Hi Stefan,
since last night your two receivers have been
operating without interruption, allowing to compare results from your two
grabbers.
- WSPR: Last night, 21 of my low-power
WSPR transmissions were decoded simultaneously by DK7FC/p and DK7FC. On
average, the /p receiver had a 5.67 dB SNR advantage. For my
direction, the receive loop and the T antenna seem to have performed
similarly.
Today between 11.08
and 11:24 I sent some more SNR sequences with higher power
(0.1 W EMRP), expecting a higher SNR difference in the lower daytime background
noise. However half of the transmissions were not decoded on either
grabber, and those that were picked up by both showed only a small advantage.
This is probably due to the strong QRN from flashes from a nearby thunderstorm,
which for some reason are heavily affecting WSPR decodes. It might help
to use effective noise blanking in the SpecLab instance which is feeding
WSPR. Anyway if the statics happen to ease
off I will attempt another daytime comparison later today.
- DFCW: Between the thunderstorms this
morning, my 2 mW DFCW-60 transmission was definitely picked up more clearly
on the /p receiver. However this one suffers from a large frequency drift
(10 Hz upwards), which appears to be strongly correlated with RasPi core
temperature and solar chargerate plots - so presumably just crystal
temperature. In addition, some of the dashes appeared slightly
disrupted, either by audio glitches or by fast and small LO frequency
jitter. During the storms, my impression was that noise blanking in the
narrow spectrograms could also be optimized a bit.
There are a couple of QRM lines which are always
commonly visible on both receivers (472.36 and 477.74 kHz). I am wondering
whether you could perhaps use them as references for a SpecLab frequency
drift correction?
BTW. I have taken a number of screenshots
from your grabbers which I have copied to our private dropbox
folder.
All the best,
Markus (DF6NM)
Sent: Saturday, June 27, 2015 12:39 AM
Subject: LF: Heidelberg remote MF receiver
Hi Stefan,
it looks like signals are consistently better
on your remote receiver, perhaps around 6 dB or something in that ballpark.
So it seems that all your work is finally paying off! I'm looking
forward to see a daytime comparison tomorrow.
My guess is that the main contributor to frequency
variation would be the 461 kHz LO crystal rather than the soundcard samplerate.
Anyway exchanging the 12 MHz crystal may possibly have no effect at all, because
the samplerate of USB soundcards is usually derived from the bus
master clock (ie the RasPi) and not from the internal crystal on the
dongle.
Best 73,
Markus (DF6NM)
Sent: Friday, June 26, 2015 11:22 PM
Subject: Re: LF: 476.181 kHz from indoor loop
Hi Markus,
Really? Wonderful! :-) And the stream is still running. I'll watch it
some days now to see if all Raspi problems are gone, then the next part-project
is the RX.
Meanwhilethe sky wave is present and i can see you quite strong in
DFCW-30 with your 3 mW ERP from the loop. I missed watching you on the remote
grabber in daytime. What was the SNR ralative to the city location? Same or
better? I switched back to the T antenna in the afternoon. It would be
interesting so see you on my loop. On 21:19 i switched to the loop!
Ah and
now you can see the drift of my RX which is quite visible, but still uncritical
for QRSS-30 or WSPR.
I thought about changing the 12 MHz xtal of the
soundcard. It is the cheap SMD xtal which has 100 ppm/K but there are other
versions with 30 ppm/K. Maybe an idea, they are no expensive and still
compact...
73, Stefan
Am 26.06.2015 21:04, schrieb Markus Vester:
Hi Stefan,
good to see the remote
station working nicely now.
>> but don't see you
...yes you do ;-) There is a slight
frequency offset in the remote grabber which had put me out of your
QRSS-30 band. I now switched to my "heritage" QRG 136172.5 ahem
476172.5 Hz, and voila there it is, loud and clear.
Currently still on the low Marconi, also
about 2 mW EMRP. The relatively strong coupling to the LF
grabber E-field antenna produced some aliases and noise there, which have
been mitigated by a 475 kHz Saugkreis (trap) - same as in old AM radios
;-)
73, Markus
Sent: Friday, June 26, 2015 7:46 PM
Subject: Re: LF: 476.181 kHz from indoor loop
Hi Markus,
Thanks for your DFCW-30 transmission. I can see you clearly in daytime
on my RX in the city! However i can't see you on my RX is the garden! The
remote system seems to run stable now, at least for a longer time then in the
last 3 days :-)
http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_remote_Grabber.html
I
switched between a T antenna and a loop beaming 300/120 deg but don't see you.
It appears that the hill between us is (my garden is on the hill side, the
city antenna is more distant to the hill) actually reducing the SNR. So it
also reduces QRN from the east and favours the west. But i would prefer an
omnidirectional pattern :-/
73, Stefan
Am 24.06.2015 12:11,
schrieb Markus Vester:
I am currently running a DFCW-60 beacon on 476.181 kHz, using
the same 10m^2 indoor transmit loop as previously on LF. With 35 Watts
of RF input, estimated radiated power is around 2
mW, with lobes pointing west and east.
The daytime groundwave signal is visible in the bottom panel of the
DK7FC MF grabber. Going by the CCIR plots for 3 mS/m conductivity, the
groundwave attenuation for this distance would be about 23 dB in excess off
lossless 1/r propagation, resulting in approximately 0.2 uV/m in
Heidelberg.
Best 73,
Markus (DF6NM)
