Nags Head Report #2

[Andre' Kesteloot]

As previously mentioned, we ran several separate experiments
during the Nags Head 15-17 January 1999 week-end.  These
experiments were run on a secluded beach, away from
power-line noise. Some of the experiments were powered from
motor-driven AC generators, while others were powered from
car batteries, or gel-cells.

George Lemaster WB5OYP's set up included a Rycom 3121
receiver, with 3.4kHz and 100 Hz crystal filters. This is a
transistorized frequency selective voltmeter AM/SSB device
about 20 years old that tunes from 1 Khz to 1.5 Mhz.

The audio output was connected, alternately, to a W9GR
DSP-II and a JPS NIR-10 DSP audio filter. The W9GR seems to
be superior in reducing atmospheric noise, but the sharp CW
filters are fixed frequency (The W9GR manual says it uses
the 'Widrow-Hoff' LMS adaptive filter algorithm where
signals correlated from one sample to the next are
considered likely to be speech).  The NIR-10 has a tunable
CW filter which helps pulling out beacon tones from the
noise. The receiver and DSPs were operated from a gel-cell
battery.

The loop antenna was a 8 ft. square single turn. (The loop
dimensions were scientifically determined by the length of
boards that would still fit in the old Volvo station-wagon).
The  loop was one turn of aluminum foil shielded RG-59 type
cable-television wire. The gap in the shield was a plastic
box with two F-connectors. The loop was connected to a
preamplifier input pot-core transformer with 4 turns on
primary and 80 turns on secondary. The secondary was
resonated with a varactor diode for tuning. The preamplifier
was the conventional circuit using FET and bipolar stages
with additional emitter-follower stage to the output.

A second antenna was made of two ground stakes in the sand
separated approximately 180 ft. Two 1/2 in. copper water
pipes were driven about 2 ft into the sand. The two stakes
were connected with shielded RG-59 type foil shielded
cables, Two cables were 100 ft. in length each out to the
two ground stakes. The stakes were about 180 ft apart, along
a East-West line. Only the center conductor was connected to
the ground stake to provide a 'shielded cable' feed to the
preamplifier. The resistance between the two stakes measured
approx. 1400 ohms. A voltage of about 31 mV was measured
between the stakes, so there seems to be some galvanic
action or something between the copper pipe and the
saltwater creating the voltage which increases slowly with
time. The coax cable ends were connected to the same
preamplifier input transformer as the wire loop antenna. The
signal dropped out when one ground stake was removed
indicating the connection through ground was essential for
operation.

The air loop and the 'ground loop' (or "earth loop" as John
Taylor G0AKN probably would have called it) were alternately
connected to the input of the preamplifier. An Algerian
broadcasting station (located due East of our location) on
171 kHz was approximately 7-8 db better on the air loop than
the ground 'loop'. There is no advertising of a 'ground
antenna' here, as it is possible the signal was simply being
picked up on the center conductor of the coax cable.
Possibly the coax cable looked like the air loop with a
'shield gap' similar to the air loop, although the coax
cable was on the ground. The ground connection between the
stakes could have simply been the connection between the
center conductors of the coax like the gap at the top of the
air loop.  It is not known if the path length through ground
stakes was significant for the receiving signal, but if one
stake was lifted the signal dropped completely. (An
additional test, not done, would have been to simply connect
the center conductor of the coax cable at the ends while the
coax was on the ground for comparison. That is, what would
have happened if the loop antenna were simply flat on the
ground).

Although the jury is still out as to how the 'ground loop'
operated, whatever did happen was remarkable as we were able
to clearly hear the Algerian broacasting station on it  :-)

Photographs of the loop will appear in the next issue of the
AMRAD Newsletter, and on our URL www.amrad.org

73
Andre'  N4ICK