Here is a link to an appropriate transformer design: http://www.aytechnologies.com/TechData/9-to-1_XFMR.htm This design will work well at 630m. It's a lot like the binocular core design with 73 material. Gary, K9AY --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus