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Re: LF: Heidelberg remote MF receiver

To: [email protected]
Subject: Re: LF: Heidelberg remote MF receiver
From: DK7FC <[email protected]>
Date: Sat, 27 Jun 2015 14:26:54 +0200
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Hello Markus,

Most interesting, thanks! Let us use the strong QRN to check if SL does perform better if the SpecLab NB is used in front of WSPR. Ah i could use a 2nd instance for a 2nd instance of WSPR(2), feeding VAC1 to VAC3 including the NB, while the current instance feeds VAC1 to VAC2 without a NB to WSPR(1).

Just continue to run your WSPR in 100% mode, if you like :-)

73, Stefan

Am 27.06.2015 14:12, schrieb Markus Vester:
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)
 

From: DK7FC
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
 

From: DK7FC
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)

 
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