ARLA/CLUSTER: Medir a condutividade do solo.
Fernando Dinis Silva
scacem gmail.com
Terça-Feira, 25 de Setembro de 2012 - 16:20:23 WEST
Amigo João obrigado por ter encontrado este post pois temos tanto falado
aqui nas terras e muita gente não sabe medir a *respectiva* terra.
(Nota este email não foi escrito com o novo acordo ortográfico)he..he
2012/9/24 João Costa > CT1FBF <ct1fbf gmail.com>
>
> 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 soil’s quality can mean a difference up to the factor 1000
> (city – vs rice paddy). While in practice you’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 isn’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. It’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. Don’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 – 10mm diameter of the rods is a good trade-off.
> - Aluminium might be too soft. Try to use steel
> - A distance of 50 – 100mm between the parallel rods should work well
> - Impedance should be located between 30 Ohm … 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 don’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.
>
> --
> António 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 informações/opções 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>
>
>
> _______________________________________________
> CLUSTER mailing list
> CLUSTER radio-amador.net
> http://radio-amador.net/cgi-bin/mailman/listinfo/cluster
>
>
-------------- próxima parte ----------
Um anexo em HTML foi limpo...
URL: http://radio-amador.net/pipermail/cluster/attachments/20120925/de0d8bb4/attachment.htm
Mais informações acerca da lista CLUSTER