Have you tried putting a real telly in the same place as the Mac and using it with the antenna set up exactly the same? Also, one thing i've learnt from my digital TV experiences (i know you're using analog, but this still applies to a point), consider the polarisation of your antenna. Try and find out what polarisation the local TV stations use, and orient your antenna that way. For example, here in the Wide Bay, the TV stations use vertical polarisation, which means that for me to get a decent picture, i must orient my antenna vertically. I know its hard to do this with a pair of rabbitears, but thats what I use. Connected to my digital STB, I have a TV antenna cable i made up with male RF connectors on each end of an oldskool antenna ribbon cable, which goes to a female to female gender changer, which then goes to a pair of rabbitears i've got leaning up against a window pane.

May/may not help, i know, but it might come in handy.