Thank you Alan for your reply.
One more difficulty could be the different radiation pattern of the two
configurations. So the signal difference at a specific location must be
seen with care. If the signal decreases by say 6...10 dB at different
RX stations (in N, S, W, E) one could get a doubtless information which
configuration is better. But probably the results will not be so clear ;-)
73, Stefan
Am 13.10.2010 00:02, schrieb Alan Melia:
Hi Stefan. I believe this antenna is through trees and foliage. This may
mean the measurements maybe not what you think they are. The voltage on the
grounded end configuration will probably be lower. this may mean that you
have less of the power going though the tree foliage. In the ungrounded
state you have a lot more of your power leaking away from the foliage before
it gets to the end. It might be that this shunt path lowers the apparent
resistance. The only way to test the efficiencies of the two configurations
is to get a relaible comparative field strength measurement. It probably
doesnt really matter if the receiver is not accurately calibrated provided
you can get a a reading of the dBs difference. This doesnt affect the
resonance tests you have done to determine how to get the most current
flowing in the wire.I dont think you can sensibly calculate ERP in this
sytem. Of course the length is now a significant portion of a wavelength (~
3/8th ) so this has implications too.
Interesting experiment I await the recieve measurements with interest.
Alan G3NYK
----- Original Message -----
From: "Stefan Schäfer"<[email protected]>
To:<[email protected]>
Sent: Tuesday, October 12, 2010 9:32 PM
Subject: LF: Questions and resonating tests and measurements on a 700m wire
antenna at 137 kHz
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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