Development of microwave dielectric materials has been expected on the wireless communications in the high speed communication society. There are three important directions for research and development of microwave dielectric materials. We have been studied on these three directions based on the crystallography. First direction: tungstenbronze-type like compounds, Second direction: homologous compounds and third direction: silicate compound such as forsterite.

The microwave dielectric properties of alumina (Al2O3) ceramics were studied. Our objective was to improve the large negative temperature coefficient of resonant frequency (τf = -60 ppm/°C) of Al2O3 ceramics by the adding rutile (TiO2) with a large positive τf (+450 ppm/°C). A near-zero τf (τf = +1.5 ppm/°C) with excellent microwave dielectric properties (Q·f = 117,000 GHz, ε r = 12.4) was obtained in 0.9Al2O3-0.1TiO2 ceramics sintered at 1350°C for 2 h, followed by annealing at 1000°C for 2 h in air.

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A novel type of high pressure microwave discharge has been investigated to feed the microwave power at the centre of the compact high pressure discharge lamps using the antenna effect. This method of microwave discharge is named as the antenna excited microwave discharge (AEMD). The 2.45 GHz microwave of around 50 W from the solid state microwave generator can sustain a stable plasma column in the small gap between a couple of antennas fitted on the compact lamp filled with discharge gases at a pressure higher than atmosphere. The AEMD has been applied to a compact metal halide lamp and an extremely high pressure mercury discharge lamp. As a result, the metal halide lamp showed high luminous efficacy of around 130 lm W−1. The excellent lamp properties obtained here can be explained by the low heating loss at the antennas and the lamp wall. The profiles of the microwave electric field in the lamp and the microwave launcher have been numerically calculated to consider the microwave power supply into the lamp.

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