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Dick, conventional wisdom says that the soil
resistance becomes lower as the soil becomes wetter. However your
measurements indicate the antenna ground loss becomes greater with increased
wetness. Perhaps there is another coupling mechanism here. For example, if one
had a loop antenna suspended in free space, would the ground loss resistance
become zero?
Ralph Lane
----- Original Message -----
Sent: Friday, December 13, 2002 7:39
AM
Subject: LF: Measuring earth
resistance
To All from PA0SE
There were some
requests for info on the way I measured the earth resistance of my aerial
system.
In 1988 I made an impedance bridge with a noise source.
In the first version of this e-mail the circuit diagram was included but it
did not pass the reflector.
Most bridges of this type have a
transformer between the noise amplifier and the bridge circuit. But I found it
impossible to make the bridge frequency independent up to 30MHz. By putting
the transformer between the bridge circuit and the detector I managed to make
the readings reliable up to 30MHz.
For measuring earth resistance I
connect the bridge between the bottom end of the loading coil and the earth
connection, after disconnecting the transmitter from the coil. The
X-control is left at the zero position
The detector (receiver = "ontvanger
" in Dutch) is tuned to 136.5kHz and the vacuum capacitor in parallel with the
tuning coil and the R-control of the bridge tuned for minimum noise. The
minimum is so frequency-dependent that it becomes deeper as the bandwidth of
the receiver is reduced!
The total resistance of the aerial system can
then be read from the R-control. But for better accuracy I measure its
value with a digital multimeter.
From the value so found I subtract
the loss resistance of the loading coil. What remains I call the earth
resistance. Of course in that value are also included the radiation resistance
and the resistance of the aerial and earth wire, But these quantities are
negligible as compared to the earth resistance.
In the earth resistance
will also be included losses in the aerial insulation and in the capacitance
of the aerial to surrounding objects like trees (no trees in my case). But
these losses are impossible to separate from the real earth
resistance.
Measuring earth resistance at 50Hz has no meaning for
the loss at 136kHz because the earth resistance (impedance is more correct) is
frequency dependent. At low frequencies earth acts like a resistor, at
high frequencies like a capacitor in parallel with a resistor.
I find
the loss resistance of the loading coil with its tuning capacitor in parallel
as follows.
I use my W & G signal generator and tracking selective
voltmeter type PSM-5. The generator is connected to a one-turn loop that is
positioned at some 35cm from the bottom end of the coil. An oscilloscope probe
is connected to the level meter and hung near the top end of the coil. The
aerial is replaced by a variable capacitor (broadcast receiver type). The
PSM-5 is adjusted to 136500Hz and the variable capacitor tuned for maximum
voltage over the coil and its value noted. The PSM-5 is then tuned higher and
lower to the frequencies where the voltage over the coil has dropped to 0.707
(-3dB) of the maximum. The frequency difference between these two frequencies
is B (I use a frequency counter to improve accuracy; B is rather small). This
yields Q = 136.5/B (Q = 350 in my
case).
The value C of the variable capacitor is then measured. This yields
the reactance of the capacitor
X = 6.28 * f * C
ohms. Because there was resonance this is also the
reactance of the loading coil in parallel with its tuning capacitor.
We
now find for the loss resistance of the loading coil: R =
X/Q.
My measurements support the findings by Alan
Melia. The earth resistance is higher in winter than in summer and is maximum
when the earth is soaked with water. When the water freezes the resistance
goes down.
I hope this info is of some use.
73, Dick,
PA0SE
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