Rick at Amtronix is now selling a bargain-priced return loss bridge - $69 kit, $109 assembled and tested. Covers 400 kHz to 1 GHz. Directivity spec is 30 dB. No, it doesn't turn your TG/SA into a $50k VNA, but for a little over a Franklin, it looks like quite a deal.
Details and specs on Amtronix web site (www.amtronix.com). I haven't used one, so I have no comment on its performance, just passing along the info. --- Jeff WN3A