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Re: LF: RE: Ground loop antennas, how to tune best?

To: [email protected]
Subject: Re: LF: RE: Ground loop antennas, how to tune best?
From: DK7FC <[email protected]>
Date: Fri, 03 Aug 2018 14:54:13 +0200
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Hello Jim,

Thanks for your contributions.

Well, the PA is not clipping, also it was fused at 5 A and it can deal with 10 A easily. The limiting factor was the available transformer which started to saturate at 2.97 kHz at the given number of primary turns. Also the output impedance was not well matched to the antenna load, and finally the load impedance had a reactive component. All this leads to a poor efficiency and thus i decided not to run even more power. But now, for the coming experiment i'm better prepared with 3 different transformers which should operate well in the ULF spectrum, at least in the upper half of it.
Am 02.08.2018 18:03, schrieb [email protected]:
If the above applies, one solution might be to characterize the transformer and 
load with a swept-sine (or impulse) signal, and then compensate with lumped L's 
and C's; but I have not done that with large VLF earth-loops or with large VLF 
buried copper-wire loops because I suspect that some of the parasitic L/C 
effects that I have seen with oscilloscope connected to the PA and matching 
network in those cases might be from distributed capacitances and 3-D 
distributed inductances in the earth; and compensation might therefore be 
difficult.
Thus it may be necessary to add external reactive components, not only the series -C to compensate the inductive part on the fundamental frequency but also considering the impedances (and the radiation resistance!) on harmonic frequencies. Means, adding a filter :-) But in the next experiment i will try to compensate the inductive component of the antenna only and then watch the V/I curves...
It is a most interesting project for me and i am glad that the antennna
already showed such a good efficiency that the 8.27 kHz signal showed up
very clearly on Paul's grabber, a distance almost 20x higher than the
previous VLF distance record from a transmit earth antenna.
So i'm hoping to become able to carefully determine the inductance of
the loop at various frequencies, which tells something about the
imagined loop dimensions.
Also think about the 'records' down on the way to DC :-) Crossing the
far field border on a certain band makes the main difference between
'local tests' and serious RF radiation. It is a different league, at
least in my personal view. Another league may be the crossing a distance
of 1 wavelength.
On that QTH, the distance to my own grabber is 55.6 km so i can reach
the far field on any frequency down to 870 Hz! So far, the lowest
frequency detected in the far field (all by amateurs of course!) is
still 2970 Hz! And last week i got an easy detection at 2970 Hz over
that distance, even in 424 uHz and despite heavy QRN! So, with a 3...6
dB stronger signal, maybe starting to transmit a few hours earlier, i
assume that i can crack my own record and move a step deeper, down to
2.47 kHz.
Whether or not the above applies, I wondered if swamping parasitic-L/C-induced 
I/V peaks at the PA output with a lower-impedance PA (and perhaps some parallel 
resistance) might keep the PA transistors in their SOA, and enable 90% 
efficient operation closer to your goal of ~ 50W PA power. Using a 500W linear 
PA to provide 50W (at 90% efficiency) would be expensive, and inexpensive Class 
D is more susceptible to I/V peaks, but I thought I would mention this 
possibility just in case.
Yes, certainly i will reach the limits very soon. But for a next experiment it is worth to run the current setup. It will depend on the results and the common interest if and how i proceed. I could borrow a generator an run > 1 kW... Then, and also for the best flexibility down on the way to DC, a H bridge class-D, driven with a variable supply voltage and steered by some IR2113 for example, may be the choice for me. Then i will not need an output transformer at all! But i would have to spend a low-pass filter of course...

73, Stefan

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