Its best to provide:
log4j.appender.f.File = ./myapp.log
. represents the current folder (usually the project root folder). This also works on different OSs. In case when you use ${catalina.home} you might not be running a web application through tomcat.
This post helped me go through the issue:
http://www.matjazcerkvenik.si/Site/Java::Log4j_Properties.html (updated link http://www.matjazcerkvenik.si/developer/java-log4j.php)
Cheers!
