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LF: Questions and resonating tests and measurements on a 700m wire anten

To: [email protected]
Subject: LF: Questions and resonating tests and measurements on a 700m wire antenna at 137 kHz
From: Stefan Schäfer <[email protected]>
Date: Tue, 12 Oct 2010 22:32:12 +0200
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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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