On 10/15/2015 6:13 AM, Brian Howie wrote:
In message , Jeff Liebermann
writes
On Wed, 14 Oct 2015 14:34:10 -0400, rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.
So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.
The shielded loop reduces local noise pickup by eliminating much of
the electric component of that noise in the near field. Since the
ability of a small loop antenna to hear properly is primarily an
exercise in improving the SNR, any reduction in noise levle, without a
corresponding reduction in signal level, is a very good thing. More
detail:
http://electronics.stackexchange.com...-if-anything-m
akes-shielded-loop-antennas-so-great-at-rejecting-local-nois
I've built small loops that were not shielded and measure the SNR of
some stable signal, such as WWV. I then wrapped the loop in aluminum
duct tape, leaving a gap to prevent a shorted turn problem, retuned,
and found that the baseline noise level had decreased and the SNR had
improved. It works.
I've a 5 foot Octagonal loop for MF. The shield is copper water pipe,
with a gap , 7 turns inside plus a coupling winding. It does a good job
eliminating local noise (mostly ASDL hash from the phone lines) compared
with a vertical. However the capacitance between the shield and turns
seems to load it quite a bit meaning I can't get the tuning range I'd like.
I assume there is nothing to space the wires from the pipe other than
the insulation. Maybe you could use wire with thicker insulation? Or
if you are using straight pipe, could you use a fabricated spacer at the
corners? I guess that might be hard to assemble with soldering the
joints.
--
Rick