Microwave Patch Sensor

Technique Basics:

In the near-field mode, a microstrip patch antenna has been developed as a resonant sensor for microwave NDE.  The resonant frequency of the sensor is indicated by the minimum in the return loss, Figure 1.  A mathematical relationship has been developed that relates the resonant frequency of the patch antenna and the permittivity of the material under test, placed in contact with the sensor.  In this way, changes in material permittivity can be detected that may indicate material degradation or anomalies.

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Figure 1: Modeled and measured return loss of a microwave patch sensor in air. The resonance shifts depending on the permittivity of its environment and, in this way, degradation or anomalies in low-conductivity materials can be detected.

One particular application of such a sensor is the evaluation of aircraft radome structures, Figure 2. The radome houses the aircraft’s weather radar and is fabricated out of low-loss, low-permittivity composite materials, such as fiberglass with foam or honeycomb core. For accurate operation the radome must appear as “electrically transparent” as possible to the radar. Any variations in electrical permittivity caused by excess resin, water ingression, excess paint, or impact damage for example, will hinder the radar’s ability to sense accurately.

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Figure 2: Severely damaged aircraft radome.

 

A resonant patch sensor operating in the X-band (8.2 to 12.4 GHz), Figures 3 and 4, has been designed and tested at CNDE.  The sensor detects defects in particular layers of a multilayered dielectric structure, such as an aircraft radome, in the form of permittivity variations.

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Figure 3: Front view and back view of the resonant microwave patch sensor designed to inspect aircraft radome sandwich structures.

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Figure 4: Resonant microwave patch sensor inspecting a Saint Gobain radome sample.

 

Results:

To test the patch sensor’s performance in detecting water ingression in a glassfiber-honeycomb-glassfiber sandwich structure, a 2-D scan test was conducted over a region containing a single water-filled honeycomb cell (around 0.25 cc), Figure 5.  A honeycomb core with fiberglass skins similar to that used in a typical radome was used as the specimen for the water detection test. Water was injected through the fiberglass shell using a hypodermic syringe.

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Figure 5: Photograph of fiberglass-honeycomb-fiberglass test-piece top surface with water injected into 1 honeycomb cell (within the circle). The imaged region is indicated by the dashed rectangle, using the sensor geometric center as the origin.

 

In the image result of the sensor resonant frequency measured over the scan region, Figure 6, the area of water ingression (blue) can be clearly observed. It can be concluded that this sensor shows good NDE performance in detection of water present in this simulated radome structure.

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Figure 6: Sensor resonant frequency measured over the scan region. The actual position of the water-filled honeycomb cell is indicated by the white dashed line. Scan region length x width= 52 = 42 mm2.

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