Never heard of them. I have seen frequency shifters, but those were mainly a gimick. I think the only real use they found was for competition hunters to keep from cross-talking to same-model detectors.
Are they still making these? I don't remember seeing one in many years.
hello hftrsrskr , welcome to the forum
they do still make these for some detectors and some guys will swear by them . what detector did you want it for i might be able to help you locate some .
most of these you see are just standard ferrite bead inline filters and will reduce some emi from high power tension wires but most of your emi issues are under that khz and they won't do much if anything to help with that . you could give them a try its your buck.
This is my experience with them Dan. They do help a little directly under high voltage lines, but have no effect most of the time. They are cheap, and radio shack carries two different types. Honestly, I wouldn't waste my money them.
Would there be a gain loss with an inserted device in-line? I'd think that there'd possibly be a 3db hit, half power with the extra connectors. Just a thought.