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This paper presents the design, construction, and testing of grounded frequency selective surface (FSS) array as a diffuser for destroying millimeter wave coherence which is used to eliminate speckle in active millimeter wave imaging. To create stochastically independent illumination patterns, we proposed a diffuser based on random-phase distributions obtained by changing the incident frequency. The random-phase diffuser was obtained by mixing up the phase relations between the cells of a deterministic function (e.g., beam splitter). The slot length of FSS is the main design parameter used to optimize the phase shifting properties of the array. The critical parameters of the diffuser array design, such as phase relation with slot lengths, losses, and bandwidth, are discussed. We designed the FSS arrays with finite integral technique (FIT), fabricated by etching technique, and characterized the

Free-space active millimeter wave
(mm-wave) systems have gained more and more attraction during the last few
years due to their indoor security applications. There is no incoherent mm-wave
source, and highly coherent mm-wave sources produce speckle in active mm-wave
imaging of conceal objects because of interference phenomenon [

Reflect array with patch antenna
requires tight fabrication tolerances to achieve desired phase value, as the
patch size versus phase curves are extremely nonlinear [

This paper is organized as follows. Section

The
design of W-band quasioptical filters, consisting of periodically perforated
slots on metal backed Roger 4003C substrate, is considered. As shown in Figure _{r} = 3.38, and loss
tangent 0.0027. Before metallization, a 10

Unit cell of rectangular slot grounded FSS.

We considered three grounded FSSs with slot lengths of
896

As
discussed in our paper [_{11}) at
resonant frequencies are −2.17 dB, −2.53 dB, and −2.68 dB for curves
(i), (ii), and (iii), respectively, which prove that the FSSs reflect in full
W-band, and the total W-band phase is useable in reflection mode. The
realization of slot FSS in compare to the patch demonstrate that for FSS, the
maximum transmission occurs at the resonant frequency. Therefore, there are
losses at resonance since some portion of the energy is lost in the area
between FSS and the ground plane. The FSS suffers higher losses at the
resonance in compare to patch, but the phase variation with frequency of the
FSS is more linear than patch antenna. This property is more advantageous than
the higher reflection gain.

Measured
reflection amplitudes of different slot length grounded FSSs. (Slot length of curve
(i) 1076

Figure ^{˚}, −176.6^{˚}, and
−79.16^{˚}, respectively. So, the phase delay between the curve (i) and curve
(iii) is 180^{˚}. At 91.61 GHz, the slot length variation of 106 ^{˚} phase variation in the
W-band. This value can be increased by decreasing the slot length and by
introducing higher order modes.

Measured phase versus frequency
curves, slot lengths as variable parameter. (Slot length of curve
(i) 1076

Till now, we have shown how phase
delay can be introduced by using FSS of different slot lengths. This investigation
demonstrates that each FSS cell with unique slot length is equivalent to a
delay element, and the amount of delay is determined by the slot length. As an
example of deterministic function realization, mm-wave beam splitter by FSS
array is consider in this section. The idea of this design is to introduce
variable phase delay in constant phase of a coherent mm-wave plane wave to
split the beam in two directions to observe the phase-delay effect. The
schematic diagram of the beam splitter array is shown in Figure

Slot lengths,
corresponding phase values, and phase delays of beam splitter. The patterns of the left most column of the table represent
different columns of Figure

Phase (degree) | Slot length
( | Delay (degree) | |
---|---|---|---|

−138^{˚} | 1097 | −61^{˚} | |

−128^{˚} | 1048 | −54^{˚} | |

−116^{˚} | 1013 | −47^{˚} | |

−103^{˚} | 990 | −40^{˚} | |

−89^{˚} | 972 | −34^{˚} | |

−75^{˚} | 959 | −29^{˚} | |

−61^{˚} | 949 | −23^{˚} | |

−47^{˚} | 941 | −17^{˚} | |

−30^{˚} | 931 | −11^{˚} | |

0^{˚} | 917 | 0^{˚} | |

32^{˚} | 903 | 32^{˚} | |

49^{˚} | 895 | 49^{˚} | |

63^{˚} | 888 | 63^{˚} | |

78^{˚} | 879 | 78^{˚} | |

91^{˚} | 870 | 91^{˚} | |

105^{˚} | 858 | 105^{˚} | |

116^{˚} | 846 | 116^{˚} | |

127^{˚} | 832 | 127^{˚} | |

138^{˚} | 812 | 138^{˚} | |

147^{˚} | 785 | 147^{˚} |

Schematic drawing of beam splitter FSS array. (Patterns
follow the values listed in Table

The radiation patterns of the arrays were
measured with backward-wave oscillator (BWO) in motorized setup. The measurement
was carried out with 45^{˚} angular position of the antenna axis to the BWO
millimeter wave source axis (in measurement Figures ^{˚}. At 94 GHz measurement, two lobes were obtained at 130^{˚}
and 118^{˚} due to the phase variation of beam splitting. The measured rectangular and polar radiation
patterns of the beam splitter are shown in Figures

Measured near field radiation pattern of random phase diffuser: (a) rectangular plot (b) polar plot.

Measured near field radiation pattern of beam splitter array at different incident frequencies: (a) rectangular plot (b) polar plot.

The measurement radiation patterns at
90 GHz, 94 GHz, and 100 GHz frequencies are presented. The results show that at
each frequency, we get two main lobes on both sides of the antenna broad side
axis. The position of lobes changes with frequency as the phase values of the
FSS cells changes with frequency. Figure

As explained for beam splitter in Section

Schematic drawing of
coherent destroying diffuser. (Patterns correspond to the phase values listed
in Table

As already mentioned, the amounts of phase delay listed in Table ^{˚} to 150^{˚} is shown in rectangular plots of Figures

We have designed a coherence destroying diffuser system using passive FSS
array. We explained the slot length dependence phase variation properties of
FSS, and also showed how phase delay can be controlled by changing slot
lengths. We presented the design of mm-wave beam splitter as deterministic
function which split the coherent beam in two directions and then shown the
coherent destroyer by reshuffling the cell columns of beam splitter array. This
coherent destroyer can also be used as multifrequency diffuser system. In
multifrequency diffuser system, at each frequency, the radiation pattern is
different, and also the reflection is diffused reflection, that is, the
reflected signal is incoherent. This type of reflector array is capable to
destroy the coherence of the coherent mm-wave sources as the array behaves like
a phase modulator which introduces random
phases. In multifrequency diffuser approach, the frequency difference
should be large enough, since a small difference generates quite similar
speckle distributions [

This work was partially funded by the Vrije Universiteit Brussel (VUB-OZR), the Flemish Fund for Scientific Research (FWO- G.0041.04), and the Flemish Institute for the encouragement of innovation in science and technology (IWT-SBO 231.011114).