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VLF:Ground loop antennas, how to tune best?

To: [email protected]
Subject: VLF:Ground loop antennas, how to tune best?
From: DK7FC <[email protected]>
Date: Wed, 01 Aug 2018 18:59:39 +0200
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Hi all,

Today i played a bit with my linear VLF/ULF PA, some output transformers, a 50 Ohm dummy load and a 3 mH air cored coil (a smal one, from LF). I thought this could represent the ground loop antenna during tests in the shack.
With the switchable C-network, i managed to tune to resonance between
8.27 kHz down to 0.97 kHz. A usual R-L-C network. The goal was to
proove that the system can tune to and run 1 A antenna current on each
of the desired frequencies.
However, i did not manage to reach more than 500 mA 'antenna' current.
Above, there appeared significant distortions / harmonics. Why?
When i connect the dummy load directly to the output transformer (just R
instead of RLC), i can easily tune to 1 A 'antenna' current and the sine
wave almost looks perfectly. So it cannot be an issue of a saturating
transformer. Also the coil cannot saturate.
So, where are the distortions coming from?
The main component of the overlayed distortion voltage is maybe 10x higher in frequency.
BTW in the first test with the 900m ground loop the voltage showed some
distortions as well and i wondered how it comes from. Then i thought it
has to do with saturation and a far-from-ideal matching of the PA output
impedance...
Back to the test in the shack:
Eventually there are further resonance frequencies, coming from the stray inductance of the transformer and the 'internal' capacity of the coil.
Now if this can happen with discrete elements in the shack, it can
happen on the ground loop anyway! This antenna will certainly have an
interesting frequency response, TX-wise.
So my thought is: Maybe it is even better not to series resonate the
loop but to parallel resonate it! This will lead to a higher feed point
impedance, which will be frequency dependent, so it is a more complex
thing. But the parallel C should help to avoid transmitting on harmonics.
Or, i could series resonate the antenna on the frequency of interest and
then add a parallel resonated LC circuit. This is easier to do because
the parallel LC can be tuned before connecting the antenna. Then the
antenna can be connected...
Or, even something like a T-filter (2 series L and one parallel C) but
that will be complex to align when sitting in the car which is full of
electronis stuff anyway... Also it is a challenge on ULF :-)
And certainly it helps to minimise the stray reactances, e.g. by using
just as less primary turns on the output transformer as possible. But
then it is not usable on all frequencies and will need taps on the
primary and secondary side.
Any ideas / comments? :-)

73, Stefan

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