Just a comment/correction about JT65:
I don't know who is claiming 20-dB improvement over CW, but the
creator of JT65, Joe Taylor - K1JT has stated that "about" ten dB
improvement can be realized. I can hear -18 dB JT65 tones from eme
signals. This fits well with the theory of receiving weak-signals,
be they CW or digital.
The typical (or maybe better than typical) CW op can hear copy
signals down to 17 dB below what is SNR = 0 dB in a 2.5 KHz
bandwidth. How that is determined is by reducing bw to 50-Hz. The
improvement in SNR from 2.5 KHz to 50-Hz is given by 10Log(2500/50) =
10 Log(50) = 17 dB.
JT-65 signals can be decoded down at -28 dB referenced to 2.5 KHz,
thus about 10-dB improvement over hearing CW.
I find I need to reduce radio bw to 100-Hz when trying to copy
very-weak CW; maybe my ears/brain adds another 3-dB filtering to get
to 50-Hz (maybe not as I seem to need SNR> 6 dB to copy CW. But I
can make out tones at SNR=0 (or below).
Narrowing radio bw to 50-Hz does not help me as the filter ringing
confuses what my brain hears. Having severe hearing disability does
not help, either.
Nearly all the improvement in decoding signals comes from narrowing
bw. JT-65 digital bw is about 5-Hz using a sequence of tones that
vary in audio frequency so the FSK signal takes about 180 KHz bw in
Tx. Note 10Log(2500/5) = 27 dB.
One more note: JT-65 is run at SSB bw (typ. 2.5 KHz) since all the
weak-signal work is done in sw. Some eme JT-65 stations run 4-KHz bw
to watch several frequencies. I run MAP65 at 60-KHz bw to
simultaneously watch the whole eme sub-band 144.095-144.155. My
system does not use the K3 2nd IF or roofing filters. I've written
before about that so visit my website if you want to know more about that.
73, Ed - KL7UW
---------------------------
From: brian <als...@comcast.net>
To: "Elecraft@mailman.qth.net >> 'Elecraft Reflector'"
<elecraft@mailman.qth.net>
Subject: Re: [Elecraft] AGC INFORMATION
Message-ID: <582c591c.10...@comcast.net>
Content-Type: text/plain; charset=windows-1252; format=flowed
Hi Matt,
I used to think I could copy CW signals below the noise.
However when I put SPECTROGRAM in line and actually measured the S/N
ratio the results were disappointing.
According the SPECTROGRAM, those "in the noise" signals actually had an
+8-10 dB S/N ratio.
Try it and see for yourself.
Likewise those WSPR and JT65 guys have been mislead by the -dB (e.g.
-20) edited signal strengths. They need to know that those are
calculated with a 2.4 - 2.8 KHz assumed noise power. In reality, many
of the negative "dB" value signals can be copied by ear.
73 de Brian/K3KO
73, Ed - KL7UW
http://www.kl7uw.com
"Kits made by KL7UW"
Dubus Mag Business e-mail:
dubus...@gmail.com
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