"Earth" Antennas

[André Kesteloot]

> G0AKN wrote:
>  Earth Bipole Notes by John Taylor G0AKN   <wireless@rmplc.co.uk>
>
>     >    1. An earth base whose earth rods are only 230 metres apart is capable of radiating electromagnetic energy at frequencies of 73 kHz and 136 kHz, despite the fact that the wavelength of radiation at these frequencies is 4,100 metres and 2,200 metres respectively. This has been demonstrated on numerous occasions. There is some evidence for directionality. The bulk of the line is on or under the ground. It has been shown experimentally that the impedance presented to the output of the transmitting amplifier when 136 kHz is used is much lower than at 73 kHz.
>     >    2. The resistance between the rods at 50Hz is around 28 ohms at the G0AKN site. A more detailed examination showed that this resistance (more properly called impedance) steadily increases with the frequency of current fed to the rods, reaching a maximum. If the frequency continues to increase the impedance decreases to a minimum and then rises again. Readings appear to vary with weather conditions. There are multiple current paths in the soil - the depth of these depend upon the skin depth at the frequency in use. Adding capacitance to the circuit has increased the current sometimes - but not always. There may be other factors such as inductance of the ground and skin effect in the wire.
>     >    3. The maximum and minimum impedances vary with the length of the base. For example, measurements taken in April 1997 showed that the first maximum peak of impedance occurred at a frequency of approximately 200 kHz with an 80 metre base, whilst on the same day with the base of 230 metres it was at approximately 75 kHz.
>     >    4. The behaviour of a long line close to the earth resembles that of a transmission line. A transmission line shorted at one end has points of maximum and minimum reactance spaced one quarter of a wavelength apart. The reactance is alternately inductive and capacitative.
>     >    5. Using this approach the 80 metre base represented l /4 at 200 kHz. 230 metres represented l /4 at 75 kHz. Thus the 200 kHz radiation had a wavelength of 320 metres in the ground and the 75 kHz radiation a wavelength of 920 metres in ground.
>     >    6. The velocity of electromagnetic radiation in a medium is dependent upon the dielectric constant of that medium. The dielectric constant of a vacuum is 1. Geology texts give the value of the dielectric constant for soil to be between 3.9 (dry soil) and 29 (moist soil). That of clay varies from 7 to 43. Water has a dielectric constant of 81. If u = velocity of electromagnetic radiation in the medium and d = dielectric constant, u = 300,000,000/Ö d. If f = frequency then because l = velocity of radiation/f we can calculate the dielectric constant. Substituting the wavelengths obtained above we find that 'd' is 22 and 19 respectively - a very good agreement. A more precise measurement of maxima and minima will shortly take place at G0AKN. A rough value for the dielectric constant of the soil at the QTH on that day will be taken to be 20.
>     >    7. In order to get a low impedance presented to the amplifier, the length of base should therefore be l /2. Using d = 20 this means that the base length should be about 250 metres at 136 kHz and about 460 metres at 73 kHz.
>     >    8. The dielectric constant will vary with the nature of the ground and its water content.
>     >    9. The circuit may require added inductance or capacitance to allow more current to flow.
>     >   10. A chance discovery recently in a 1918 'wireless' text showed that there had been experimentation with transmitting through long wires close to the ground in 1911 (except that the wires were either connected to the earth plates via 2nF capacitors (Leyden jars, of course) or left unattached). The aerials were said to be very directional.
>     >   11. Signals on 136 kHz from a 0.85 mile base (that was not correctly matched to the amplifier output) this summer were received clearly on a 1 metre loop system at 250 km. It was noted at the time that the base impedance was also higher at 73 kHz than at 136 kHz.
>     >
>     >  Current loops in the ground may achieve l /2 or greater even with very short bases.