Serious British Experimentation !

[André Kesteloot]

> John G0AKN wireless@rmplc.co.uk wrote
>
> Report on Earth Tests, South Devon.  August 1998
>
> The aim of these tests was to investigate the transmission properties of a
> much longer earth base than the 230 metres in Oxfordshire.  A colleague
> kindly provided 1000 metres of 32/0.2mm wire and a further 500 metres of
> slightly thinner stranded wire was located in my ‘stock’.  G3HMO had looked
> over maps of the chosen area and had found a long private road whose owners
> he knew.  This was to minimise the risk of damage to the wire from
> passers-by.
>
> We visited the site on 1st. August and commenced hammering the 1.2 metre
> earth rods into the soil.  It was only possible to get the rods in a few
> centimetres owing to the local rock formation so we drove two rods about 5
> metres apart into a nearby bank of earth at a shallow angle.  It was
> difficult to salt and water these rods and the resistance between them was
> shown to be 500 ohms.   We searched around the site and found one other
> place where we were able to drive a rod fully in vertically.  This was
> salted (1kg.) and 25 litres of water applied.  For good measure we connected
> the other two rods to this rod.  This was to be the site of the station.
>
> 1,500 metres of wire was then laboriously laid out along the ground by the
> side of the twisting Devonshire lane.  At the far point – part way down a
> hill – the ground at the side of the road rose in a steep bank.  A plateau
> was found where it was possible to get a rod in vertically.  Only a metre
> entered despite a hefty sledgehammering.  The area was salted and watered
> but the lie of the land meant that the solution drained off rapidly.  We had
> no more rods so returned to ‘base’ to measure the resistance.  The distance
> between the earth points was measured by GPS receiver as 0.85 mile.  Using
> my portable 50 volt AC system, resistance was found to be 190 ohms.  The
> wire accounted for approximately 35 ohms of this.  I am sure that this base
> resistance could be substantially lowered by better contacts with the
> ground.
>
> On the 2nd. August in the early morning drizzle we set up the transmitter
> (the 1kW amplifier feeding a custom built switchable output transformer)
> powered by the Briggs and Stratton 2.2 kW generator. The generator’s output
> was fed through a spike protector extension lead and then through a most
> useful device made by school education suppliers Philip Harris that measured
> power consumption.  A Fluke meter was placed in series with the amplifier
> output.  We transmitted for 45 mins on 136.5 at 10.00 local and for a
> further 45 mins at 11.00 local on 72.05 kHz.  It was possible to exceed
> 630mA at 136.5kHz into the earth system and over 1A at 72.05kHz. The
> amplifier drew 600 watts at 136.5 kHz and 800 watts at 72.05kHz.  One
> amusing phenomenon was the roar of the generator’s exhaust being CW
> modulated!  I tape-recorded this for posterity.  It was gratifying to hear
> later that our signals on 136.5 kHz had been heard by G4GVC portable at
> Woburn and by others at the Crawley club G3WSC station. Perhaps we have been
> heard by others?  The WX was excellent later that day.  I experimented also
> with 6kHz and was able to send 1.5A into the ground.  The system power
> consumption  was 1.2 kW at that frequency.  Tests using the equipment with a
> kite aerial scheduled for the 3rd. August had to be abandoned owing to a
> change in the weather.
>
> I intend to experiment further with my ‘portable’ station in areas where I
> can get a decent earth connection.  The equipment has proved to be most
> reliable.  I am quite certain that what is needed is very good contact with
> the ground (which we did not achieve this time) but not a water-logged base.
> This method is not for the fainthearted, though.  Hammering rods into the
> ground, carrying 100 litres of water and generators, and laying out and
> reeling up 1.5km of wire is hard work and G3HMO and I were tired at the end
> of the day.
>
> John G0AKN  wireless@rmplc.co.uk