COMPLEX PERMITTIVITY MEASUREMENT SYSTEM FOR DETECTION OF SOIL CONTAMINATION
Table 1 shows the average of complex permittivities and standard deviations of air measured by the
short and long sample holders in three frequency ranges, i.e. 50 -200 MHz, 200-400 MHz, and 400-500
MHz. The averages were compared with the value reported in the literature (εair=1. 0006, Rao 1977).
The deviation between the measured and literature values in the frequency range of 50-200 MHz was
slightly larger than those in other frequency ranges. The deviations in 200 MHz to 400 MHz range
were –0.25% and +0.14% for the short and long holders, respectively. In the higher frequency range of
400-500 MHz. the deviations were –0.92% and –4.8% for the short and long holders, respectively. The
increased deviation in the long holder measurement indicates the beginning of dielectric dispersion. It
can be noticed from the results that in the frequency range of 200-400 MHz the complex permittivity
measurements had the least deviations for both long and short holders compared to other frequency
ranges.
TABLE 1- Calibration of sample holders with air
ε’air=1.0006 (Rao, 1977)
f(MHz)
ε’(short)
ε’(long) (ε’(S)-ε’air)/ ε’air% (ε’(L)-ε’air)/ ε’air%
50 -200
0.9873
1.0020
-1.72
0.14
200-400
0.9980
1.0020
-0.25
0.14
400-500
0.9914
0.9526
-0.92
-4.8
Calibration Step 2: Measuring the Complex Permittivity of Polyethylene
Polyethylene is a polymer consisting of two carbon atoms bonded together with four hydrogen atoms
(Callister 1997). It has excellent dielectric properties. Polyethylene has high hardness, excellent water
resistance and is inflammable. It has applications in various fields of high frequency such as radar cables
(Von Hippel 1995).
The 2nd step in the calibration of the complex permittivity measurement systems for both short and long
sample holders was carried out by measuring the relative permittivity of a polyethylene sample with a
known relative permittivity (εPE = 2.26 at 24 oC , Von Hippel 1995). The polyethylene sample used in
the measurement was 35mm long with an annular shape that fills the cavity between the inner and outer
conductors of the sample holder cell. The relative permittivity of the polyethylene sample was measured
six times using the short holder. The repeatable measurements were highly consistent with a standard
deviation of 0 008. The results of the measured complex permittivity versus frequency are shown in
Figure 4 for both long and short holders. The resonance was not observed within the applied frequency
(0.3-1300 MHz); this is consistent with Eq.2, which predicts the first resonance frequency will occur at
5532 MHz.
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