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Ground Parameters For Hams Ground Parameters For Hams
Do we have a problem and can we do
something about it?
Rudy Severns N6LF
rudys@ordata.com
www.antennasbyN6LF.com Note
Copies of this presentation are available on
my web site:
www.antennasbyN6LF.com
Because of time limitations this
presentation is very short but a detailed
multi-page discussion of the issues,
equipment examples, test data, calibration
information, etc, etc, is available on my web
site.
Both are in .pdf format. NEC Modeling
Antenna modeling on a computer has assumed an
important role in antenna design.
The values for ground conductivity (sigma) and relative
permittivity (Er) are key inputs especially for verticals.
However, there are some problems:
most of the time the values selected are little more
than a wild guess,
fixed "typical" values are used which do not reflect
reality as the frequency is changed,
and it is seldom appreciated that for a given sigma,
there may be a wide range of Er values. A major problem
Soil parameters by nature vary widely:
from site to site
laterally within a site
vertically
soil is often stratified
over time due to rainfall and dry seasons
Can we actually make useful
measurements? Conventional idea
Simple low frequency sigma measurements with
something like a Wenner array represent a good
approximation to sigma on the lower HF bands. The idea is based on IEEE Std. 356-2001 comment
"The curves of conductivity and relative
permittivity in ITU-R Recommendation
527-3 exhibit no dispersion in the band
3-30 MHz, whereas measured values
show significant dispersion in the band
for which surface soils typically can
show characteristics from lossy
conductors to lossy dielectrics ........
Therefore, the ITU values for the HF
band are inconsistent with the results of
complex variable theory and are in
error." Typical HF conductivity plots
0.004
0.006
0.008
0.01
0.012
0.014
0.016
1
2
3
4
5
6
7
8
Frequency (MHz)
S
o
i
l
c
onduc
t
i
v
i
t
y
(S
/
m
)
Rose garden
comparison between rose garden and antenna hill
Hill
3/07/05 Typical HF Er plots
24
29
34
39
44
49
54
59
64
69
74
79
84
1
2
3
4
5
6
7
8
Frequency (MHz)
R
elati
ve d
i
electr
i
c co
n
stan
t
Rose garden
Hill Comment on high Er values
The following quote is from the King and
Smith, Antennas In Matter.
"For some time, the high values of permittivity
and the dispersion at these lower frequencies
were thought to be artifacts of the measuring
procedure; that is, it was thought that they
were caused by electrochemical effects at the
interface between the metallic electrodes and
the sample of rock or soil. Measurements
made using several different materials for the
electrodes, however, indicate that there is a
high permittivity associated with the
geological material apart from any electrode
effects." Ground measurements for hams
Whatever scheme is adopted for ground measurements
by hams has to have certain attributes:
it has to use simple and low cost mechanical
apparatus
and
the instrumentation should be no more advanced
than an MFJ or AEA or similar impedance analyzer
the measurement procedure must be quick and easy
with data reduction on a spreadsheet.
Vector network analyzers, like that by N2PK which are
appearing in many shacks, are great but should not be
required. Comments on measurement accuracy
For horizontally polarized antennas more
than ¼-wave above ground, the ground
parameters are not critical.
However, for verticals, close to ground, the
parameters are important.
Fortunately we do not need 1% accuracy.
+/- 25 % is just fine! Using the decay rate of the E-field
It is very common in the BC field to measure the rate of
decay of the electric field along a radial line away from
an antenna.
For soils dominated by resistivity this can give a
reasonable average value for conductivity by curve
fitting to the data.
However,
at HF where the capacitive component matters, there
are often several different combinations of sigma and
Er that fit the curve
in addition this approach doesn't give good
resolution close to the antenna Soil probe technique
It is possible to insert a probe into the soil and measure
the impedance of the probe to determine soil parameter.
The idea is that the probe can be represented as either a
simple capacitor or an antenna in the soil and soil
parameters can be deduced from the impedance.
There are many kinds of probes but the two most
commonly used are:
a single rod inserted into the soil with a ground
screen, called a monoprobe
and
a two conductor open transmission line, called an
OWL. Monoprobe example Typical ground probes Typical OWL probes Advantages and limitations of probes
Both the monoprobe and the OWL can give quite
accurate values for conductivity and Er.
In practice the measurements are quick and easy.
But these values are only for a cylinder of soil
surrounding the probe and a short distance below it
The longer the probe the deeper you can go but in most
cases you can't get down a full skin depth as you would
like.
The longer the probe, the lower the maximum frequency
for simple calculations.
What you are measuring are the properties of a skin of
earth. The bottom line!
Are ground measurements with ground probes
worth doing?
Do they give us any useful information?
I believe they are vastly better than nothing and
are worth doing at least up through 40 m.
But we must not fool ourselves into thinking they
are more than a rough guide.