Beam width estimation of microwave antennas using lock-in infrared thermography

• A fast method for determining beam widths of microwave antennas in E- and H-planes using lock-in thermography is proposed.
• Amplitude images are used to determine the half power points in both E- and H-planes.
• From these E- and H-plane beam widths the directivity is then calculated.

The beam width of microwave antennas can be easily determined using infrared thermography. A thin absorption screen made up of some carbon loaded polymer is placed in front of a microwave antenna. The electromagnetic waves impinging the screen are partially absorbed by it, resulting in temperature rise of this screen. This temperature rise is monitored by an infrared camera. These infra red images of the temperature distribution are identical to the power distribution of the incident electromagnetic field at the screen location. The distance between half power points on the main lobe of the radiation pattern at the screen location in two orthogonal directions (E- and H-planes) can be easily found. From these distances the half power beam widths in both E- and H-planes can be calculated. Further from these beam widths, directivity is also calculated.The experiments were carried with a patch antenna radiating at 2.45 GHz. In order to show the repeatability of the results, experiments were carried for different distances between patch antenna and absorption screen. A great agreement is seen between simulation, measurement and experimental (thermographic) results.The microwave source is modulated at a low frequency so as to avoid the lateral spread of heat on the absorption screen and to permit lock-in detection of temperature distribution.