Dear LF,
Today i have done some measurements on my "earth antenna" at 137 kHz.
The antenna was used as a ground loop antenna (far end grounded) and as
a inv-L antenna (far end ungrounded).
Winthin the last days the antenna was optimised. Now the first 250m are
about 8 m above ground (average). The wire was replaced by a
olive-green military wire (steel-copper-silver, UV resistant
insulation) which makes the antenna rather invisible. The rest of the
antenna is still in a height of about 4m (average). I assume these 4m
height difference could make a significant difference in the efficiency
on LF (not so much on VLF).
The H Bridge PA was used for the tests. This PA can fed the antenna
from 10 Hz (!) to 200 kHz as seen today. The applied power (DC input)
in the tests was about 25 W. A 5 pole low pass filter was applied on
the PA output.
First i wanted to measure u(t), i(t) and the phase by an oscilloscope
but this is not really running sufficient when supplied by the
generator (trigger and display problems). So i choose the LF tuning
meter (ON7YDs website) which i built in 2003.
When configuring the wire (abt 700m length and abt 600m electrode
spacing) as a ground loop antenna, it is resonated with a series L (not
C!) of about 800 µH. Then, its Z = 840 Ohm. No matter what the
radiation resistance is. Field strength measurements on my LF grabber
make probably no sense due to the low distance. I have to compare both
configurations in a further test, maybe this weekend.
After disconnecting the ground rods at the far end the antenna (now
inv-L config) was resonated by using a L of just 250 µH (estimation).
The Impedance Z was then measured to be 440 Ohm! (U=73,3 V * I=166mA).
What does these values tell us? I have not yet completely understand
the whole dependency i think. So i am looking forward to your ideas
and comments!
It seems as if the losses have been reduced in the inv-L case. But
what about the radiation resistance? How can we compare those different
antenna types?
The ground losses on VLF and down to 10 Hz (quasi DC) are about 700
Ohm, today. Since the antenna is not short against lambda (i.e.
it is even longer than lambda/4), the antenna acts not like a pure loop
and not as a usual back garden LF antenna. Additionally, the ground
losses are very high here! The ground electrode on the fed point could
be about 350 Ohm, so 50% of the total loop losses on VLF. But,
theoretically, the radiation resistance could be some Ohms (?) due to
the antenna length. If it will become longer (...lambda/2), the ground
electrodes current decreases, and so the losses? Quite difficult and
interesting, isn't it?
My idea is just to do further tests, hoping to be copied at many LF
stations, since this is the final goal :-) A simulation seems to be
very difficult, due to the many unknown parameters...
I want to find what is the best configuration (loop or inv-L) and i
want to increase the antenna to 1000 m length! :-) Then i will build a
tuner with fixed components. This will be an easy job since the
voltages and currents are moderate! :-) The impedance should be
transformed to 50 Ohm (sincei have a 25m long RG58 cable to the class E
TX that should be used later) and i hope i can improve the RX for that
antenna.
73, Stefan/DK7FC
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