I just got a passive Terk loop for mw. It does quite well; my Sangean 400 with the TERK during daytime testing was similar in sensitivity to my Sangean PR-D5. The PR-D5 itself improved modestly; but not enough to make it worthwhile for that set. The 400's selectivity is not sufficient to fend off local pests from weak signals 10 KHz away...if anything, that problem was made a little worse by the TERK. On my 1981 vintage SONY ICF-E10W set (a mono, speakerless set about the same size as the 400, but slimmer), it's excellent selectivity handled the TERK very well...I was able to boost signals 10 KHz away from locals with ease and without amplifying the pest. This on signals that the 400 cannot seperate without the TERK. The E10W also had slightly better sensitivity in concert with the TERK than the PR-D5 barefoot!
The TERK is available online from Radio Shack, and is mdelivered for free to your nearest store. $39.99