VLF,
In the evening i got the idea to make a comparison of the signal level
when using the LOPT and another reference signal. The levels were
measured and compared in SpecLab. I used an E field probe some 10m
distant, well shielded to prevent overloading of the input stage. No
noise reduction tools were used (connected to L1).
As the reference signal is used my netbook and again SpecLab to
generate a 8970 Hz sine wave (which looks perfect on the scope). To get
a higher voltage i took 2 of the described audio transformers (1:10
types) and switched them in parallel on the primary side and serial on
the secondary side. With a portable scope i measured 25 Vp. This
produced a signal level of -49 dB. When using the LOPT i can measure
-15 dB.
So my measurement and assumption to have 2 kV rms on the antenna is
wrong. It is just 900 V rms :-((
After the DFCW-600 test in the afternoon, the LOPT was warm, but just
the ferrite, not the windings. So there must already be some saturation
of the material. I was just running 15 Watts or so. Now i did some
changes on the drive circuit and brought the LOPT outside on the roof
again. Since it was cold first, the current consumption was just half
the value as i stopped the transmission. However the current was rising
continuously.
My new conclusion is that this kind of transformer is not really the
best choice, even for local tests. What i like to have is a compact
loading coil that can be covered and protected from rain, for permanent
tests.
Anyway it was an interesting test and i will further investigate. BTW i
received my DFCW-600 signal in 3.0 km distance when using the E probe
in the forest. S/N was 15 dB in 3.8 mHz. Should be better anyway
73, Stefan/DK7FC
Am 06.04.2012 16:54, schrieb Stefan Schäfer:
VLF,
The LOPT still works, seems to be no problem. Just a FET was sk.
Somehow all the previous problems are gone now and i am getting about 2
kV rms from the transformer. So i stopped my LF beacon on 137 and QSYed
to 8.97000 kHz. I also found the problem why my last transmission had
so many phase glitches. Now the PC is locked to GPS again. The PA takes
about 15 W input power, no not much DX is to be expected, just a local
test. I do some tests with the VLF active E probe now, receiving my own
signal in some distance and compare the S/N from the forest and a flat
field.
I think Paul can't receive my weak signal but we've often been wrong
with such assumptions :-)
BTW i'm transmitting DFCW-600 centered at 8970.00000 Hz, so i should be
outside eventual other VLF tests by DF6NM.
73, Stefan/DK7FC
PS: If all works well with the transformer over a longer period, i
intend to run a stable carrier over several days or transmit ma full
call in DFCW-60000 or so :-) But the signal is 20 dB stronger with the
big coil and another 20 dB stronger with the 300m kite...
Am 01.04.2012 23:47, schrieb Stefan Schäfer:
Hi Roger,
Well, they are not as suitable as i expected. Just a few kV can be
generated and applied to the wire. But that's fine for local tests. I
didn't measure the L but rather tried to find a resonance. I took two 1
nF capacitors and a 50 Ohm resistor in series to make an equivalent
circuit of my antenna for tests in the shack. I built a class D PA out
of the transformer, running at low input voltage and tried to find the
resonance. But there was no clear resonance. The output waveform looks
very different to a sine wave :-) Probably this is due to internal
winding capacitances, no idea. At least i spend a lot of time and do
not want to spend even more time on the LOPT project.
A few weeks ago i tried some local tests with WOLF10 and got a decode
in 950m distance, which is close in the near field, so not really
exciting. I just want to go on a bit with these tests but i won't go on
much longer with the tests. The standard QRSS or DFCW or MFSK analog
modes, readout by a human is just the best in my opinion.
73, Stefan
Am 01.04.2012 22:35, schrieb Roger Lapthorn:
Stefan et al,
Not being too familiar with LOPTs what inductance are the windings? Are
there particular non-encapsulated types that would be more suitable for
compact loading coils for lower powered 8.97kHz E-field TX antennas?
73s
Roger G3XBM
--
http://qss2.blogspot.com/
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
https://sites.google.com/site/sub9khz/
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