Topband: MFJ 1025

[W2pm at aol.com]

In a message dated 1/8/2007 1:07:27 PM Eastern Standard Time,  
R.Gorski at Astronautics.com writes:

I would  be interested in the types of "noise antenna" used for the MFJ
1025  canceller together with results.

Ron  N9AU

_______________________________________________


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!