ARLA/CLUSTER: Medir a condutividade do solo.

Segunda-Feira, 24 de Setembro de 2012 - 10:42:51 WEST

 How to measure Ground Conductivity

[image: Picture of a Measuring tape]

Over the last years I spent a lot of time simulating antennas. One
annoyance I encountered over and over was the unavailablity of values for
ground conductivity and the relative permittivity. Two figures which have
quite an impact on the antenna performance, especially on vertical
antennas. This finally led to the conclusion that both figures need to be
measured. In early 2012 I built a measurement kit and finally determined
ground conductivity and relative permittivity at our contest station
ED1R <http://www.ed1r.com/>
.

Motivation

Antenna literature only provides a few generic curves for the estimation of
conductivity and the relative permittivity of soil (see below). Being
unsure about the soilâ€™s quality can mean a difference up to the factor 1000
(city â€“ vs rice paddy). While in practice youâ€™ll have a rough idea if you
are in the desert or rather close to the sea, everything in between is far
from being clear. The better the data you feed into the simulation, the
closer your results match in reality.

Some theory

Two conductors (metal rods) with known length and diameter are pushed in
parallel into the soil. At the top, the impedance is measured. The setup is
considered as a two-wire line with an open end. The impedance between the
open ends (located in the soil) is transformed as a function of its
geometrical properties and the mediums (here: soil) porperties along the
wire. Since the geometrical properties are known, the mediums properties
can be calculated with a bit of math.
Fortunately, I could base my measurements on the intensive work of Hardy,
DL1GLH <http://www.technik.dhbw-ravensburg.de/~lau/>who published a
detailed summary of his research paper Measurement of the ground
conductivity and relative permittivity with high frequency using a open
wire line (OWL)<http://www.technik.dhbw-ravensburg.de/~lau/groundconductivity.html>
on
his website. If you are interested in the math behind this, please check
out his website.
Online Calculator

While I had to to calculate the results in Matlab, Hardy recently published
an online ground conductivity
calculator<http://www.technik.dhbw-ravensburg.de/~lau/gc.html> which
does all the hard work for you. Therefore a Matlab license isnâ€™t required
anymore.
Setup

I measured the impedance with my DG8SAQ VNWA3 Vector Network
Analyzer<http://www.sdr-kits.net/VNWA/VNWA_Description.html>,
my good old MacBookPro and a homebrew two-wire line (see pictures below).
Once the impedances where measured and written down, I calculated the
conductivity (sigma) and relative permittivity (epsilon) with Mathworks
Matlab <http://www.mathworks.com/>.

Two square bars are functioning as a guide to ensure that the metal rods
are pushed in an equal distance into the earth.

Once the metal rods are pushed into earth, the square bar guides are
removed. This is done by removing the nuts. The upper square bar is then
replaced by the connecting bridge. The nuts are tightened and the VNWA is
connected with a coax cable to the bridge.

Best practice

There are no perfect length, distance and diameter for the two-wire line,
but when you create your own measurement kit you might find the following
advices helpful. They are the outcome of my experiments:

   - Avoid rods longer than 300mm. Itâ€™s quite difficult to penetrate
   agricultural soil for more than 300mm without using much force.
   - Just use your foot or hand to push the rods into the soil. Donâ€™t use a
   hammer or stone to push the wire-line into the ground. It is crucial that
   they remain exactly parallel!
   - If the soil is too hard, try to push the rods in a 45deg angel into
   the soil
   - 5 â€“ 10mm diameter of the rods is a good trade-off.
   - Aluminium might be too soft. Try to use steel
   - A distance of 50 â€“ 100mm between the parallel rods should work well
   - Impedance should be located between 30 Ohm â€¦ 500 Ohm

Performing the Measurements

I performed the measurements at two different spots. Both spots are
agriculturally used fields. Spot1 is a household garden and spot2 is a
grain field. Find below some pictures taken during the measurements.

  Results

Find below the results of the measurements taken at the two spots
Results Garden (Spot1)  Frequency [MHz] Impedance [Ohm] Conductivity [mS]
Permittivity 1.85 236.3-j51.4 12.2 26.6 3.60 208-j66.1 13.1 21.8 7.1
170.5-j74.6 14.5 17.4 14.2 125.7-j72.3 16.7 14.2 21.1 96.5-j60.6 19.1 13.3
28.2 85.7-j40.3 22.9 11 Results Grain field (Spot2)  Frequency [MHz] Impedance
[Ohm] Conductivity [mS] Permittivity 1.85 338.8-j157.0 7.3 33.4 3.60
249.3-j161.8 8.4 28 7.1 167.3-j138.6 10.4 22.8 14.2 105.5-j99.2 13.7 18.3
21.1 79.5-j72.3 16.8 16.2 28.2 66.0-j48.6 21 14.8

Discussion

   - When comparing the results with the charts taken out of the standard
   broadcast-engineering literature, they seem to be plausible. Considering
   that fertilizers are applied to to both fields it is reasonable to find the
   values close to *Rich agricultural land*.
   - I suppose that more fertilizers are applied to the garden, which could
   explain the better conductivity.
   - The measured conductivity & relative permittivity explain the good
   results we achieve on the lowbands during contests. We always have the
   impression to be loud on the low bands.
   - Conductivity & relative permittivity can vary (significantly) between
   parcels of land. The two measurements are an indication but donâ€™t paint the
   full picture. Consider that the area most influential is the nearfield and
   then the farfield up to a distance of 10 Lambda.
   - Soil penetration is also an important fact which needs to be
   considered, especially with *bad* ground. Depending on the conductivity
   and frequency, the current can penetrate up to 100 meter deep the
   underlying ground. Make sure you know the various layers of soil you are
   standing on.

Conclusion

The two-wire method is an easy way to determine the conductivity and
relative permittivity of soil. With amateur means, fairly precise results
can be achieved. Measurements should be taken at various points since the
ground characteristics can change significantly between two parcels on
land. The soil quality at ED1R is close to *Rich agricultural land*. Now
further antenna simulations can be improved by using the exactly determined
values.

-- 
AntÃ³nio Matias

CT1FFU - CR5A

www.dxpatrol.com

QRV: HF, 6m, 4m,2m,70cm,23cm
SSB, CW, MGM
DX, EME, Contest,Sat.

POBOX-38
2504-909
Caldas da Rainha
Portugal

-- 
Para mais informaÃ§Ãµes/opÃ§Ãµes visite o site:
http://groups.google.com/group/ct-comunicacoes-e-tecnologias<http://groups.google.com/group/ct-comunicacoes-e-tecnologias?hl=en?hl=pt-PT>
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