radial stub microstrip rf choke

[BasketballJones]

Can anyone point me to any equations for a radial stub RF choke. This is the kind that is etched out on a microstrip board. Is it as simple as a 1/4 wavelength with a 90 degree radius, or is there more to it?

 

[uWaveFDL]

http://www.geocities.com/gregsdownloadpage/circuit_design.html

Try this site.