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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