VLF,
That 3rd harmonic is on the 17 km band. Do we have any experiences
about local diurnal propagation?
Sooner or later there will be misunderstandings and some will say i am
transmitting on 17.265 kHz although the TX runs on 5.755 kHz!
I had a thought about that. 3rd harmonics are generated by all HF
amateurs, but we all ignore them. We actually can ignore them if they
are well attenuated but that doesn't mean that they do not exist! BTW
has someone ever tried to decode a 3HD signal (that name, 3HD, is just
for fun, like the Dreamers band, a shortcut for 3rd harmonic detection,
reminds on HD quality movies or 3D glasses ;-) ) in OPERA mode?
Example: When someone transmits a legal (!) QRO signal of 750W
(in DL) on the 160m band using a typical lambda/4 vertical (which is
radiating the 3HD signal because there it is lambda 3/4 and thus low
impedant too), using a 60 dB attenuating low pass filter, which is
quite a good supression, then there should be a 750uW signal on the 53m
band, which could be detected in some distance, probably several 10km
when using slow modes like OPERA-4 (WSPR will not work so easily here).
Receiving this 3HD signal is certainly not absolutely strictly
forbidden :-) So actually the amateurs can do legal QRPP tests in slow
modes on 53m, can't they? Would you think that it is ethical
unjustifiable to try to receive such a signal if it is there anyway and
legally generated?
Example2: PA0WMR is often TXing OP8 on 478.x kHz, making 2000km
distance. If someone in say 30 km distance can receive him on 1434 kHz
(if there would be no AM BCD stn), close to the decode limit, would
something be wrong with this test?
I know from WSPR QRP transmissions from DL to VK on 15m with 0.1 W TX
power!! So i assume a 3HD signal could be detected in a few 100 km,
maybe?
Below 8.3 kHz there is no TX power limitation and we can certainly
assume that no interference is caused to anyone at 3HD, just like in
the above example (53m band ( i never tried it)).So why not trying to
do tests like these today? The noise floor is much lower there which
improves the situation :-)
73, Stefan/DK7FC
Am 02.02.2014 18:15, schrieb Markus Vester:
Stefan,
your third harmonic is clearly
visible in Holland
and here
Started spot-on 17265 Hz, now
another peak is coming up 15 mHz up.
Good luck,
Markus (DF6NM)
Sent: Sunday, February 02, 2014 3:27 PM
Subject: Re: LF: DX VLF experiments in 3HD
Markus,
I am 16 dB stronger now!
Am 02.02.2014 14:52, schrieb Markus Vester:
So what should be expected
here? Scaling the fieldstrength from 1 km to 180 km (1/r^3 to
lambda/2/pi, 1/r thereafter), the signal should be 82 dB weaker here on
5.755, and 63 dB weaker on 17.265 kHz. Neglecting ionospheric contributions (not really
correct), and assuming similar noise levels (though 5.5 kHz is much
affected by railway QRM here), my SNR in 0.47mHz should theoretically
have been
35 - 82 + 10 dB = -37 dB on 5 kHz
and
50 - 63 + 10 dB = -3 dB on 17 kHz.
So while a detection on the
fundamental seems pretty hopeless, the third harmonic should already be
getting close.
Yes, the QRN and QRM is much lower in 3HD :-) Why didn't we get this
idea earlier???
What is the inductance of the
loading coil?
Ehm , i didn't really measured it, it's just trial and error.
Would be interesting to see a
picture of the scope trace picked up near the antenna ;-)
Looks quite good but could cause misunderstandings...
The amperemeter in the coil shows 350 mA now! There is some kind of
selfoscillations, audible. Thus i do not rise the power even more. Have
to search, find and fix this problem later. The AC voltage from the
vari-transformer is 110V now, so there is room for at least another 6
dB. Power is in the range of 100W now.
I'm going to measure the strength at home now. Hopefully the signal is
audible :-) Yesterday it was still weaker than the mains hum, so far
away from any interferences!
73, Stefan
Best 73,
Markus (DF6NM)
Sent: Sunday, February 02, 2014 2:06 PM
Subject: Re: LF: DX VLF experiments in 3HD
Markus, VLF,
Am 01.02.2014 20:34, schrieb Markus Vester:
Stefan,
running two 478 µHz "6000"
windows centered on 5755 Hz (on the grabber, but much QRM here) and
also 17265 Hz (offline) which seems more quiet. So hoping for a long
carrier...
Well, i hope that 16 hours key down were long enough ;-)
On which of the two frequencies
would you expect to radiate more power?
I don't know. But the harmonic signal is very weak and cannot cause
interferences for JXN or HWU or SAQ or GBZ...
I've done measurements at home, 950m distance, of that 5.755 kHz
transmission and saw that the signal was very weak, see attachment.
Just 50 dB SNR in 4.5 mHz at 3HD. The TX power was just 15W at the end.
Also the frequency response of the active antenna + soundcard is
attached.
The experiments are very interesting here and i like to do the
measurements. Despite rain the coil was on the top of the building and
on air. It was just covered by a trash bag. The voltages are very
small...
I just hang out a 30cm long wire connected to a probe of the
oscilloscope and so the voltage waveform of the antenna wire. It nearly
looks as expected :-)
Now i got another idea. There is a 2.2 uF WIMA MKP-10 capacitor in
series to the output of the H bridge, just to protect it against damage
in case of a DC output. Now i've disconnected the antenna and tuned for
the series resonance of this C and the L of the primary winding of the
loading coil. The resonance is at 1.77 kHz. This allowed me to
calculate a much smaller additional series C which helps to attenuate
unwanted frequency components, i.e. makes the spectrum cleaner and
provides a lower impedance on the wanted frequency :-)
There is really no alternative to that H bride PA which works from 10
Hz to 100 kHz :-)
in a few minutes i will be on air again for a longer time, hopefully
with a much stronger signal.
Maybe you can detect something of that 5.755 kHz transmission.
73,GL, Stefan/DK7FC
BTW Today I have updated SpecLab
from version 2.79 b08 to b10 (Jan 26), and it seems that GPS 1-pps sync
is finally working well now, even with 42 µHz resolution.
Good luck for the test!
73, Markus (DF6NM)
Sent: Saturday, February 01, 2014 5:46 PM
Subject: LF: DXl VLF experiments in 3HD
VLF,
This evening and night i'm planning to transmit on 5.755 kHz,
using the large antenna, the large coil and the large PA :-)
I don't know what can be expected and how the system works at that
frequency. Actually it is just a first test for playing arround and
doing some measurements :-)
If some nearby stations want to give it a try, maybe even in 3HD,
then don't hesitate :-)
73, Stefan/DK7FC
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