Performance of the Porcelain-Metal Substrate at Microwaves

The performance of the porcelain-metal substrate at microwave frequencies is described. Commercial electronic grade porcelain-steel and porcelain-copper-steel (a special substrate) were investigated for their microstrip transmission line properties. It was found that for the standard thickness of porcelain, the 50-Omega microstrip transmission line width was 0.01 inch. The velocity of propagation was determined to be 45 percent of the free space velocity for the 50-Omega impedance line. The attenuation of a characteristic impedance line on a porcelain-steel substrate was measured as 0.8 dB/in normalized to 1 GHz. The attenuation of the porcelain-copper-steel substrate, however, was only 0.33 dB/in normalized to 1 GHz. In comparison, an alumina substrate of the same thickness with chrome-gold metalization has a loss of only 0.28 dB/in normalized to 1 GHz. Two test circuits, a low-noise transistor amplifier and a high-power transistor amplifier, were designed and fabricated on porcelaln-metal substrates and tested. The results were comparable to the same circuits on much thicker alumina substrates.