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JT4 DT Window

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  • Rex
    I am looking for advice on the timing window within which JT4 is designed to decode. As background I note that when W5LUA is using his flex radio SDR there is
    Message 1 of 3 , Jul 10, 2014
      I am looking for advice on the timing window within which JT4 is designed to decode. 

      As background I note that when W5LUA is using his flex radio SDR there is a processing delay of 2.3 to 2.4 seconds and that this can extend the DT at the maximum Moon distance to over 5 seconds.  Perhaps 5 seconds is beyond the DT window for JT4 and this explains the lack of decodes or even good syncs.  The table below compares results on 24 GHz.  The DT at VK7MO is within the correct range with an analogue receiver but that at W5LUA seems to be extended due to the use of an SDR receiver.
       

      Date

      Moon Distance

      (km)

      DT Based on Moon Distance (secs)

      DT at VK7MO

      (secs)

      DT at W5LUA

      (secs)

      30 June

      405653

      2.70

      2.9

      Nil Decodes

      8 July

      378562

      2.52

      2.4 to 2.7

      4.8 to 4.9

       

      Based on the data for 8 July there is a delay in reception by W5LUA (which appears to be due to the use of a Flex radio SDR) of around 2.3 to 2.4 seconds.  It is noted also that the interrupt process on WSJT can cause variations of up to +/-0.2 seconds.  Thus on 30 June there is the potential that the signal after W5LUA’s SDR was delayed to 2.7 +2.4+0.2 or 5.3 seconds.  The question then is whether 5.3 seconds is within the timing window for JT4?. 

      73 Rex VK7MO


    • Joe Taylor
      Hi Rex, My recollection is that JT4 probably stops trying to decode at about DT=5 s. A processing delay as large as 2.3 s seems much larger than necessary. It
      Message 2 of 3 , Jul 11, 2014
        Hi Rex,

        My recollection is that JT4 probably stops trying to decode at about
        DT=5 s.

        A processing delay as large as 2.3 s seems much larger than necessary.
        It seems likely that for some reason W5LUA has configured his software
        with some very steep-edged filters, thereby causing large delays. For
        the sort of things he is doing -- e.g., microwave EME -- there would
        seem to be no reason to do this. Perhaps be can reconfigure his SDR
        software to reduce the delay. (My Linrad setup, as an example, has
        processing delay less than 1 s.)

        -- 73, Joe, K1JT

        On 7/10/2014 6:07 PM, Rex rmoncur@... [wsjtgroup] wrote:
        > I am looking for advice on the timing window within which JT4 is
        > designed to decode.
        >
        > As background I note that when W5LUA is using his flex radio SDR there
        > is a processing delay of 2.3 to 2.4 seconds and that this can extend the
        > DT at the maximum Moon distance to over 5 seconds. Perhaps 5 seconds is
        > beyond the DT window for JT4 and this explains the lack of decodes or
        > even good syncs. The table below compares results on 24 GHz. The DT at
        > VK7MO is within the correct range with an analogue receiver but that at
        > W5LUA seems to be extended due to the use of an SDR receiver.
        >
        > Date
        >
        >
        >
        > Moon Distance
        >
        > (km)
        >
        >
        >
        > DT Based on Moon Distance (secs)
        >
        >
        >
        > DT at VK7MO
        >
        > (secs)
        >
        >
        >
        > DT at W5LUA
        >
        > (secs)
        >
        > 30 June
        >
        >
        >
        > 405653
        >
        >
        >
        > 2.70
        >
        >
        >
        > 2.9
        >
        >
        >
        > Nil Decodes
        >
        > 8 July
        >
        >
        >
        > 378562
        >
        >
        >
        > 2.52
        >
        >
        >
        > 2.4 to 2.7
        >
        >
        >
        > 4.8 to 4.9
        >
        > Based on the data for 8 July there is a delay in reception by W5LUA
        > (which appears to be due to the use of a Flex radio SDR) of around 2.3
        > to 2.4 seconds.It is noted also that the interrupt process on WSJT can
        > cause variations of up to +/-0.2 seconds.Thus on 30 June there is the
        > potential that the signal after W5LUA's SDR was delayed to 2.7 +2.4+0.2
        > or 5.3 seconds.The question then is whether 5.3 seconds is within the
        > timing window for JT4?.
        >
        > 73 Rex VK7MO
        >
        >
        >
      • Rex
        Thanks Joe That does look like the explanation and I have passed your reply on to Al. For info I am on the mainland at GippsTech and did a demonstration of
        Message 3 of 3 , Jul 11, 2014
        Thanks Joe

        That does look like the explanation and I have passed your reply on to Al.

        For info I am  on the mainland at GippsTech and did a demonstration of small station EME on 24 GHz last night which was very successful under good conditions.  Since we did our paper I have upgraded my power from 9 watts to 17 watts with a pair of DB6NT PAs phased up with the Magic Tees that Charlie G3WDG reported on in the recent DUBUS.  And I can now work Al under average rather than needing good conditions.  The following compares results with the 9 watts and the 17 watts over tests with various levels of total loss due to Precipitable Water (PW) at both ends plus degradation and spreading loss.



        As you can see we still failed under the worst conditions on 30 June but this might have been due to the DT window or just that we were at the limit with 17 watts so I expect I still need JT4W for the worsts conditions and low elevations to Eu.  I will be doing a grid square tour on the mainland on 24 GHz over the next 2 months and be very keen to test JT4W under the worts conditions if you need some on air results.

        73 Rex VK7MO



        On 12/07/2014 1:23 AM, Joe Taylor joe@... [wsjtgroup] wrote:
         

        Hi Rex,

        My recollection is that JT4 probably stops trying to decode at about
        DT=5 s.

        A processing delay as large as 2.3 s seems much larger than necessary.
        It seems likely that for some reason W5LUA has configured his software
        with some very steep-edged filters, thereby causing large delays. For
        the sort of things he is doing -- e.g., microwave EME -- there would
        seem to be no reason to do this. Perhaps be can reconfigure his SDR
        software to reduce the delay. (My Linrad setup, as an example, has
        processing delay less than 1 s.)

        -- 73, Joe, K1JT

        .


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