K.L. Ford

Design Methodology for a Miniaturized Frequency Selective Surface Using Lumped Reactive Components

The design methodology is described for a miniaturized frequency selective surface (FSS) using lumped reactive components. Capacitive and inductive elements are utilized in metallic patches to create current loops similar to that observed in bandpass aperture type FSS. It is shown that the resonant frequency of the FSS is controlled mainly by the values of the lumped components and to a lesser extent by the distance between the components. It is insensitive to the periodicity. Issues such as the effects of component loss and oblique incidence are reported using full field predictions. Measurements are shown for an FSS with unit cell periodicity of ¿/36.

Oblique Incidence Performance of a Novel Frequency Selective Surface Absorber

Oblique incidence performance of a novel two-layer absorb/transmit frequency selective surface (FSS) is investigated. The FSS has good frequency stability for both horizontally and vertically polarized waves incident normally or at oblique angles. Due to its transmission for 900/1800/1900 MHz mobile bands and good absorption for 5 GHz waves, it has the potential as a security wall or isolator for 5 GHz WLAN systems. The absorption in the stop band helps reduce additional WLAN multipath fading caused by conventional reflecting FSS designs. The first layer of the FSS consists of conventional conducting cross dipoles having a circular aperture in the centre, while the second layer uses resistive cross dipoles. Moreover, the conducting cross dipoles have been sandwiched between two dielectric sheets to achieve a stable response for different angles of incidence. The periodicity of both FSS layers is the same while the distance between the two layers is reduced to one eighth of the free-space wavelength. This reduction leads to a more compact design compared to the conventional Salisbury screen, while still achieving acceptable absorption in the stopband. Both theoretical and experimental results are presented to confirm the performance of the absorb/transmit FSS.

Switchable Frequency Selective Surface for Reconfigurable Electromagnetic Architecture of Buildings

A frequency selective surface (FSS) that is electronically switchable between reflective and transparent states is tested. It can be used to provide a spatial filter solution to reconfigure the electromagnetic architecture of buildings. The FSS measurements show that the frequency response of the filter does not change significantly when the wave polarization changes or the angle of incidence changes up to ±45° from normal. The FSS is based on square loop aperture geometry, with each unit cell having four PIN diodes across the aperture at 90 degree intervals. Experiments demonstrated that almost 10 dB additional transmission loss can be introduced on average at the resonance frequency, for both polarizations, by switching PIN diodes to on from off state.

Independently Tunable Low-Profile Dual-Band High-Impedance Surface Antenna System for Applications in UHF Band

An independently tunable low-profile dual-band high-impedance surface (HIS) antenna system is presented which employs a single layer cross bow-tie dual-band tunable HIS ground plane with a printed wideband monopole antenna designed to cover the UHF band. The unit cell of the proposed HIS ground plane is 19.71 mm by 19.71 mm with thickness of 1.74 mm. Simulations of the HIS ground plane have illustrated that either single or dual-band reconfigurable reflection phase resonances can be achieved between 0.95 GHz and 2.45 GHz by controlling C1 (4.55 pF~0.6 pF) and C2 (4.55 pF~0.6 pF) independently which represent the two groups of varactor diodes mounted on the surface elements of the HIS. Simulations with the wideband antenna 4 mm above the HIS ground plane illustrate an effective tunable operational bandwidth from 1.07 GHz to 2.75 GHz with a dual-band capability. Parametric studies indicate the radiation efficiency of the system increases as the antenna height is increased and the diode resistance is reduced. The radiation efficiency also increases with the frequency in general over the tunable frequency range and with the optimum parameters typical values from -4 dB to -2 dB can be achieved. The proposed HIS ground plane and the wideband antenna have been fabricated with Toshiba 1SV245 varactor diodes and it has shown an effective tunable bandwidth from 0.9 GHz to 2.8 GHz approximately with the dual band capability.

Improvement in the Low Frequency Performance of Geometric Transition Radar Absorbers Using Square Loop Impedance Layers

A technique is described for improving the low frequency performance of geometric transition (GT) radar absorbers based on lossy foam pyramids. The technique makes use of the fact that at high frequencies, only the geometric transition region of the absorber is utilized whereas at low frequencies, the whole absorber thickness interacts with the incident wave. Hence the low frequency performance may be improved, without compromising that at high frequencies, by electrically loading the absorber base layer using one or more frequency selective surfaces (FSS) whose elements are typically in the form of single or nested loops. Other advantages of this technique include minimal increases in weight and manufacturing costs. The paper includes comparative predictions of unmodified and loaded GT absorber reflectivity at both normal and oblique incidence and discusses the effect on absorber performance of tolerance variations in the dimensions and location of the loading FSS elements. Finally, free-space reflectivity measurements on unmodified and loaded commercial absorber blocks are made over the frequency range 1-10 GHz and these confirm the validity of the technique.

Active frequency selective surface using PIN diodes

A novel active frequency selective surface (FSS) is presented for 2.45 GHz applications. It consists of circular aperture elements with four PIN diodes placed orthogonal to each other on FR4 substrate. The negative dc biasing is provided with cross shaped supply lines from the reverse side of the substrate, while positive biasing is provided from the front side of printed circular structure. This active FSS design enables transmission to be switched on or off at 2.45 GHz, providing high transmission when the diodes are in OFF state, and high isolation when the diodes are ON. The design also provides very good stability to oblique TE incidence, i.e perpendicular polarisation.

Application of Impedance Loading to Geometric Transition Radar Absorbent Material

This paper describes a novel, lightweight technique for significantly improving the low-frequency reflectivity performance of conventional geometric transition radar absorbent materials as used typically in anechoic chamber facilities for electromagnetic compatibility testing. The improvement is achieved by the inclusion of impedance-loading elements within the base region of the absorber, and these are implemented in the form of one or more frequency-selective surfaces (FSS). The discussion covers the design of the FSS using computer simulation technology microwave studio, its predicted effect on absorber performance at both normal and oblique incidence, the effect of manufacturing tolerances, and the fabrication and characterization of a prototype-loaded absorber panel.

A miniaturised dual band Artificial Magnetic Conductor using lumped components

This paper describes the design of a dual band Artificial Magnetic Conductor (AMC) with the use of lumped reactive components. The AMC comprises of a single layer of metallic patches, which have unit cells with capacitors between the patches. Dual band performance is achieved by having two different capacitors on alternating unit cells. The performance of the AMC is evaluated using simulations in CST microwave studio. The impact of the values of the capacitances in the AMC is assessed, with respect to frequency band separation. The AMC is designed to operate at 430MHz and 900MHz.

Novel Planar Band Pass Lump-Loaded Frequency Selective Surface

A novel planar bandpass lump-loaded Frequency selective surface (FSS) is presented. Based on the artificial transmission line current loop realized by the lumped loaded elements, this novel bandpass FSS has overcome the traditional resonant type FSS limitation of the mutual dependence between the dimensions and bandwidth. A lumped-loaded band pass FSS whose unit cell is only lambda/20 with 18.6% bandwidth is manufactured and tested.

Secure Electromagnetic Buildings Using Slow Phase-Switching Frequency-Selective Surfaces

The concept of a secure electromagnetic building (SEB) which can successfully prohibit wireless communications is presented. Wireless security is achieved using a slow phaseswitching technique and can be realized by time-varying the transmission properties of a frequency-selective surface (FSS) to increase the bit error rate (BER) of the unwanted signal. Results are presented which demonstrate that a technique of phase switching at rates much lower than the baseband data rate can be used successfully. The system has been implemented using a reconfigurable dual-polarized dual layer FSS incorporating varactor diodes where over 100° of phase change can be achieved for voltage changes of ±0 - 3V . A vector signal analyzer was used to evaluate the BER performance of the system for a GSM signal operating at 2 GHz. BERs are shown to be as high as 36% which are sufficient to successfully prohibit wireless communication. The solution is also shown to be robust over a wide range of incidence angles, which is important for real-world applications where the location of the prohibited wireless source may be unknown or mobile. Furthermore, as the system is reconfigurable, the building can be switched between secure and nonsecure modes.