Dear Ed and friends, LF group,
You wrote...
...I start to understand that my ARGO in horizontal mode shows continuous
lines -
completely not such beautiful what I saw in Internet?? (any FAQ?)
A big antenna like you have will give very high receive signal levels -
when operating using a 100m high Decca mast a couple of years ago, I found
attenuation of about 30dB at the RX input gave good signal to noise ratio,
and also gave good protection to the RX against overload and damage from
static - especially if you use big resistors for the attenuator pad.
Basically, use as much attenuation as possible while you still get a good
SNR. As well as the antenna matching network, the receiver I used had a
single bandpass circuit for preselection - I did not have any problems with
overloading during the operation.
I think the most important thing when using Argo or other spectrogram
software is to adjust the gain levels correctly - I use a procedure like
this: Switch off the receiver AGC, select the narrowest bandwidth filter
and set the RF gain to give a comfortable audio level that does not
overload the IF or audio stages. In Argo, I switch the AGC option off,
select the "high" visual gain setting, and set the contrast control to the
middle of the scale.Then use the PC "volume control" to adjust the sound
card mic or line input gain so that the signal level indicator on the left
hand side of the display peaks at about 1/2 or 1/3 of full scale. Then I
set the Argo sensitivity control so that noise "snow" is just visible on
the display. If a strong signal appears, I reduce the gain using the RX RF
gain control to get a clear display. Also, make sure you have set the
center of the Argo frequency scale equal to the BFO pitch - I have
forgotten to do that a few times, and spent a long time wondering why I
could see no signals! I always avoid using the RX AGC - I find it causes
the signals to break up, as the gain varies because of noise spikes
At my QTH, apart from the band background noise "snow", it is possible to
see many horizontal lines in the display, which are due to Loran beacons
(called Chayka in Russia?). Usually I null these out using a loop antenna,
but there are still a few lines from other sources. QRN crashes cause
vertical streaks on the display. The usual local QRM problem is 50Hz noise
from mains operated equipment - to cure this, you can try moving the RX
antenna, or find and switch off the equipment causing the noise. The mains
noise may cause lines of QRM to drift across the display, or may just
increase the "snow" on the display.
Of course, a big antenna is not essential for receive, a simple long wire
with a resonating coil is adequate. So it might be a good idea to test the
receive system at your home QTH before the next expedition. - There are
several of us on this reflector who can transmit some test signals for you
- let us know when you would like them.
Cheers, Jim Moritz
73 de M0BMU
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