Near field focusing for nondestructive microwave testing at 24 GHz - Theory and experimental verification

- The impact of near field characteristics of antennas on high resolution microwave based NDT is analysed.
- Antenna concepts for microwave based NDT were evaluated regarding spatial resolution, penetration depth and return loss.
- Near field simulations are verified using the Electromagnetic Infrared Method.
- Horn antennas with perforated metal aperture are able to produce high resolved images at a certain depth of the DUT.
- The focusing characteristics of horn antennas with delay lens and reduced aperture size are permittivity independent.

This paper describes the development of different novel antenna concepts for improving the spatial resolution of microwave based non-destructive testing (NDT) at 24 GHz. In a great number of applications the antenna of the sensor can be brought very close to the device under test. In these cases, the near field characteristics of the antennas are crucial for a high resolution. However, common sensor heads offer either a high image resolution or a high penetration depth. In order to combine both of the characteristics different antenna concepts have been developed. The objectives were to obtain a high return loss combined with a sufficient high dynamic range and a near field focusing of electromagnetic waves in order to yield a high resolution. Altogether, three antennas have been set up. Each antenna has been calculated analytically, followed by a FEM simulation, near field measurements and an experimental verification.