Ok, ok. I said it wrong. When I think of 160, I say “night,” It was about
4:00 pm local time.
Chuck W5PR
On Mon, Dec 24, 2018 at 5:46 PM JC <n4is@n4is.com> wrote:
> Jerry
>
> The new mode FT8 is not all that new, actually, there are several aspects
> to
> consider, like detect the signal, decode the signal detected, make a
> decision to accept the decoded signal. The improvement on signal to noise
> ratio concept is very old, just the internet made it possible with time
> synchronization. The decode uses new algorithms and some very intelligent
> way to guest the decoded signal.
>
> Check this out. 1975 Sept QST; Coherent cw test! Experiments show 20 db
> Signal Boost over QRM,
>
> http://www.arrl.org/files/file/Technology/tis/info/pdf/7509026.pdf
>
> The improvement on signal to noise ratio is just because a narrow
> bandwidth.
> The gates opens at the right millisecond window. On FSK the secret salvage
> is time synchronization. You can record the audio and play it back, the
> decode will happen only if you synchronize the time of the recording with
> the time in ms of the PC clock.
>
> I did that, and it worked, I have a SDR QS1R and using HDSDR software to
> record the I/Q file, RF file. I used to record rare DX expedition signal
> and
> the bandwidth is 50 KHz, I can see the FT8 guys on 1840, My question was ,
> can I decode them from the digital file recorded several month ago?
>
> I started plaining the file at the top of the second count, and voalah!!!,
> The WSJT-X decoded several station, weak as -21 db. The weak signals are
> there, buried in the noise on my old digital recorded file.
>
> Then I decided to test my HWF, the practical result measuring cw signal is
> that the signal to noise increase around 20 db, 10 db due the directivity
> RDF 11.5 and another 10 db from the polarization filter. The Horizontal WF
> attenuation on vertical signals is over -90 db. The manmade noise vertical
> polarized is reduced below the MDS of the receiver and cannot be amplified
> by the receiver.
>
> The IC-7800 has two identical receivers. I connected my HWF on receiver
> MAIN
> and the TX antenna on the receiver SUB, I installed two instances of the
> WSJTX program, one for each receiver. After 15 minutes the number of
> decodes
> on the HWF was 20 or times more than the vertical full size vertical, my TX
> antenna 120 Ft high.
>
> Signals decoded around -21 db on the vertical was decoded on the HWF 0 to
> +1
> db. Signals less the -5db decoded on the HWF was not decoded using the
> vertical, The HWF was decoding hundreds of signals that would be -40 db on
> the decode using the vertical.
>
> I think the s/n reported by the program as ball part is actually very good
> and close to the real s/n improvement of 2 Hz BW, depending on the mode.
>
> The only real way to increase signal to noise ratio is increasing the
> directivity of the RX antenna, more real RDF means real signal to noise
> ratio improvement. I used real because it is very easy to destroy the
> directivity with integration, leaking, intermodulation, low noise figure
> etc.
>
> One bad concept, bidirectional unterminated beverage with two lobes one in
> the back and one front, it just does not work because the RDF is 6 db down
> a
> terminated beverage. Same for BOG's the RDF is bad, a K9AY works better
> because has more RDF. A simple Flag can deliver 9 db RDF is tis easy to
> hide
> too. Two Flags in phase 11.5 db and four Flags 14 db RDF, and a very clean
> pattern besides real broadband from 1 MHz to 10 MHz
>
> As you can see on the ARRL 1975 article, there is nothing new about
> improvement of signal to noise ratio reducing the bandwidth. On the
> article,
> the test was CW at 12 wpm and 9 Hz filter BW , no ring using WWV as time
> source for the synchronization.
>
> That was state of the art back in the early 70's, almost 50 years ago.
>
> 73's
> JC
> N4IS
>
>
> -----Original Message-----
> From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of K4SAV
> Sent: Monday, December 24, 2018 3:10 PM
> To: topband@contesting.com
> Subject: Re: Topband: FT8 - How it really works
>
> Although I have finished my FT8 testing, there is one final thought I would
> like to leave with you, and also to correct one statement I made earlier.
> Someone thought FT8 measured the noise in the interval when the FT8 signals
> were off, and I replied that would result in a real S/N number. That is
> not
> true as you will see in the info below. You would get a real S/N number if
> the RF was sampled, but not if the audio is sampled.
>
> I spent many years designing electronic circuits professionally, so I still
> think that way. So for a few minutes lets think about a circuit that can
> decode something below the noise floor .If you think about FT8 or anything
> similar, from a designers point of view, you suddenly realize that making a
> statement of "the circuit can decode down to X dBs below the noise floor"
> is
> almost an impossible task, that is, if you are talking RF noise floor as
> most people will be assuming.
>
> Since you will be dealing with audio, not RF, the receiver will convert the
> RF into audio and compress it into something that has a lot less dynamic
> range. How much less? Say the volume is set to a level such that the
> strongest signals do not clip, then how far down is the noise?
> You can expect that to vary on each band too.
>
> Now comes a real complication. If you were taking samples in the RF world,
> you could see the noise level on your S meter and estimate it relative to
> the strongest signals. However your circuit will be dealing with audio.
> Surprisingly, when the signals disappear, the receiver AGC voltage drops
> and
> the receiver gain increases. That produces a lot more audio signal. The
> audio noise in the case of no signals becomes higher than the audio level
> for strong signals if you are using USB bandwidth and receiving something
> similar to FT8. That condition is not nearly as pronounced when using a
> narrow CW bandwidth. Even if you put the receiver into AGC slow mode it
> won't hold for the 3 seconds when FT8 is off, so you still get the
> increased
> audio in the off period. Then there will be a sudden increase in audio
> when
> the first signal reappears, until the ACG kicks in and lowers it. This
> happens even with fast AGC selected. It's fast enough that you don't notice
> it when listening, but if you put a scope on it you can see it. Yeah, all
> that surprised me too when first thinking about it. Take a close listen
> and
> see if you agree. If you can't hear it, put it on a scope or anything that
> displays an audio waveform and it will become very obvious.
>
> If you made a statement that this circuit can decode X dBs below the noise
> floor, most people will be thinking RF noise floor. So what is it in the
> audio world that represents the noise floor in the RF world, and what would
> your statement mean?
>
> Of course you could turn off the AGC and decrease the receiver RF gain and
> that would make the audio very low when the signals disappear. That would
> also severely limit the dynamic range for your circuit since you would no
> longer have the compression supplied by the receiver.. Your circuit would
> have to cover a much wider dynamic range, similar to what a receiver does.
> So your circuit would need what? maybe 100 dB dynamic range to cover the
> strongest signals to the weakest noise floor, forgetting about decoding
> below the noise floor. Actually that wouldn't really happen because
> receivers can't produce a dynamic range of 100 dB in the audio. They may do
> it in the RF world, but not in audio.
> Receivers have no need to do that.
>
> Jerry
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