[Elecraft] RF Noise canceller ?

[G3VVT at aol.com]

In a message dated 03/04/05 12:11:33 GMT Daylight Time, DL2FI at qrpproject.de  
writes:

There  have been several schematics using such technics over the last 20
years.  Most of them use a seperate short antenna, feeding the signal pase
shifted  into  a combiner which combines the main Antenna Signal with  the
phases shifted extra antenna signal. This construction can reduce a lot  of
man made noise. 
I remember there was a commercial available stand  alone unit sold by a small
english company.



-------------------------------------------
 
The Antenna interference cancelling unit in the UK was from SEM a company  in 
the Isle of Man (GD). No longer seem to be in business. There is one from MFJ 
 still on the market and believe a model may have come from a company called  
NIR (now part of Timewave? cannot find any info on the Timewave web site)  
though could be wrong on this latter unit.
 
I built my own from a design from G4WMX for a "Null Steerer" that was  
circulating in the UK in 1989. I can remember my late father in law  Ted, G3UUA 
building one at the time and was impressed with how well it  worked, taking his S9 
level of interference out totally. About two years back  when suffering from 
local electrical interference I had a go at building the  same unit. There is 
a PCB produced by Geoff Steedman, M0BGS available now as  well as a kit of 
parts to populate the bare PCB. For the kit of parts with PCB,  I paid £25 (US$47 
approx). Built it up into a diecast box and have it  permanently in the RX 
antenna feed. Easy for me as I have a separate TX/RX on  the QRO rig. 
Alternately has PTT switch feed to enable it to be used in the  antenna feed of a 100W 
HF transceiver. With a suitable auxiliary antenna it can  null out quite high 
levels of local interference, the limiting factor being  the amount of wanted 
signal also picked up by the auxiliary antenna. With the  power off the unit is 
totally bypassed.
 
The downside with mine is that with the unit active, the wanted  signal is 
attenuated somewhat and even more when nulling out interference. The  upside is 
that it can still make a vital difference in hearing signals  buried under 
local interference. I believe Geoff, M0BGS has modified more recent  PCB to add 
an emitter follower to the outgoing main antenna feed to offset  through 
losses. Will have to try to modify my PCB to add this.
 
I have the scanned circuit and notes somewhere in my PC for the original  
unit before the emitter follower was added. If anybody wants to have a go  at 
constructing this device and needs the info, contact me direct.
 
Bob, G3VVT