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DIgital DX News from VK 30 march 2003

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  • Rex Moncur
    FSK441 ACTIVITY SESSION Welcome to VK1ZQR who was copied by Dave VK2AWD direct. Other stations on this weekend were, VK2IE, VK2FZ, VK2JKK, VK3HY, VK3KAI,
    Message 1 of 1 , Mar 29, 2003

      Welcome to VK1ZQR who was copied by Dave VK2AWD direct.
      Other stations on this weekend were, VK2IE, VK2FZ, VK2JKK, VK3HY, VK3KAI,
      VK3FMD & VK7MO.  Next weekend will be Type A on both Saturday and Sunday.


      A number of stations are tryng out JT44 terrestrial (troposcatter) on
      144.225.  Average signal levels in dB are as follows (where two figures are
      provided these are for separate days):

      VK3CY to VK7MO 788 km average -19, -22
      VK3AUU to VK7MO 560 km average -15 to -16
      VK3AFW to VK7MO 596 km average -10 to -14
      VK3BWT to VK7MO 621 km average -11 to -15

      VK2FLR to VK3CY 743 km average - 15
      VK2KU to VK3CY 690 km best -6
      VK2KU to VK3BWT 431 km best -5

      Mike VK2FLR has completed with VK3CY using just 10 watts.

      JT44 will typically decode 100% correct at -18 dB with useful decodes at -22
      dB. Using the averaging technique signals as low as -27 dB can be used to
      help build up a message over time.  For comparison SSB signals can just be
      copied at -6 dB.

      It has been found that meteor pings often confuse JT44 and this is a
      particular problem at distances of 650 km and more.  The solution is to:

      * run the clipping function hard at "5",
      * use the freeze facility to limit the frequency range in which you look for
      the reference tone,
      * keep the frequency tolerance as narrow as you can for the drift between
      both stations eg 50 Hz; and,
      * keep tracking the centre frequency by clicking on the peak on the red line
      each time you get a signal you are sure of. Avoid clicking if you hear a
      meteor ping in the last RX period or see one on the green signal strength

      On weekday mornings VK7MO (144.225) is beaming Melbourne from 0745 to 0845
      and Mallacoota/Sydney from 0845 to 0830.


      The following table compares JT6M and FSK441 on 6 meters using McKinley's
      equation 9.4 page 240 (ie for underdense pings).

      Advantage of JT6M over FSK441 on 6 meters in dB

                                        Number of Characters in Message
                                   2            4            8         16
      Distance Km
      500                    10.5        8.1        3.1       -6.8   -14.2
      1000                  12.1        11.3      9.5         6        3.4
      1500                  12.5       11.9     10.8        8.6      7.0
      2000                  12.8       12.5      11.2       9.3      7.9
      The table indicates that the full advantage of JT6M requires distances of
      1500 to 2000 km and that at short distances such as the VK3AXH to VK2AWD
      tests JT6M will have no advantage.

      JT44 EME

      Mike VK2FLR reports as follows:

      One new one on JT44 EME I forgot to mention was G4FUF on 13 March. He has
      a single 15 element yagi and peaked sync 5 -22dB here. He was running a
      4CX1500 which is probably good for as many watts; 400 watts would have
      produced signals of -28dB, suggesting a contact would have been possible
      given enough time. The situation would have been much the same as HL1/ON4MU
      with his 200 watts and two yagis, when I had complete callsigns but ran out
      of common moon before we could complete.


      Please look at the VK2 VHF DX site 
      http://www.vhfdx.oz-hams.org/    then
      click on "Digital Modes"

      73 Rex, VK7MO

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