Hi Stefan,
I noticed I wrote ALC, but obviously this must be AGC….
I installed Speclab, and did some tests with the 879. With the ACG off I
adjusted the gain just to the threshold of (audible) distortion of the DCF39
signal (500 Hz filter) and read-out the level in Speclab. Then I dialed to
137.700 and read out the (average) noise level. This turned out to be a 40
dB lower, however I could hear NO bandnoise at this setting of the manual
AGC… only the receiver noise.
From this I inferred two things:
A. DCF39 is more than 40 dB above the noise (but how much I cannot measure
with this method)
B. The 879 is a really lousy receiver (But I already knew that)
But: there is also good news! I did see (but not hear) your CQ at 17:40 HR
CET, see attached screenshot. My first RX on 137! So at least the receiver +
software works.
In the screenshot you can see your first CQ recorded with the FT897. At
17:32 I switched to the TS130. At first glance the signal looks better on
the TS130. Also notice that this trace is a bit off-frequency and drifts
visibly. The 130 has an analog VFO, and the frequency readout is in 100 Hz
steps. The drift might also be attributable to the SA612 mixer I use (now
with a 10 MHz xtal)
This proves that at least QRSS3 QSO’s will be possible, so I have no excuse
not finishing my PA and variometer.
Regards,
Minto pa3bca
--------------------------------------------------------------------------
Ceterum censeo Carthaginem delendam esse
-----Original Message-----
From: Stefan Schäfer
Sent: Sunday, August 07, 2011 11:56
To: [email protected]
Subject: Re: LF: Re: CW sked on 137 friday morning?
Hello Minto,
Am 07.08.2011 00:10, schrieb Minto Witteveen:
Just a question: How do you use speclab to measure the signal level of for
instance DCF39 AND the noisefloor.. By measuring both and comparing?
I don't use a commercial HF TRX for LF but a down-converter (LF-125 kHz)
so i am not limited to a SSB filter bandwidth. I can display 125...149
kHz when running the soundcard at 48 kS/s. So a slight difference here.
But generally you must switch off the AGC and ALC and noise blanking and
all that affects the output AF level. Then you can measure the absolute
levels, e.g. -10 dB for DCF and -80 dB for the band noise and -90 dB for
the RX noise.
Just read it out on the right side.
I did a audio recording during the QSOs. My loop was beaming N/S, so not
optimally for DCF-39. Anyway it was 85 dB S/N in 488 mHz (i use 488 mHz
for QRSS-3). So this is an example how to simply read out the SNR:
http://dl.dropbox.com/u/19882028/LF/DCF39%20in%20488%20mHz.png
Anyway there could be 10 dB more sensitivity on my RX to have best
reception in a quiet sunday morning in winther...
- but then I would have to disable the ALC of the FT817?.
Not sure but i would disable anything ;-)
I can try /p sometime, go out in the fields far away from man-made noise,
with the miniwhip, a 12 meter fiberglass pole, a 7Ah SLA, the transverter
and the 817.
If you are outside in a flat field, you maybe do not need to lift the
antenna to 12m! 3m could be reasonable. Yes, beeing outside will at
least show you if its all caused by local QRM...
73, Stefan/DK7FC
Regards, Minto pa3bca
--------------------------------------------------------------------------
Ceterum censeo Carthaginem delendam esse
-----Original Message----- From: Stefan Schäfer
Sent: Saturday, August 06, 2011 16:51
To: [email protected]
Subject: Re: LF: Re: CW sked on 137 friday morning?
Hello Minto,
Am 05.08.2011 23:59, schrieb Minto Witteveen:
Stefan (et al),
I did listen this morning on 136.8 to see if I could copy you or any
other station, but the results were disappointing: I heard nothing.
Thanks for trying anyway.
Well, i was in QSO mit PA0A who must be close to you, so something must
be wrong...
I have an FT817 and an FT879 as possible receivers. Both are not known
for their superb receive quality but with a 5 pole filter + additional
amplification I got them to work quite reasonable on 500 KHz.
Both rigs are very bad on LF. Very insensitive and prone to heavy
intermodulation, especially at night.
So this weekend I build a quick-and-dirty 4 MHz uptransverter with a
SA612. The Yeasu’s are much more sensitive at these frequencies. To keep
strong LF and MF stations from causing intermodulation had to add 7-pole
Chebishev filter between a PA0RDT miniwhip and the transverter. For
details see:
http://www.pa3bca.nl/index.php?option=com_content&view=article&id=23&Itemid=17
Looks good, that page.
This morning (when there was almost no man-made QRM form TV’s etc) DCF39
was S9+30dB on the 897. On (4000+) 136.8 the noise was S 5 with the 2.4
KHz SSB filter, and S1 with the 500 Hz filter.
Now I know that the FT’s S-meter is not very reliable, especially from
S1-S9. But between S9 and S9+40 it is “reasonably” accurate.
If you use SpecLab for receiving, you directly get the signal level in
dB (i.e. dB below full scale of the ADC). Then you can measure the DCF39
level and the noise floor level. Meanwhile i am getting DCF39 at 80 dB
above noise in 488 mHz with the RX loop.
As far as I could find out DCF39 generates 50 KW ERP. If Stefan generates
0.5 Watt ERP (My guestimate)
Should be a few dB more ;-)
and the distance to my QTH is approximately the same, I would expect
Stefan to be 50 dB weaker. But 50 dB below DCF39 would still be above the
noise floor? So why didn’t I hear him (or for that matter, PA0A who is
closer and has 1 Watt ERP).
Maybe the S meter is worse than you think, maybe.
Before I finish my 1 kW TX I first have to make sure that I will be able
to copy any other station.... :-)
I had the same problems. But i immediately received DF6NM audible. So if
you don't even receive PA0A, there seems to be lack of sensitivity.
What happens with the noise floor if you disconnect the PA0RDT? How many
dB does the noise floor decrease?
Can you do /p tests?
Don't expect to much activity on LF. But there are new stations coming
on the band and you could be one of them and help making the band alie
again.
CU there :-)
73, Stefan/DK7FC
capt110807174157.jpg
Description: JPEG image
|
