I heard that microstrip antennas are only good from 100 MHz and up. What's the design limitation for lower frequencies in regards to microstrip antennas?

Greetings Qacer:

The frequency limit is governed by the wavelength of your signal(s) in the substrate. A wavelength calculator is available here. Enter your frequency of interest and the dielectric constant of the substrate.

For example, FR4 has a dielectric constant of around 4.5, so the wavelength of 100 MHz in it is about 1.4 meters (55 inches). If you want to implement any passive circuitry that requires a wavelength shift along the line (as is necessary for filters, power splitters, baluns, etc.), very large substrate would be required.

If all you need is a substrate to implement 50 ohm (or other another impedance) transmission lines to interconnect components, then microstrip can be used at any frequency.

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- Kirt Blattenberger
RF Cafe Progenitor & Webmaster

an introduction on microstip antenna:

http://www.4shared.com/file/742081/ae5d ... strip.html

Thank you all!