Topband: MFJ-1025 Noise Reduction Unit

[Tod -MN]

> If anyone has had good experience with one of these and would 
> mind talking to me about it over the phone, send me your 
> phone number (lower 48 states) and I will be happy to give you a call.
> 


Brad:

I don't have time for a phone call this week. Here are a couple of
suggestions-

1. Go to my web site and look at the sections that deal with the MFJ-1026.
Your 1025 is missing the preamp that is found in the 1026. That may mean you
will have to acquire or build a preamp for the noise antenna. 

2. The critical thing in getting a null is to get the signal amplitude of
the noise to be about the same in the noise antenna input [aux] and the RX
antenna input. Then the Phase and Null controls will have a chance of
working effectively.

Your on the ground antenna may simply not have enough signal to do the job
or it may produce a noise signal that has a phase which is sub-optimum for
the MFJ unit.

I suggest you start the null process by reducing the gain for both inputs to
zero. The add about 1 unit of input to the regular antenna and listen to the
'noise' part of the signals in the receiver. Next reduce the regular antenna
input to zero and increase the aux antenna input until the noise is about
the same as you hear for the regular antenna. 

Next change the gain for the regular antenna back to the 1 unit and see if
you can get a null at any adjustment of the phase and null controls. You can
use the reverse switch if needed. 

If you do get a null then keep the settings there and slowly increase the
regular antenna input 1 unit at a time and then the aux input to the point
that the null is best for the increased reg antenna input. You may need to
'tweak' the controls slightly from the position that was the best null for
the very weak reg antenna input. DO NOT CHANGE THEM BY VERY MUCH. Repeat
this until you run out of aux input to generate the null or until the reg
antenna input is at max. It may be that you will want to quit before you get
to the maximum on the reg antenna input because the signal to noise ratio
sounds better with less regular antenna input. Try to get to the place where
you can copy signals best even though the regular antenna input may not be
at maximum gain.

If you find NO null with the regular connection of the antenna and aux
inputs try reversing them and going through the nulling process again. A
null may not remove all of the line noise because there may be more than one
source or more than one location that is sending out RF from the same single
source. You will have to accept that unless you have multiple nulling units
you cannot null all of the noise sources --- only one.


I hope that helps you get started. The MFJ units work well but they do
require that the noise antenna provide a signal that produces a good noise
signal at a phase that can be nulled. It may be that even a small piece of
wire [10 feet or so] as a vertical at a different point in your yard will
provide a good aux signal which has a phase that the MFJ can handle well.

73, Tod. K0TO