The Simple RX Loop works awesome on 600m and freq's around there if you build it as a multi band antenna. I built two of them years ago when Brian first showed us the article. Here is my portable version: http://we0h.us/505kc/RX-Loop/Portable-RX-Loop-small.jpg A LM741 opamp works fine if you can't find the OP27 opamp. Mike-WE0H /16