> From: owens@stout.atd.ucar.edu (Chip Owens)
Hi Chip,
> I have tried shielded loops, both horizontally and
> vertically polarized. These don't help much.
The reason they don't help is if you shield an antenna from the
time varying electric field you also shield it for any time varying
magnetic fields or radiation fields. All shielding does is make the
loop's "shield conductor" become the actual antenna, instead of the
stuff inside the shield.
Same with a noise source. It contains both electric and magnetic
induction fields, as well as a radiation field.
> The noise blanker in my OMNI-V helps some, but still
> the average noise level hovers around the S-7 level.
Noise blankers only work on sharply rising pulses with short duty
cycles. Light dimmer (and computer noises) are more like regular
carriers with buzzy modulation.
> Has anyone had any success in minimizing the effects of
> light dimmer noise using the DSP boxes? or by other
> means?
Brian Beasly had an article in QST about low phased dipoles. If you
copy it and make sure balance is maintained, it might help. It was
one of the best ideas I've seen in print.
DSP boxes help, especially ones with multiple manual filters, but
the best solution (other than turning off the source) is to use a
phasing system with a sense antenna to cancel the noise. I've been
using a variable phase combiner for many years now, and MFJ just
copied the design for production. It's surface mount machine
assembled so QC should be excellent. It has good dynamic range, low
noise, nearly perfect phase rotation, and excellent amplitude
flatness. I have used several boxes like this to combine several
antennas nulling noise or peaking signals from several sources at
once for years now.
But even a good noise canceller will have restrictions. It
either requires the signal come from ONE source, or if you have
multiple sources of noise the canceler will work ONLY if they are all
from the same basic direction and all noise sources arrive at both
the sense and main antennas with the same signal level ratios. If
the sources fit these requirements, the ultimate attenuation is
near infinite!!!
Other than removing the dimmers or phasing antennas, your
probably stuck. Shields won't do the trick at all.
73, Tom W8JI
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