I have used the MFJ 1025 for about 6 years now but it really came to light when we moved to the new QTH in SW Florida. First off I did mod the noise antenna input since I have never had enough property to place the noise antenna far enough away from my transmit antenna so there is a relay on the input of that port so when I am transmitting that input goes to ground, no worries about blowing that little wheat bulb. I have tried different type of noise antenna's and this is were you should throw there manual away cause a vertical is the best overall noise antenna to us so far, tried a 30ft vertical but at this time am using a 40ft vertical wire inside a spiderpole, this has worked best especially on the lower bands. I can take an S9 noise level created usually from a nearby 70Kv lines lightning arrestor gone mustang and take it down to S1 or 2, now when there are several of these arrestors that have gone south that noise is compounded and you may not get that overall best performance but it is still better than nothing.
Is there some attenuation on the received signal, yes but it far outweighs not being able to copy the signal at all buried in the noise plus after you null the noise you still have residual signals that the vertical does pick up that at times give you the effect of both your horizontal antenna and the vertical antenna so there seems to be some diversity reception going on. The vertical noise antenna also works well enough on 20m and above to lower the noise floor to copy weak signals being covered but hi levels of static. On 20m and above the line noise from power lines get more aggressive and the amount of level you can back it off on is not as great, maybe a shorter noise antenna would work best but the improvement on the lower bands is outstanding, the differences on the spectrum display is amazing, night and day.
I would assume that the box could be used on phasing two RX only antennas as well but here again you would have to cut off the signal coming in from both antennas while you transmit on your main station antenna.