Here's another one that's pretty cheap:

http://mcm.newark.com/NewarkWebCommerce/...0&QText=33-6925

If you have a couple 75ohm/300ohm matching (i.e. "balun")transformers in your junk box here's something to try: Hook the 300ohm ends of two transformers together, back to back in other words. Then insert this assembly into your cable feed line using the 75ohm ends. This works, I've done it and it can be a quick, cheap and dirty way of troubleshooting the problem before spending any money on a fancy-schmancy isolation transformer. (I wouldn't recommend it as a permanent fix though.) Good Luck!
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Tekdredger