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 <n...@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-boun...@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 > _________________ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > > _________________ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector