DIAGNOSTICS OF HETEROGENEOUS POLYMER
MATERIALS USING NOISE-LIKE MM-WAVE RADIATION
B. Kapilevich , B. Litvak, T. Ben Yehuda and O. Shotman The College of J&S, Dept. of Electrical and Electronics Eng. The Israeli Center for Radiation Sources and Applications, Ariel, Israel
ABSTRACT. The paper describes preliminary study depolarizing effects taken place in dense medium consisting of polymer granules. We have compared forward scattering losses when medium is illuminated by coherent and incoherent (noise-like) mm-wave radiations. In case of coherent illumination we have observed strong depolarizing effects leading to increasing forward scattering losses while incoherent illumination leads to averaging depolarization resulting in relative reduce forward scattering losses. In order to perform incoherent experiments we have designed and assembled the wide band noise sensor operating in the W-band (75-110 GHz).
Introduction.
Investigation of complex permittivity spectrum of heterogeneous materials using electromagnetic wave scattering in non-homogeneous media is a powerful tool for estimation of the dielectric relaxation process, the activation energy of the conductivity, the polarization mechanisms and motions of charge carriers. Based on such a study it is possible to predict electrical properties of bulky samples fabricated from polymer powders [1]. Another area associated with microwave characterization of powders is nanoparticle-filled polymers that provide improving electrical and termo-mechanical properties of dielectric materials [2]. For example, the composites fabricated with ~0.1 µm Cabot BT-8 hydrothermal powder, showed a broad relaxation over the measured frequency range [3]. The reduction in particle size and subsequent modulation of the domain structure are responsible for the shift in the relaxation frequency. The dielectric properties of ceramics and powders of PbTiO3, composited with paraffin, were measured from 1 MHz to 18 GHz. All samples exhibited evidence of relaxation phenomena in their dielectric spectra [4, 5].
However, when electromagnetic properties of a nonhomogeneous material are described by an effective complex propagation constant, there is assumed a quasi-homogeneous structure of the medium. For a two-component material composed of a discrete scatterers embedded in a background (host) medium, the effective propagation constant depends on the other additional factors [6]:
• Volume fraction occupied by scatterers;
• Electromagnetic properties of the scatterers and background medium; • Statistical description of the position of the scatterers;
• Polarization and coherency of electromagnetic waves.
There are a limited number of experimental studies of heterogeneous dense media when the roles of depolarization and coherence effects are taken into consideration [7]. Such effects can be easily observed in mm-wave range when particle dimensions are comparable with wavelength. In this paper we present a study of mm-wave forward scattering losses in Nylon granular material illuminated both coherent and incoherent radiation. Such additional experimental information will be useful in development and testing new theoretical models describing wave propagation through heterogeneous media.
Description of the experimental setup
Since we are going to compare forward scattering losses for coherent and incoherent illuminations, the two experimental setups have been assembled both using a free space technique. Figure 1 shows the setup employed for coherent forward scattering experiments. It consists of the foam polystyrene container being transparent for mm-waves and filled with granular polymer materials placed between transmitting (Tx) and receiving (Rx) identical horn-lens antennas. They have the gain about 30 dB and the beam width about 5 deg. on the 3 dB level. The coherent transmitter is assembled of Agilent signal generator and Agilent mm wave source module which are used to produce the W-band signal. In the Rx part we use an isolator to prevent reflections of the W-band signal and than the signal is received by Agilent 8757D scalar network analyzer.
Fig.1. Rx-Tx configuration for coherent measurements in 75-110 GHz range. Similar setup was used for incoherent measurements but we replace Tx part on full W-band Farran noise source, modulated by a pulse generator at the frequency of 1 KHz.
To receive a noise-like signal the wide band mm-wave sensor has been developed. Receiving antenna (Rx) is connected with the W-band LNA (gain about 15 dB), then the signal is down-converted by mixer with IMPATT LO (94 GHz). The IF signal is amplified by wideband amplifier (gain about 70 dB) with the bandwidth about 6 GHz so that the total double-side bandwidth is 12 GHz. The signal is detected by a wideband HP- 423A detector and amplified again using a video amplifier (gain about 10 dB). Tektronix Digital Scope is employed for recording forward scattering signals, Fig.2. Granular polymer materials under study have been placed into the foam polystyrene container transparent for mm-waves in both experimental setups that permitted to compare the role of depolarisation effects more accurately. The two shapes of granules were studied: spheroids of the diameter d = 3/32" (Nylon, Teflon), and d = 1/8" (Polypropylene).
Fig.2. Rx-Tx configuration for incoherent measurements in 80-100 GHz range. Granule distributions within the container are similar to investigated in [8] and shown in Fig.3a where the diameter of container D = 27mm. Both single and multi layers distributions have been investigated in the experiments described below. The general view the experimental setup is depicted in Fig. 3b.
Fig.3. Examples of granules distribution within the container (a) and general view of (coherent) experimental setup (b).
(a) (b)
Results of testing W-band sensor
To perform incoherent experiments we have designed and assembled the wide band noise sensor operating in the band 88-100 GHz. It was tested using different modes of operation to estimate the role of video and RF components and their contribution in the system noise performance. Noise performance has been estimated using both DC and AC output signals. All measurements were carried out without averaging. Both above parameters were measured as shown in Fig.4: Vdc – output DC
Fig.4. Illustration of the measured parameters: Vdc – output DC voltage of rectified
noise and Δ – output AC voltage of the same noise.
To estimate a contribution of each element of the sensor into the system noise performance, they were independently switched "off" – "on" by means of stet-by-step procedure as follows:
• Video amplifier ( see Fig. 5a);
• Video amplifier + IF amplifiers ( see Fig.5b );
• Video amplifier + IF amplifiers + Local oscillator ( see Fig. 5c);
• Video amplifier + IF amplifiers + Local oscillator + W-band LNA ( see Fig. 5d). • Video amplifier + IF amplifiers + Local oscillator + band LNA + Farran
W-band noise source ( see Fig. 5e).
It should be pointed out that the final experiment with the Farran W-band noise source has been done with additional attenuation - 7.4dB at its output port. Based on the above measurements we have estimated the noise performance of designed mm-wave sensor using Y-factor method [9]:
• Estimation of the Noise Factor using DC output
1. Output power ratio Y = Vdcout /Vdcin = - 6.56V/-4.72V = 1.39 2. Actual ENR = 13 dB - 7.4 dB = 5.6 dB or E = 3.63
3. Noise factor F = E/(Y-1) = 3.63 / (1.39 – 1) = 9.3 or 9.7dB
• Estimation of the Noise Factor using AC output
1. Output power ratio Y = Vdcout /Vdcin = 480mV/360mV = 1.33 2. Actual ENR = 13 dB - 7.4 dB = 5.6 dB
or E = 3.63
3. Noise factor F = E/(Y-1) = 3.63 / (1.33 – 1) = 11 or 10.4dB
Both approaches have demonstrated close results and prove an efficiency of the designed sensor for incoherent experiments.
Experimental results – single layer case
Examples of measured forward scattering losses for spheroid shaped granules are shown in Fig.6: Nylon (A), Teflon (B) and Polypropylene (C). The measurements were done for coherent illumination, co-polarized alignment of Rx and Tx antennas
explained by bulky material behavior in frequency domain. For example, Polypropylene granules (N=100) has demonstrated strong variation of co-polarized loss more than 10 dB in the range 75-110 GHz. Similar experiments have been carried out for cross-polarized alignment of Rx and Tx antennas. As an example, measured
forward scattering cross-polarized losses are depicted in Fig.7 for Polypropylene granules. There is observed decreasing cross-polarized loss for N=20.
(a)
(b)
(c)
(d)
(e)
Fig.5 Illustration a contribution of the different elements of mm-wave sensor in the system noise performance.
Loss, dB
(A)
(B)
(C)
Frequency [GHz]
Fig. 6. Measured coherent forward scattering losses for different spheroid granules (A - Nylon, B - Teflon, C – Polypropylene) after averaging in co-polarized antennas
alignment with number of particles N = 20, 60, 100. Loss, dB
Frequency [GHz]
Fig. 7. Measured coherent forward scattering losses for different spheroid granules Polypropylene after averaging in cross-polarized antennas alignment and number
particles N = 20, 60, 100
In general, forward scattering power component Pforward consists of the two
parts: co-polarized power Pco-pol and cross-polarized power Pcross-pol resulting in Pforward
= Pco-pol + Pcross-pol. The observed behavior clearly indicates existing depolarization
effects depending on the ensemble structure – different N number.
The experimental setup used for incoherent forward scattering experiments depicted in Fig. 2 allows observing the forward scattering signal in frequency domain when we have connected Anritsu Spectrum Analyser at the IF amplifier output of the sensor. Some typical results recorded for spheroid granules (Polypropylene) illuminated by incoherent radiation are shown in Fig.8. Comparison with the coherent experiments of the same material reveals considerable reduction of forward scattering losses when incoherent source has been applied. Since depolarization effects are frequency sensitive, the application of incoherent source leads to their averaging in
Loss, dB
Frequency [MHz]
Fig. 8. Recorded forward scattering signals in IF frequency domain for Polypropylene granules illuminated by incoherent source
Experimental results – multi-layers case
We have also investigated forward scattering losses in multi-layers Nylon granules of arbitrary form with size variations within 2-4 mm. Multi layers configurations were composed of granules filled with different thickness inside the container. Figure 9 shows
Layers contribution -14 -12 -10 -8 -6 -4 -2 0 0 0.5 1 1.5 2 2 Thickness [cm] .5 Loss, dB
Fig. 9. Measured incoherent forward scattering losses for heterogeneous granular Nylon as a function of granules thickness.
the measured forward scattering losses as a function of averaged layer’s thickness marked by black boxes. Measurements were done using frequency limited incoherent illumination in 88 – 100 GHz range. A similar experiment was performed with averaged layer of 1 cm thickness illuminated by coherent signal scanning within 88-100 GHz range, Fig.10. The container with granules was rotated on the angles 0, 45, and 90 deg. in the plane perpendicular to the direction of wave propagation in order to estimate depolarization effect. This effect causes non-monotonic change of forward scattering losses with increasing frequency resulting in a variation of losses within 19-25 dB. The layer of same thickness illuminated by like-noise radiation had about 7 dB losses only.
1 cm thickness -26 -24 -22 -20 -18 88 90 92 94 96 98 100 f [GHz] Loss, dB
Fig. 10. Measured coherent forward scattering losses for heterogeneous granular Nylon with thickness of 1 cm and different angular orientation of the container with granules.
Conclusion
Comparative studies of forward scattering losses in dense heterogeneous medium have been done using coherent and incoherent radiation in mm-wave range. Tested materials were polymer granules having size comparable with wavelength. Depolarizing effects leads to additional increase of forward scattering losses for coherent illumination while incoherent illumination demonstrates much less attenuation for the same materials.
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