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Antenna comparisons

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  • Rex Moncur
    Hi All The following is the results of some tests to compare a small and large yagi on 2 meters with WSJT. In summary the small antenna performed better by
    Message 1 of 1 , Mar 6 9:04 PM
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      Hi All
       
      The following is the results of some tests to compare a small and large yagi on 2 meters with WSJT.  In summary the small antenna performed better by between 60% and 80%.  The improved performance is believed to be due to the wider beamwidth antenna seeing more sky and picking up more meteor signals.
       
      ARRANGEMENTS FOR TESTS
       
      The tests involved 5 stations roughly in line TXing for a half an hour (30 seconds on and 30 seconds off) and receiving at one site with two FT-847s with separate computer set ups -  one connected to the large antenna and one to the small.  The distances of the transmitting stations varied from 800 km to 2000 km.  Each station transmitted a set of three characters in a group and a valid signal was counted if three of the same were received on one ping or burn.  The Large Antenna is a 10 element yagi 3 wavelengths long and the small antenna a 6 element yagi one wavelength long.  The power of the transmitting stations varied from 60 watts to 400 watts and their antennas from 8 to 12 element single yagis.  The results were averaged over three days.
       
      CONCLUSIONS
       
      1.  If both antennas have the same background noise then a small 6 element yagi will outperform a larger 10 element on MS ( based on these limited tests by around 60 to 80% more readable signals)
       
      2. On rather limited results Conclusion 1 seemed to be independent of the distance in the range 800 to 2000 km.
       
      3. Where a lower gain antenna picks up more background noise due to its wider beamwidth this can seriously degrade its performance.  It is easy to check this by swapping antennas on WSJT and watching the RX noise level in the absence of a signal.  (on the basis of the limited tests it seems a 2 dB increase in background noise level is tolerable but at 6 dB the number of readable signals may drop to as little as 20%).
       
      4. A dual diversity reception system with a small and large antenna can provide a useful improvement in performance ( again while the tests were limited they demonstrated around a 20% increase for dual diversity over a small antenna on its own.)
       
      Thanks to VK4TZL, VK2EI, VK2FLR, VK2FZ, VK3ANP, VK3AXH, VK3AEF and VK5AKJ for contributing to these tests and to Scott VK7HSE for loaning the second FT-847.
       
      73 Rex, VK7MO
       
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