TopBand: Re: Noise nulling

Mon, 20 Oct 1997 09:41:23 +0000

> From:          sander@aud.alcatel.com (dick sander)

> I recenctly picked up a JPS ANC-4 Noise Canceller at a flea market.

> I installed a wire outside from my tower out of the center of RG213 for
> a sense antenna. The wire is about 8 ft long and runs from the 12 ft
> level towards ground about 45 degrees and is tied off (insulated) to a
> chain link fence.

I never have seen an ANC-4, but I have used self-designed devices 
like that here for many years. Speaking in general terms, I use mine 
daily. About 50% of the time it makes a noticeable improvement in S/N 
ratios. 

I use mine in the peak mode, adding the signals from a Beverages to 
my phased loops, or from the phased loops and the phased verticals, 
and sometimes from ALL three at the same time (my unit has multiple 
antenna inputs, it can sum or subtract up to five input channels at 
the same time and has memories for gain and phase settings).

Independent of the type of unit, the concept is  exactly the  same. 
All you have is a variable phase control, and a variable gain control 
that lets you phase two antennas together.

In any of these units, BOTH antennas must hear the same signal at the 
same time to be effective. If you want to peak a signal, it must be 
clearly audible on BOTH antennas. If you want to null a signal (like 
noise or QRM), it must be be audible on BOTH antennas at the same 
time.

If the noise sense antenna is located where it hears a new noise 
source, it will only ADD that new noise to the system. If the desired 
signal is in the same direction as a dominant noise source, and the 
S/N ratio of each antenna is the same, you can't do a thing with 
anything. The same is true if the noise is arriving from multiple 
directions.

I don't think it would be anything more than pure blind luck if you 
got a system like this to work without some planning and 
experimentation.

By the way, the DeMaw (now SK) QRN squasher  system does not rotate 
phase at all, so it would really be luck if it worked. It's either 
zero or 180 shift on 160. 

I've never measured an ANC-4, so I have no idea how well it shifts 
phase (but if someone loans me one for a week or two, I'll be happy 
to measure it and post the results). The MFJ unit does shift phase 
quite well, on 160 it rotates phase through all but 30 degrees of 360 
degrees.

The key to success is having the unit shift phase without changing 
signal level in the process, and having the correct antennas on the 
inputs. 3 dB is the typical improvement if you are in a quiet 
location, but it can be 40 dB or more if you have a strong local 
noise source in one direction.

73, Tom W8JI

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