Late in December 2000, I built a reasonably large " Octoloop" ( as described by Will Payne)
It has ~ 4 metres dia , 100pr.phone cable (200turns), built into 32mm copper pipe.
A large number of experiments were conducted during the ensuing 6 months. It was found
to be best suited to operation in the 1kHz.....22kHz frequency bands. It was also used
with moderate success for frequencies as low as 2Hz, and was able to receive the lower
4 Schumann resonances if conditions were favourable (no wind, loop movements).
The total of 200 turns is actually split into symmetrical groups of 20,30,and 50 turns each.
This allows series-parallel connection of various groups for experimentation.
The final configuration is now 2x parallel groups of 50 turns in series with another 2x 50
turns group. This gives a centre tapped 100 turn coil of low resistance (higher Q, also
allowing lower op-amp. input noise performance).This resulted in a total resistance of
85 Ohms and 38 + 38mH inductance.
This configuration is now mostly used to receive VLF/ ELF spectra and proved quite
successful for "whistler" detection and recording. High order mains harmonics are a
limiting factor in this location, extending well up to 4kHz (!)
The loop coils are connected to a balanced input low noise pre-amp.which is located
directly under the Octoloop. The input of the low noise amp. is preceded by a 22kHz
Whistler detection is a bit difficult in my location, living in "semi-suburbia" means
having to put up with power mains harmonics up into the 6kHz region ! Various tests
using a 2.5m vertical showed this approach as being totally un-usable, even when
filtering first with 22kHz LPF and 2.5kHz HPF, the harmonics just too strong.
This is where the balanced input Octoloop really shines, it makes regular whistler
reception possible in an area where simple verticals are hopeless antennas. Saves
me having to make long trips into the outback away from power lines......
Still, the lower 2.5kHz are usually too high in harmonics, so I limit the range from
~ 3kHz to 14kHz. Therefore my spectrograms show (apart from whistlers) the navy
station VL3DEF, and sometimes the Russian alpha nav. stations.
OBSERVATIONS
For almost 6 months of the year there are no whistlers observable other than very
faint ones. These are often not even audible, but still show up faintly on the spectro-
gram. Solar events / geomag. events seem to have only minor, "modulating" effects
on the quality of the whistlers .During the northern winter months (our summer) the
local sferics can be collossally strong, making faint whistlers unobservable anyway.
At that time "hooks" and "tweeks" also reach extreme strength and are very high
in numbers.
Over the last 18 months I have built up a library of ~ 2000 whistler spectra and also
many hours of wav. files (directly recorded onto CD ROM). Much of the wav. files have
yet to be evaluated, a job which can best be done during the "whistler- drought" time
of the year, which is ~ Oct. - March ,for my location.
Here are some typ. samples : (most spectrograms ~ 155kB)
