The gain of the 15 foot noise reducing vertical is about -15 dB
reference:
http://www.kongsfjord.no/dl/Antennas...%20rev%202.pdf

In a brief review of the article by the name found in the Subject
line, I came across this bare quote offered above. It contains not
one, but two unsupported claims that seem to be commonly encountered
in the Short Wave Listening community.

The first that is more easily supported or denied is the gain claim of
-15 dB. Where does this loss come from? Without any substantiation
beyond the inference of comparison to "active antennas," it seems to
be lost to indirect references in other writings. However arrived at,
this claim is suspicious in the extreme - unless it is a vague and
offhand substitution for antenna system gain which goes to the heart
of the matter of a poor ground system. If so (the loss is found in
the absence of an adequate counterpoise), that is indeed low hanging
fruit that has been left rotting on the limb.

As for the noise reducing claim, this, too, appears to arrive through
indirection or muddied with discussion of active systems. I can only
surmise that the lowered noise was the noise of the added circuitry of
the active antennas. If you discard the amps, I suppose you can claim
you've improved the noise which brings us to the Gordian knot of the
low gain needing those amplifiers - most curious writing.

At least one person has claimed that noise reducing antennas are
noisy. But when I quizzed him about his implementation,
it turned out that he had not implemented the antenna correctly.
If you do not follow the instructions,
then you may end up with a noise increasing antenna like he did.

Good reporting would have described the defect so that the solution
could be observed as rational rather than prescribed.

much to my amazement, that long coax (50 feet) lead
often degrades 2nd order intercepts of active
whip antennas by 20 dB or more and degrades
3rd order intercepts of active whip antennas by up to
10 dB, depending on the type of active whip antenna.

It would appear that "depending on the type of" antenna begs the
question why coax is the culprit. I can see how this kind of writing
spawns a new superstition of the superiority of twin lead.

More could be said about Common Mode suppression (which the designs on
this page do NOT entirely address).

73's
Richard Clark, KB7QHC