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The art of noise
nulling
Top Band mailing list, May 2000
Pete,
W2PM: Does anyone have views or comments on the MFJ 1025
canceller vs. the 1026 which has the optional "active antenna
preamp"?
I recently acquired the 1025 as my ANC4 was fine except on the
topband. I knew the MFJ phasing circuit has the range to cover
160 and also has a main antenna attenuator which can be very
useful in certain scenarios - the ANC4 does not. However, it
seems the MFJ 1025 version doesn't have the noise antenna gain
to perform well, except on the most raucous and strong S7-plus
noise sources.
Dave NR1DX: The trick with any of the noise cancellers
is to get a noise signal of equal amplitude into both the
main antenna input, as well as the noise antenna input.
In my experience folks who have had problems getting the cancellers
to work have not put enough thought into getting a good antenna
for the noise antenna.
For example,
if you look at the relative efficiency of a full size inverted
L on 160 vs say a 10 ft piece of wire or the 1' whip that comes
with the canceller it doesn't take much to figure out that the
ability of the smaller antenna to pick up a noise source a few
blocks away may be down by as much as 60db as compared to the
larger antenna.
A 20db preamp in the MFJ1026 won't make up the difference enough
to get a good null. If the source of noise is a leaky pole pig
in your own yard the small noise antenna may do it.
The procedure I use with my 1026 is I first note the S-meter
reading of the noise with the "main antenna gain" all the way
up, and the noise antenna gain all the way down.
I then turn main control to zero.
I then turn up the gain of the noise antenna until I get the same
S-meter reading.
Next I turn the main antenna gain all the way back up and adjust
the phasing control for a null.
Careful tweaking of the noise gain and the phasing control then
tames the beast.
If I can't get an equal level through the noise antenna side,
I either live with the noise or try a different antenna as the
noise antenna.
Successful noise nulling is an art and it takes a good
noise antenna plus patience to tune out a noise source as the
controls can be quite sharp. Also if you are dealing with more
than one noise source your out of luck as the noise canceller
can only get rid of one noise at a time as the arriving phase
of the two sources is different.
Dave
NR1DX: The trick with any of the noise cancellers is
to get a noise signal of equal amplitude into both the main
antenna input, as well as the noise antenna input.
Pete Ferrand, WB2QLL: Yes, this point cannot be overemphasized.
The main thing to point out is that the desired signal is not
a factor here, but both channels must have the same amount of
noise in 'em.
Also, if the noise aerial doesn't have enough signal pickup, you'll
have to cut the gain down on the main antenna input, which once
you've nulled your noise you may not have enough desired signal
left since you've turned the gain down.
Dave
NR1DX: Note the S-meter reading of the noise
with the "main antenna gain" all the way up, and the noise antenna
gain all the way down. I then turn main control to zero. I then
turn up the gain...
Pete Ferrand, WB2QLL: This works, of course, but wears
out fingers and pots. I had to replace one pot in my 1226 already.
It's far easier to just have a couple switches on the inputs that
are some kind of toggle switches that can flip the signal off
to each input in turn for the adjustment.
I took an old antenna switch so I didn't have to drill UHF connector
holes and added a couple automotive-type switches 'cause I had
them around and it works just fine. Each switch just disconnects
its input and grounds the antenna side. An old antenna switch
with four coax connectors already on it makes it easier.
If you want to do it fancier you can replace the pots with push
on/off switching pots on the 1226 itself.
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