eBay and other sources list a range unpowered (not active) antennas for general-coverage use. These are electrically-short whips or loops that are ostensibly impedance-matched to 50-ohm coax and feed the 50-ohm antenna input of receivers. They boast flat response from kilohertz to maybe 30 megs. Does anyone here have actual experience with these, to the extent that you can judge the relative performance against long-wire or active antennas? Any info on what sort of matching network is used on these would be welcome too.