Neoklis 5B4AZ just released version 1.5 of xnec2c, which incorporates my F/B ratio patch as well as a few other small fixes. I have tried it and it works fine on Fedora 15. http://www.qsl.net/5b4az/pages/nec2.html
xnec2c-1.5-1.fc15 has been submitted as an update for Fedora 15. https://admin.fedoraproject.org/updates/xnec2c-1.5-1.fc15
Package xnec2c-1.5-1.fc15: * should fix your issue, * was pushed to the Fedora 15 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing xnec2c-1.5-1.fc15' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/xnec2c-1.5-1.fc15 then log in and leave karma (feedback).
xnec2c-1.5-1.fc15 has been pushed to the Fedora 15 stable repository. If problems still persist, please make note of it in this bug report.