To All from PA0SE
Ralph Lane wrote:
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?
I am also surprised that wet ground results in higher earth resistance
than dry ground. It is also remarkable that the resistance decreases when
the wet ground is frozen. I don't feel competent to
offer an explanation. Alan Melia has observed the same phenomena
and he brought forward some ideas that I repeat here.
Hi all, Finbar EI0CF measured one of
our spiral evolutions under several
different weather conditions. Finbar's site is right on the shore of
Trawbreager Bay so he has sea water close at hand. When we had increased
the top load the ground loss was ultimately reduced to about 20 ohms in
normal (dry) conditions. As one might expect when it was actually
raining, a
high loss resistance was measured. (There would have been a water film
over
the plastic coated wire used. ) This loss reduced immediately the rain
stopped, but the loss resistance still remained about twice the normal
"dry"
value. Even long after the rain has ceased the loss stayed high whilst
the
ground was still freshly wet. The measured loss did not return to normal
for
about 24 hours afterwards.
I assume fresh rainwater has a high permittivity and also a high
resistance.
Over time salts will disolve into the wet mass reducing its
resistivity.
These measurements are still in my file as I had not yet decided how to
explain the results. However in view of "corroborating"
evidence I will see
if I can pull then together. All measurements were done using a simple RF
bridge described on my web site.
Cheers de Alan G3NYK
[email protected]
http://www.alan.melia.btinternet.co.uk
I think that a loop
antenna, suspended over ground, will suffer less loss because its near
field is mainly magnetic and I assume this results in smaller currents
generated in the earth than an "electric" antenna will
do. But there are experts on loops on the reflector and they may
care to give us their view.
73, Dick, PA0SE
- ----- Original Message -----
- From: Dick Rollema
- To: LF-Group
- 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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