vlf_elf

The "antennas " :

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One is simply a loop in the ground. A 40-core telephone cable was buried in a 400mm

deep trench, shaped as a 25 x 53 metre rectangle. All the cores are connected in

series (with a centre connection after 20 turns), resulting in a rectangular coil of 20 + 20 turns, with an effective window area of 53,000 squ.metres. The overall loop inductance is 427mH, total resistance 546 Ohms. Although it is capable of picking up signals up

to 25kHz into the VLF region, its principal use is for frequencies below 12Hz.

Next I constructed a 200 turn ,4m dia. "Octoloop" . Its effective window area is

2650 squ.metres . The winding was made with a length of 100 pair telephone cable,

several groups of cores can be selected to function as a 30 turn up to a 200 turn coil.

The Octoloop was operated at various times as high as VLF , and as low as 2 Hz.

Its principal use is the range 3 - 14 kHz, but more recently it was actually my first

antenna with which I was able to detect and record the Schumann resonances.

The major drawback of the Octoloop is its sensitivity to slight movement by wind.,

causing "microphonics" .

Finally I had to construct the type of device with which the "professionals" use to

study the Schumann resonances, namely a large induction coil.

I wound my coil of 69,300 turns on a 800mm length of 50mm dia. PVC pipe ,using a

long threaded rod through the coil former (pipe). This was at one end clamped into

the chuck of an electric drill, which in turn was held in place in a bench vise.

The opposite end of the threaded rod was located through a hole in an improvised

bearing bracket bolted to the workbench. Finally the rod's end connected to a mech.

turns counter. The electric drill was powerded by a VARIAC transformer, the best

voltage for my particular drill turned out to be around 60...70 Vac. I started and stopped

this set-up with a foot switch. It took several hours of continuous high speed winding

to get about 8Kg of 0.3 mm dia. enamelled copper onto the former.

The finished coil's resistance is 3.64kOhms , its inductance 10.52 H .

10 lengths of 3mm thick flat steel bar of various widths ( 16...40mm) , each 2 metres

long, were then put through the pipe centre to increase the coil's magnetic permeabi-

lity. This steel mass just about fills the available clear inside coil aperture completely. I could not measure its final inductance, but I guess it must be several hundred Henries.

The steel bars are insulated from each other, similar to transformer steel laminations.

The finished assembly was then enclosed in 90mm dia. PVC pipe and endcaps for

weather protection.

Each antenna has its own pre-amplifier and signal conditioner , powered by two

12V / 18Ah sealed lead acid batteries, which are recharged weekly. All signal out-

put cables go ~ 70m underground back to the house.

Back - end signal processing :

The signals can be directly switched to the output buffers (13dB) or filtered

via low pass and / or high pass filters. Each filter consists of a six pole Butterworth

type, switchable in 10 steps, a feature which can be helpful at times.

The final output normally drives the line-in socket of the PC's S/B card for

signals > 5Hz. Signal analysis is done with the aid of SPECTROGRAM,

a very useful spectrum analysis program. Around the clock monitoring

can be implemented by running a screen capture graphics program in the back-

ground (as decribed by IK2QFK ) called 20 / 20 , which is freeware.

Each spectrum screen display is automatically saved at selectable time intervals

in the form of .jpg files for later viewing.

I also use a standard A/D converter from PICO Technology , the ADC-11, which

is a 11 ch. 10 bit converter. This ADC is packaged within a DB-25 plug shell and

connects directly into one of the LPT- ports of a PC. The software is called

PICOSCOPE , which also contains an audio spectrum analyzer screen with

selectable FFT formats ,etc. I mostly use this for observations below 20Hz.

Signal processor >> front view rear view

The emphasis here is on the detection of natural electromagnetic radiation , with

my particular interest being the Schumann Resonances from 8 ....... 45 Hz.

Schumann resonances are generated by the numerous lightning discharges around

the world, injecting shock energy into the spherical space ("cavity") enclosed between

the earth's surface and the ionosphere.

In depth material about Schumann resonances can be found at :

Oulu/Finland Stanford Uni. U.of Alaska also Alfven resonances

This mechanism is much the same as generating microwaves within a metal cavity,

like a waveguide, by means of small electric spark discharges (spark transmitter).

The earth-ionosphere cavity is physically extremely large, therefore its resonant

frequencies are not in the microwave region, but at sub-audio frequencies.

The fundamental frequency is ~ 7.8Hz , with several harmonics and other "wave-

guide modes" making up the range of 7.8, 14, 20, 26, 33, 39 and 45 Hz.

The height of the ionosphere varies according to the (local) time of day, etc., and

this alters the exact frequencies. The Schumann resonances are fairly broad, unlike man-made signals, which are normally nice and narrowband.

The reception of Schumann resonances proved to be surprisingly difficult, and in

the course of the efforts to build equipment capable of reliable performance a

number of systems were constructed and operated . These are outlined as follows,

together with technical details and observational results.