A.1 Microstrip Antenna/Transmission Line Substrates

The selection of a substrate material is a balance between the required electrical, mechanical and environmental performance required by a design versus economic constraints.

Generally, if one has the available design volume to use air as a substrate for a microstrip antenna, this is a good choice. The antenna efficiency is high, the gain is maximized as is the impedance bandwidth of a conventional microstrip antenna. The surface wave loss when air is used as a substrate is minimal.

When a dielectric substrate is selected, one is interested in a material with the lowest loss tangent (tan ?) available. The loss tangent is a metric of the quantity of electrical energy which is converted to heat by a dielectric. The lowest possible loss tangent maximizes the antenna efficiency (decreases the losses) and is expanded upon in section 2.8 of Chapter 2.

The relative dielectric constant ? _r of the substrate determines the physical size of a patch antenna. The larger the dielectric constant the smaller the element size, but also the smaller the impedance bandwidth and directivity, and the surface wave loss increases. The use of substrates with higher dielectric constants also tightens fabrication tolerances. ^[1]

The tolerance of the dielectric value is also of significant importance in manufacturing yield. A Monte-Carlo type analysis using the cavity model is a good method of estimating antenna manufacturing yield for a rectangular microstrip antenna when etching tolerance, substrate thickness tolerance, feed point location tolerance and...