Hi,
we use 4ports-RX-splitter
https://www.ebay.com/itm/4-WAY-HF-ANTENNA-SPLITTER-COMBINER-RX-0-1-50-MHz-SO-239-connectors/322564884873?hash=item4b1a5d8989:g:JJ4AAOxyM89Sbujo:rk:1:pf:0
73! de Eugene RA0FF
http://www.qsl.net/ra0ff/
>Вторник, 25 декабря 2018, 8:17 +11:00 от Chuck Dietz <w5prchuck@gmail.com>:
>
>I think I understand much of what you are saying, but I know that I was on 160
>meter FT=8 two nights ago with the speaker up fairly loud. I only heard noise.
>I set the AGC off and adjusted the RF gain so that it did not overload. Still
>no hint of any signals, but I decoded two stations!
>
>Just sayin’.
>
>Chuck W5PR
>
>Sent from Mail for Windows 10
>
>From: K4SAV
>Sent: Monday, December 24, 2018 2:10 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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