performance of active FSS using loops

More recently, work has been carried out at Warwick University using the same elements but with surface mount capacitors and inductors in place of the diodes, and also with varactor diodes, in each case using fully populated arrays [Ref 12]. Initial measurements were carried out in a waveguide simulator, but subsequently an FSS consisting of 16 x 16 unit cells containing 512 varactor diodes was constructed. As the reverse bias voltage on each diode was increased from 0 to 30V, the reflection null frequency in Figure 10 increased from about 1.9GHz to 2.0GHz, a 6% change, but the FSS remained reflective in the frequency range measured.

Figure 10: Performance of active FSS using loops and varactor diodes

3.1.2 Phase switched screens


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dielectric spacer. By switching the FSS between two states, a phase modulation is imposed on the scattered signal. The energy in the signal is redistributed in frequency over a much wider bandwidth, beyond the bandwidth of the detection systems, and hence the energy at the illuminating frequency is much reduced [Ref 13], [Ref 14].

The FSS array used in a recently published version [Ref 13] is dual polarised, with four PIN diodes in each unit cell (Figure 11), so in section 4.1.3 below the parameter n = 4. An aspect of this array topology is that it combines parallel and series feed paths, and is tolerant to limited diode failure (section 4.1.7 below). The reflection performance of this screen was synthesised from measurements made with two FSS, with open and closed conditions at the diode loading points, to represent the active FSS in its two states. Each array was 18cm square, with 144 diode locations, and was set 8.0mm from a conducting back plane using a low loss foam dielectric spacer. Measured reflectivity data were used to synthesise the reflectivity performance of an active PSS modulated by a periodic square wave, with the results shown in Figure 12. A deep null appeared at the same frequency for all three polarisations, and this frequency could be steered by varying the mark-space ratio of the modulating waveform.

Figure 11: Phase switched screen

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Figure 12: Performance of phase switched screen

3.1.3 Optically switched elements

There are several US patents filed in the 1980’s which relate to the concept of optically addressing arrays containing active elements, [Ref 15], [Ref 16]. The motivation of that work was partly beam steering and partly the design of active radomes, which would become transparent to signals from an enclosed antenna when required. The conductivity of the active elements is a function of the incident light intensity.

More recently ([Ref 17], [Ref 18]) some modelling work has been carried out following the same theme. In [Ref 17], an FSS consisting of dipole slots deposited on a 50µm thick silicon substrate had a band pass at 46GHz. Optical illumination increased the plasma density N in the silicon and hence decreased the relative permittivity, which in turn modified the wave propagation constants used in the Floquet modal analysis of the structure. Increasing N had little effect on the FSS transmission response until it reached about 1013cm-3, when the pass band transmission levels began to fall. The transmission coefficients fell to about -26dB when N had reached about1016 cm-1. The pass band had totally disappeared. Figure 13 refers to a dipole version [Ref 18]. For the FSS to exist for about 1ms, (i.e. with a carrier lifetime of 1ms in the substrate), optical illuminating power levels of about 1W/cm2 would be required. The concept implies rapid rescanning of the substrate to maintain the presence of the FSS.

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Figure 13: Performance of optically illuminated dipole FSS

3.1.4 Localised heating effects

There has been a proposal to incorporate materials such as silver iodide (AgI), which have heat sensitive conductivities, into local areas of arrays, to vary the microwave reflectivity. [Ref 19] describes an investigation of the FSS sketched in Figure 14, which consisted of a ceramic plate 10cm square perforated with holes into which pellets of silver iodide were inserted, each in contact with a heating element. The conductivity of this material increases as its temperature rises. By heating the silver iodide from about 50°C to 180°C the reflectivity could be varied by about 10dB in 500msec at 9.4GHz Apparently there exist other materials capable of operating similarly at lower temperatures.

Figure 14: Variable FSS using localised heating

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3.2 Variable substrates

3.2.1 Ferroelectric substrates

The resonant frequency fr of an FSS array is dependent on the permittivity of the supporting dielectric substrate. If the array is embedded in dielectric, it falls from a free space value f0 to f0 /√ εeff, where εeff is the mean relative permittivity of the dielectric layers on the two faces of the array. If εeff can be altered by the application of a stimulus of some kind, it might be possible to shift fr sufficiently so that, at a particular frequency, a high reflectivity is replaced by high transparency. Unfortunately, in the frequency responses available from simple array configurations, the rate of transition between reflection and transmission bands is very slow, and the required dielectric tuning range requirements are unattainable in practice.

Nevertheless, ferroelectrics are a class of material which offer significant tuning range, and their performance is slowly being improved. Their relative permittivities have very large values (500 or more) but they can be altered by the application of an electric field. [Ref 20] gives the results of a computer simulation of switching configurations which employed two cascaded slot arrays of simple ring elements etched on ferroelectric layers, and separated by a spacer of constant permittivity, as shown in Figure 15. The slots were etched into a conducting sheet, providing a continuous conducting path and allowing a biasing voltage to be applied between the two cascaded arrays. The ferroelectrics were barium strontium titanate (BSTO) compounded with either magnesia or alumina.

Figure 15: Ferroelectric FSS

The tuning range reported for these compounds can be as high as 60%. Application of the field bias reduces the permittivities. Figure 16 shows some results for slot ring elements, where εr in layers 1 and 3 in Figure 15 has been switched between 718 and 460, using a BSTO/magnesia compound, which has a low loss tangent, quoted to be about 0.001. This results in a theoretical loss of less than 0.5dB at the resonant peak. Alumina-based designs will have higher losses (with a tanδ of typically 0.015).

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The labels N and R refer to the operating bias mode; in the normal mode N the unbiased state corresponds to high transmission at the frequency concerned. The operational bandwidths were small, typically 2%.

Figure 16: Performance of ferroelectric FSS using ring slots

By modifying details of the structure in Figure 15 and adding a layer of absorbing material, it was found possible to devise a structure capable of switching between reflecting and absorbing states. The operational bandwidths were typically 3 – 4%.

The use of ferroelectric substrates for FSS awaits the development of fabrication methods capable of producing thin sheets of the material.

3.2.2 Ferrite substrates

Ferrites are the other well known class of material with adjustable electromagnetic properties. Results of a waveguide simulator study of an FSS with a ferrite substrate were reported in [Ref 11], [Ref 21].

The array element was a square loop, on a substrate about 1.3mm thick. The magnetic bias field was applied in the plane of the substrate, ie perpendicular to the direction of wave propagation. Figure 17 shows the transmission responses measured with and without a bias field of 4000 Gauss. Without the bias the FSS resonated at about 8GHz, with a null about 30dB deep. With it, the null vanished. An insertion loss of about 5dB remained, owing mainly to a wave mismatch at the surface of the ferrite; the magnetic permeabilities of these materials are high.

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Figure 17: Performance of FSS with ferrite substrate

The high currents required to generate the bias fields, the high mass of ferrite materials, and the impracticality of distributing the bias over the large surface area of the arrays are all major problems which imply that ferrite substrates are not a realistic option for active FSS.

3.2.3 Liquid substrates

An alternative to changing the permittivity of the substrate by the application of an electromagnetic field is simply to exchange the substrate with another of different εr. In [Ref 22], the substrate was a low loss liquid contained in a cavity underlying the array (Figure 18).

Figure 18: FSS with liquid substrate

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The resonant frequency of the FSS was again fr = f0 /√ εeff , where εeff is the mean value of the relative permittivity of the liquid on one side (2.2 in this case) and the air on the other side of the array. As the substrate was changed on one side only, the effect on εeff was comparatively small, and so the stub slot elements sketched in Figure 18(b) were used in this demonstrator. They are capable of giving narrower pass bands than are more conventional designs. The solid curves in Figure 19 show the measured transmission responses. Emptying the cavity decreased the measured transmission coefficient by 16dB at 17GHz. There was an insertion loss of 2.8dB, partly accounted for by dielectric loss in the thin retaining substrate and in the liquid, and partly by mismatch loss.

3.3 Variable coupling modes

A technique has been proposed for adjusting the frequency response of an FSS consisting of closely coupled cascaded arrays by perturbing the fields in the separation region [Ref 23].

The configuration is shown in Figure 20. Two arrays of linear dipoles were separated by a distance S (= 50µm) and the relative lateral position DS along y was varied, so that the apertures in the surface were gradually covered up. Seen from the wave incident side of the structure, the effective dipole length was increased. The incident electric field and the dipoles (L1 = 3.25mm, L2 = 4.5mm) were aligned along y. The effect of changing DS is shown in Figure 21. With no displacement the reflection resonance was near 29GHz. The maximum shift (to 14GHz) occurred when the displacement was 3mm, half the lattice period. The reflection bandwidth also increased.

Figure 19: Performance of FSS with liquid substrate

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The same technique has been applied to slot element FSS. In [Ref 24], two double layer dipole slot surfaces were constructed, the arrays being printed on both sides of a thin (0.108mm) dielectric sheet.

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Figure 20: Close coupled FSS

Figure 21: Performance of close coupled FSS

The transparency of the structure at 29GHz was not stated in the paper, but there appears to be an insertion loss of about 1dB.


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In one, the arrays were displaced perpendicular to the dipole lengths by a half period, while in the other there was no such displacement. The transmission response of the latter had a pass band at 36GHz, while in the former case there were two narrow pass bands, one at 38 GHz and another at 20GHz. The losses were high, about 8dB, in these bands, as might perhaps be expected in the light of an earlier discussion of loss-bandwidth products [Ref 25] in FSS.

3.4 Conclusions

This summary of published work on active FSS strongly suggests that, although an encouraging variety of schemes have been proposed and implemented, if such FSS were to be applied to the control of signal penetration in buildings they would be of the current/voltage controlled type discussed above.

Notwithstanding the cost issues described in Section 4 below, significant further technical work is required before implementation of such FSS structures could be considered. One issue that needs to be assessed is the intermodulation performance of active screens. Generation of spurious frequency components would clearly be highly undesirable and could potentially render the technology unusable for, in particular, mobile telecommunications applications.

4. Manufacturing costs

For the purposes of assessing manufacturing costs, it has been assumed that, if active FSS structures are to be used in controlling the signal transmission properties of buildings, they would be of the applied voltage bias type. This section discusses the contributions to the cost involved, but does not attempt to estimate a final figure.

4.1 Cost elements

The factors determining the cost of an active FSS are:

• Size of the structure required, • Device type, • Number of active devices required, • Manufacturing and assembly techniques, • Bias supply system, • Installation, • Maintenance post installation.

4.1.1 Size of the structure

Best performance will be achieved by using the active diode screen over the entire surface of a given wall. However, adequate improvements may be possible using the structure on only part of the wall.

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If large numbers of devices are to be used, it is probable that an automated system similar to that used in the population of printed circuit boards is required, with the FSS array taking the place of the circuit board.

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4.1.2 Device type

One advantage of operating at the comparatively low microwave frequencies of this study is that the devices required are likely to be less expensive than those needed for use at, for example, 10 GHz and above. 15 years ago the unit cost of components suitable for such high frequencies could easily exceed £20, but the devices used in the demonstrator at the University of Kent at that time cost about £1 each. The current price (June 2003) of the diodes used in the active FSS reported in [Ref 10] is £0.32 + VAT each for quantities of 1000+.

The reactance of the diodes in the ‘off’ state influences the length of the dipole elements required to give resonance at the design frequency. Less expensive diodes are more capacitive. The diode capacitance lowers the resonance frequency, resulting in shorter dipoles. The array unit cell size is reduced, so more unit cells and more diodes are needed to cover a fixed surface area. The resistive component in the diode equivalent circuit tends to reduce the opacity of the active FSS in its reflective state.

As an example, in background work for the field switching in [Ref 10], the diodes connecting adjacent linear dipoles had a series resistance of 7Ω and an off capacitance of about 0.3pF. The array substrate had a relative permittivity of about 3. To give a half wave resonance at 2.4GHz, the length of dipoles was 2.1cm compared with a free space half wavelength of 6.25cm. With less capacitive diodes, the dipole length increased to about 3cm. The unit cell area was reduced by a factor α of about 4.

4.1.3 Number of devices required

Suppose an FSS has a simple square unit cell and covers a wall area A. The number of unit cells in the array is typically ~ Aα / (λ / 2)2 where λ is the free space wavelength at resonance: for conducting strip dipoles this corresponds to the middle of the reflection band. In an active FSS consisting of a simple linear dipole array, the number n of diodes in each unit cell is 1. For a dual polarised design n = 2. The number of devices required for a singly polarised active FSS is therefore ~ nAα / (λ / 2)2.

On this basis, a wall 2.5m high and 3.5m long would therefore require about 18,000 active devices to cover it entirely, for operation near 2.4GHz. Even more would be required using designs such as that in Figure 11, where n = 4.

This result emphasises the need to investigate whether adequate frequency selective screening and transparency might be obtainable if part only of such a wall were to be covered by or include an active FSS. Work on sparsely populating the arrays with diodes is also relevant.

4.1.4 Manufacture


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4.1.5 Bias system

In some sample measurements, with a dipole array in the ‘on’ state, ie so that the FSS became transparent, the potential difference across each diode was about 0.7V and the forward current was about 10mA, a power dissipation of 7mW. An active wall containing 18,000 diodes would dissipate about 130Watts. In other measurements, a current of 1mA was adequate.

In the example of a wall 2.5m high, each vertical column of dipoles would contain about 80 diodes, requiring a total bias of about 60V. With about 110 columns for each direction of wave polarisation the forward current between about 100mA and 1A per polarisation would be required. A current limiting system is essential.

In the ‘off’ state, it may be necessary to provide a reverse bias across each diode, which could reach several volts per device. This would imply a potential difference of several hundred volts across each column, which may have significant implications for implementation in a real building environment.

Instead of PIN diodes, varactor diodes could be used. These have a variable capacitance with applied voltage to tune the response and as these are normally used in reverse bias, the current and power dissipation is much reduced. Varactors would also allow continuous movement of the frequency bands. However, the cost of these devices is currently prohibitive for a mass-market application.

4.1.6 Installation cost

This is likely to depend on the particular structure of the walls and cladding used in the building, and whether the FSS is to be retrofitted or installed during construction.

4.1.7 Maintenance

It is not clear how much servicing active FSS would require. In a large array, the performance of the FSS may not be sensitive to the failure of a single diode. If the diode becomes open circuit, the ‘on’ state is affected: one column in the array becomes partially reflective, with the dipoles no longer correctly joined together as a conducting strip. This failure should be detectable in the bias current. If it becomes a short circuit, in the ‘off’ state two adjacent dipoles are connected. The remaining dipoles in the array function correctly, and the array reflectivity would be largely unaffected.

[Ref 3] states that the failure histories of diodes show a preference for short versus open circuit failure by a factor of about 10 to 1, suggesting that a minor degradation of the array performance in the reflecting state is likely to be observed over time.

5. Regulatory issues

Active FSS systems may fall within the remit of two separate regulations; wiring codes and requirements for electrical devices. In this report, only the wiring codes are considered, where the device is assumed to be a “distributed wiring network”. However, if it is placed into a container of

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some kind, it may be considered as an “electrical device” and will need to comply with the appropriate regulations and practices.

There are also some issues arising from the enclosed FSS regarding fire risk as the enclosure will have to be RF transparent and therefore flammable. Comments on this have been appended where appropriate.

5.1 Diode grids

This system would comprise an array of vertical elements, each element being a set of diodes in series, with the same polarity. It is intended to act as a switchable reflector of radio signals: depending on the voltage applied across the array, the diodes would be forward biased and the array would be in its on-state, or reverse biased, corresponding to the array switched off.

It is anticipated that an array would cover part of a wall. The array would contain about 110 elements per direction of wave polarisation and, in a 2.5 m high wall, there would be about 80 diodes per element.

Figure 22: Diode grid

When switched on, about 60V would be applied across the array, which would then draw a current estimated at between 100 mA and 1A (i.e. a power dissipation of up to 60W). The array may need to be switched off by a negative potential of several hundred volts applied across it, to ensure that each diode is sufficiently reverse biased. In this state the array, should draw an insignificant current. As will be seen from the analysis below, careful design consideration will need to be given to ensure that the screens can satisfy current building regulations by minimising both current and voltage requirements.

A practical array is expected to take the form of a panel that could be mounted within or on a wall. Arrays printed upon wallpaper have also been postulated as a future development. Table 1 provides a summary of the anticipated impact of building regulations on implementation of diode grids in an office environment.

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Table 1: Impact of building regulations on diode grids

Regulation Pass/Fail Comment

A N/A

B Special measures needed to achieve compliance

An exposed enclosure of the system will need Class 0 rate of flame spread properties to comply with Requirement B2. This will probably exclude plastics materials (see note 1)

C - N N/A

Workmanship and buildability

Pass

1. If a panel is constructed so that the array is within an enclosure (i.e. a flat box, most likely also containing the power supply and controls) the latter would need to be RF-transparent. Satisfying this requirement with a plastics enclosure could introduce fire safety issues: a) if the assembly is incorporated into the wall, so as to form part of the wall surface, it may

infringe requirement B2 of the Building Regulations where Class 0 rate of flame spread properties are needed;

b) if the assembly does not form part of the wall – for example if it is mounted on the wall - it may nonetheless have to satisfy the requirements of the certifying fire authority made to control the fire load and ignitability of material within an escape route.

2. Although the Wiring Regulations (BS 7671) do not directly specify the construction of electrical equipment, they apply to the application of electrical equipment in an installation. “Electrical equipment” in the regulations includes apparatus.

3. The Wiring Regulations make a general requirement that equipment should be installed in such a way as to take account of the conditions likely to be encountered. A panel disguised within a wall, for example behind plasterboard, so that its presence is not obvious, might be regarded as unsafe if it is vulnerable to penetration by nails and the like and may deliver an electric shock.

4. An array printed on wallpaper is likely to be regarded as a wiring system with uninsulated conductors, fully within the scope of the Wiring regulations. For accessible bare conductors, the regulations permit two methods of protection against electric shock although neither is appropriate to the conditions of both the on-state and off-state of the array: a) Separated Extra Low Voltage. This effectively requires the conductors to be at a dc

voltage no greater than 120 volt and supplied by an isolating transformer whose output is isolated from earth. The supply need not be current limited and this arrangement would suit the on-state. If the diodes do not need to be reverse-biased, this regulation could also be used for the off-state.

b) Limitation of discharge of energy. Protection is deemed to be provided where the current that the equipment can supply is limited to a value unlikely to cause danger (i.e. significantly less than 50 mA). The supply voltage can exceed 120 volt and this method could provide protection for the off-state.

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See comments on Diode grids above for further details.

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5. Conceivably, a power supply designed to switch between the two conditions would satisfy the regulations.

6. However, in a number of respects, the wallpaper-array may be in conflict with the Wiring regulations: a) protection from shock by Limitation of discharge of energy, as described above is strictly

intended for equipment satisfying an appropriate British Standard; b) the regulations require every termination of live conductors or joint between them to be

contained within a non-ignitable enclosure; c) the regulations require bare live conductors to be installed on insulators.

5.2 Ferro-electric arrays

These are systems for the enhancement of wave propagation that are presently at a research stage, such that only their outline characteristics can be considered.

The operation of the arrays is due to the variation of the electric properties of the active material (ferro-electric) with applied electric field. They will need to be supplied with voltages likely to be in the range 100V to 1000V but should draw negligible current. In a practical form, they might be fashioned into demountable panels or placed in studwork walls. The devices must be kept dry and may be sealed in a protective layer. They are expected to be about 10 mm thick.

Table 2 provides a summary of the anticipated impact of building regulations on implementation of diode grids in an office environment.

Table 2: Impact of building regulations on ferro-electric arrays

Regulation Pass/Fail Comment

A N/A

B Special measures needed to achieve compliance

Enclosure of the array in a plastics enclosure may limit its deployment

C - N N/A

Workmanship and buildability Unknown

1. The minimal power demands of the array are such that risks of electric shock may be controlled

by Limitation of discharge of energy, achieved through the design of its power supply. The enclosure of the array will therefore not need to be insulating or impact resistant. It requires, however, to be RF-transparent and hermetic. If this is to be achieved with plastics materials, considerations of fire safety may limit the deployment of the devices.

2. Panels disguised in wall structures may be deemed electrically unsafe if their presence is not obvious.


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6. Overall conclusions

To the extent that the present state of development of the technology of active FSS can be judged satisfactorily from summaries published in the open literature, if active FSS are to be used in controlling the signal transmission properties of buildings, it appears that they would be of the applied voltage or current bias controlled type. Using this technology, a tuning range of about 10% may be achievable.

A variety of alternative schemes have been studied, but they either appear to be unsuited to large area applications (magnetically biasing ferrite substrates, for example), or await improvements in fabrication technology (e.g. ferroelectric substrates).

It is possible, of course, that in some cases reconfiguring the transmission characteristics may not need to be rapid, in which case a mechanical positioning of a suitable FSS screen might be more cost effective. But if the active FSS were to be used to tune out slow changes in the electrical properties of a building, caused perhaps by changes in moisture content, condensation or rain, an electronically controlled adjustment would be more convenient. It could not eliminate degraded transparency caused by changes in ohmic losses.

Various factors influencing the cost of incorporating active FSS into buildings have been addressed briefly. Sparsely populating the arrays with active elements may be a feasible way to reduce the overall numbers of active elements required, and hence the cost. If it could be demonstrated that adequate frequency selective screening or transparency could be obtained if only part of a wall, ceiling or floor were to be covered by or include an active FSS, then the price would come down, or perhaps alternative active technologies might become practicable.

From an implementation standpoint, there are obvious issues about designing safe power systems. In regulatory terms, there is insufficient information available to determine how these devices might fair; however, it would seem that there might be considerable work required to ensure compliance.

7. Acknowledgement

The authors are grateful to the Institute of Electrical Engineers for permission to reproduce some of the illustrations presented in this report.

8. References

[Ref 1] Dittrich, K.W. Multifunctional skins Proc. 6th European Electromagnetic Structures Conference, pp.1-13, 1991

[Ref 2] Chekroun, C., Herrick, D., Michel, Y., Pauchard, R, and Vidal, P. Radant: new method of electronic scanning Microwave J., pp.45-53, February 1981.

Prof E A Parker


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[Ref 3] Park, R.H. Radant lens: alternative to expensive phased arrays Microwave J., pp. 101-105, September 1981

[Ref 4] Jou, C.F., Lam, W.W., Chen, K., Stolt, N., Luhmann, N.C., and Rutledge, D.B. Millimeter wave diode grid frequency doubler IEEE Trans, MTT-36, pp.1507-1514, 1988.

[Ref 5] Lam, W.W., Jou, C.F., Chen, K., Stolt, N., Luhmann, N.C., and Rutledge, D.B. Millimeter wave diode grid phase shifter IEEE Trans, MTT-36, pp.902-907, 1988.

[Ref 6] Hacker, J.B., Weikle, R.M., Kim, M., de Lisio, M.P., and Rutledge, D.B. A 100 element planar Schottky diode grid mixer IEEE Trans, MTT-40, pp.557-562, March 1992.

[Ref 7] Sjogren, L.B., and Luhmann, N.C. An impedance model for the quasi-optical diode array IEEE Microwave and Guided Wave Lett., 1, pp.297-299, 1991.

[Ref 8] Parker, E.A. and Langley, R.J. An equivalent circuit study of a PIN diode switched active FSS Final report to British Aerospace plc., February 1990.

[Ref 9] Shuley, N.V. Diode loaded frequency selective surfaces Proc. JINA92 international conference on antennas, Nice, pp.313-316, 1992.

[Ref 10] Cahill, B.M., and Parker, E.A. Field switching in an enclosure with an active FSS screen Electron. Lett., 37, no. 4, pp.244-245, February 2001.

[Ref 11] Chang, T.K., Langley, R.J., and Parker, E.A. Active frequency selective surfaces IEE Proc., Microwaves, Antennas & Propagation, 143, pp.62-66, 1996.

[Ref 12] Mias, C. Waveguide and free-space demonstration of tunable frequency selective surface Electron. Lett., 39, no. 11, pp.850-852, 2003

[Ref 13] Tennant, A. Reflection properties of a phase modulating planar screen Electron. Lett., 33, no. 21, pp.1768-1769, October 1997.

[Ref 14] Tennant, A., and Chambers, B. Experimental dual polarised phase-switched screen Electron. Lett., 39, no. 1, pp.119-121, January 2003.

[Ref 15] McGinn, V.P. Photo-conductive element operative in the microwave region and a light steerable antenna array incorporating the photo-conductive element United States Patent no. 4,636,794, 1985.

[Ref 16] Nathanson, H.C., Driver, M.C., and Betsch, R.J. High attenuation broadband high speed rf shutter and method of making same United States Patent no. 4,922,253, 1990.

Prof E A Parker


Prof E A Parker

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[Ref 17] Vardaxoglou, J.C. Optical switching of frequency selective surface bandpass response Electron. Lett, 32, pp.2345-2346, 1996.

[Ref 18] Vardaxoglou, J.C., Lau,P.Y., and Kearney, M. Frequency selective surface from optically excited semiconductor on a substrate Electron. Lett., 34, no. 6, pp.570-571, 1998.

[Ref 19] Neelakanta, P.S., Abello, J, and Gu, C. Microwave reflection at an active surface imbedded with fast-ion conductors IEEE Trans, MTT-40, pp.1028-1030, May 1992.

[Ref 20] Parker, E.A., and Savia, S.B. Active frequency selective surfaces with ferroelectric substrates Proc. IEE, Microwaves, Antennas & Propagation, 148, no. 2, pp.103-108, April 2001.

[Ref 21] Chang, T.K., Langley, R.J. and Parker, E.A. Frequency selective surfaces on biased ferrite substrates Electron. Lett., 30, pp.1193-1194, 1994.

[Ref 22] Lima, A. C. de C., Parker, E. A. and Langley, R. J. Tuneable FSS using liquid substrates Electron. Lett., 30, pp 281-282, 1994.

[Ref 23] Lockyer, D., Moore, C., Seager, R. Simpkin, R., and Vardaxoglou, J.C. Coupled dipole arrays as reconfigurable frequency selective surfaces Electron. Lett., 30, pp.1258-1259, 1994

[Ref 24] Lockyer, D.S., and Vardaxoglou, J.C. Reconfigurable FSS response from two layers of slotted dipole arrays Electron. Lett., 32, pp.512-513, 1996

[Ref 25] Callaghan, P., and Parker, E.A. Loss-bandwidth product for frequency selective surfaces Electron. Lett., 28, p.365, 1992.

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Annex 5: Survey of Active FSS PDF, 1.6 MB