·Table of Contents ·Civil Engineering | Microwave imaging of concrete wallsG. DeshchenkoInstitute of Applied Physics, National Academy of Sciences, Minsk, Belarus Contact |
Fig 1: Geometry of synthetic aperture system for the case of concrete wall. |
(1) |
where S(x_{n,} w_{k}) - signal measured at position x_{n }and frequency w_{k}, n - speed of light at tested object [3]. Now if we store the frequency dependence of reflected signal at some points along the surface of tested objects (Fig.1), then by (1) we can determine the inner structure of objects.
Fig 2: Depth distribution of reflections, calculated on experimental data. |
(2) |
On Fig.3. presented the reflection distribution, calculated after such background subtraction. It is obviously, that level of noise reflections there are less, than on Fig.2. but the width of main peak, that corresponds to metal rod, is enough big. It is because of frequency property of used antennas.
Fig 3: Depth distribution of reflections, calculated on experimental data after subtracting the average value on every frequency |
(3) |
Such method can be used only with continues wave step-frequency radar, that is one of advantages of such device against impulse radars. The results of this operation can be seen from Fig.4. The width of main lobe became less, but noise also increase. So this method can be applied for the case, when necessary good accuracy in determining the depth of inhomogeneities in the testing objects.
Fig 4: Depth distribution of reflections, calculated on experimental data after normalization on free-space signal of used antennas. |
Fig 5a | Fig 5b |
Fig 5: Depth distribution of reflections, calculated on experimental data for test specimen of concrete with 2.5 cm air cavity on different depths from the surface ( 5 cm and 8 cm). |
The experiments with other objects also shows, that quality of received image (level of noise) also depends on the space interval between points of mesurements Dx_{n}=x_{n}-x_{n-1}. The optimal interval is depends on the frequency band of used signal and maximal depth of inhomogeneities in tested objects. So, on Fig.5. is different level of noise for the same experimental condition. The interval of 0.5 cm is optimal for depth 8 cm and frequency band 2-4 GHz.
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