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Frequency modulated thermal wave imaging for visualizing power density of electromagnetic waves on plane surfaces

In this article, Frequency Modulated Thermal Wave Imaging (FMTWI) [1–6] is introduced for the first time for determining power distribution of electromagnetic waves on plane surfaces. The advantage with this technique is that we can extract multiple amplitude and phase images from a single run of experiment. The applied excitation signal in this technique is a frequency modulated chirp signal instead of a single frequency signal used in conventional lock-in infrared (IR) thermography [7–11]. The thermal images obtained using FMTWI can be used qualitatively, e.g., to detect field leakage near electromagnetic junctions and microstrip feed lines. As a practical demonstration of this technique, an example of 2 × 2 patch antenna array at 8 GHz is considered. First, amplitude images at various modulation frequencies are obtained. Next, signal to noise ratio (SNR) values at each frequency are calculated. It is seen that SNR is lower at higher frequencies. It is observed that at higher modulation frequencies, micro-strip lines feeding the individual patch antennas of the array, are not visible in amplitude images, while at lower frequencies they are clearly visible. Mathematical modeling of the microwave absorption screen has also been carried out to show variations of incident, reflected, and transmitted powers as a function of screen surface impedance. It is also observed that the screen minimally perturbs the electromagnetic fields.


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