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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
Consultant
E A Parker
S Massey
ERA Report 2004-0072 A5
ERA Project 51-CC-12033
FINAL Report Annex 5
Client
:
Client Reference :
Ofcom
AY4464
Report edited and checked by:
Approved by:
Martin Shelley
Project Manager
Robert Pearson
Head of Antenna Systems
March 04
Ref. 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
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ERA Report 2004-0072 Annex 5
 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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Prof E A Parker
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ERA Report 2004-0072 Annex 5
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.
4.
Manufacturing costs
20
3.1
Cost elements
20
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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ERA Report 2004-0072 Annex 5
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
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Table list
Page No.
Table 1: Impact of building regulations on diode grids
24
Table 2: Impact of building regulations on ferro-electric arrays
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ERA Report 2004-0072 Annex 5
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:
•
•
•
3.1
active devices are embedded in the FSS,
substrates have variable electrical properties,
mode coupling between cascaded arrays which can be altered.
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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Transmission, dB
0
-10
Reflecting state at 2.4 GHz
Transmitting state
-20
-30
1
2
3
4
5
6
Frequency, GHz
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”
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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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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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
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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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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ERA Report 2004-0072 Annex 5
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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ERA Report 2004-0072 Annex 5
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.
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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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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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
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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4.1.5 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.
Prof E A Parker
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ERA Report 2004-0072 Annex 5
Table 1: Impact of building regulations on diode grids
Regulation
Pass/Fail
A
N/A
B
Special measures
needed to achieve
compliance
C-N
N/A
Comment
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)
Workmanship Pass
and buildability
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.
Prof E A Parker
25
ERA Report 2004-0072 Annex 5
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
A
N/A
B
Special measures needed to
achieve compliance
C-N
N/A
Workmanship and buildability
Unknown
1.
2.
Comment
Enclosure of the array in a plastics
enclosure may limit its deployment
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.
Panels disguised in wall structures may be deemed electrically unsafe if their presence is not
obvious.
See comments on Diode grids above for further details.
Prof E A Parker
26
ERA Report 2004-0072 Annex 5
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.
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ERA Report 2004-0072 Annex 5
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ERA Report 2004-0072 Annex 5
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Prof E A Parker