In a message dated 1/8/2007 1:07:27 PM Eastern Standard Time,
R.Gorski@Astronautics.com writes:
I would be interested in the types of "noise antenna" used for the MFJ
1025 canceller together with results.
Ron N9AU
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Ron -
the noise antenna is very critical to successful operation of the 1025 and
similar cancellers. However, it's hard to specify any one type because
situations can and will be very different. It's more important to know
fundementals
however. The nuller takes the same signal you are trying to hear on one
antenna, and then the same signal from the sense antenna to a mixer 180
degrees
out of phase to do the nulling.
1. The sense antenna should be fairly close to the main antenna - certainly
not on the opposite end of the house or something remote like that. It should
"hear" the same noise as the main antenna. This is key as it's very possible
to throw up a small antenna which doesnt hear the offending noise - or does
so weakly. HOWEVER, you can and should consider several noise antennas place
near know noise sources especially if you're in a dense suburban neighborhood
where the noise comes from all over. The nuller can only null on one noise
so if you have mulitple noises you won't get complete nulling but you can
select the nastiest one to null. I put antennas near one neighbors outdoor
jacuzzi - the thermostat is noise gen, another one near the the other neighbor
who
has a noisy switching PS on a satellite dish and one under a nearby electric
tower .. All three antennas are in the general area of the main antenna. I
do have noise sources in the front half of the property, but the main
antenna only hears them weakly so they are not worth going after.
DONT put the noise antenna indoors as some of the nullers suggest - you'll
pick up far too much indoor crud and the not the offending stuff.
2. The better the sense antenna the better the results. A random wire can
work but something balanced would be better or at least something with a
stable pattern and pickup gain. A random wire which can move around in the
bushes, be subject to puddles, etc. won't be able to provide a constant enough
noise sample.
3. The sense antenna should have as much noise out as possible - a very weak
noise pickup will not provide for nulling - or make nulling less effective
as you might have to turn down the gain so much on the main antenna the
resultant s/n improvement won't be worth it.
I actually use a preamplifier on my noise antennas altho I don't always need
to use it. Whenever I cannot achieve a null, popping in some amplification
often solves it. The preamp on the 1026 btw is NOT useful for this purpose as
it's hard wired to the internal whip on that model.
BE prepared for critical tuning and patience!
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