If you're anything like me you're sometimes troubled by noise. At my home QTH I only get two kinds of noise: man-made and natural. S9 noise on all the low bands and sometimes troubling on the upper bands too. I finally decided to try one of the MFJ-1025 "Noise Cancelling Signal Enhancer" box. All the online reviews seemed to generally agree that this was one piece of MFJ gear that did what it was supposed to.
The theory behind it is actually quite simple. Take the signals from two different antennas and adjust the phasing to null or enhance signals from different directions. One thing that needs to be emphasized is that the more closely matched the antennas are the better it will work. I used a 23 foot vertical as the main antenna and an 18 foot vertical whip about 30 feet away as the noise sensing antenna.
Does it work? Well, it doesn't work miracles but it does sometimes work wonders. As long as the noise that is troubling you is from a single direction it WILL be able to reduce it. How much depends on many factors, mostly how well you are receiving the noise on each antenna. One thing I did do right away was to replace the little light bulb used as an RF 'fuse' on the noise antenna input with a relay to ground the input when I transmit. The light bulb would probably be okay if I was only running 100W but a kilowatt into the main vertical popped the little bulb in short order.
It definately takes a bit of practice to get used to operating it but the procedure is pretty straight forward. Adjust the gain controls for each antenna so the noise is at the same level on each antenna and the carefully adjust the phase control for a dip in the noise level. A little tweaking of one level control, adjust the phase control for minimum noise level and you're done. It really helps if you set your radio AGC control to 'fast' during the adjustments.
This little gizmo is aleady on my indispensible list. For more info, Tom W8JI has a nice description of the theory on his web page at http://www.w8ji.com/mfj-1025_1026.htm