Hello, I've built the op-amp radio pictured at this site: Op Amp Radio, but I'm not hearing anything on the speaker. I'm using a 10-180pf variable capacitor and an 11uH inductor (about 100 turns around a paper towel roll), which, if I know what I'm doing, which I don't, should put me right in the middle of the shortwave spectrum. My antenna is about 2 feet long. Any ideas?