Often referred to as Long Wire Balun, this device is used to match the impedance of a long wire HF antenna to a 50 ohm input impedance of a receiver.
Such impedance matching usually results in a significant signal strength increase (up to 17 dB in some cases, and approximately 5 dB on average), compared to the long wire antenna connected directly to the antenna input of the receiver.
For $39.95 and some shipping you can match a Long Wire antenna to your HF radio. This makes for a great HF and SW listening post.
Check it out at http://winradio.com/home/lwa.htm