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METEORSCATTER & POLARISATION

  • Ilkka Yrjölä
  • Paloharjuntie 16, FI-45740 Kuusankoski, Finland.



  • Preferred polarisation on meteor scatter is a subject of many papers published in scientific and radio technical literature within the past 20 years. If one does not pay attention to the particular (antenna) set-up, the results seem very contraversary regarding which polarisation performs better on MS. Certain antenna systems and distances may favor vertical polarisation, but on the other hand, vertical polarisation does not work at all on distances shorter than 600 km.
     

    A series of polarisation tests was performed on 144 MHz in December 1995 and May 1996 with 3 different stations. Both back and forward scatter was utilised with signals sent using horizontal, or circular polarisation, while the receiving antenna had both vertical and horisontal polarisation.

    The list below shows how the reflections were received using horizontal and vertical polarisation. The signal strengths and duration was determined from each reflection. Limit of acceptance was drawn to -6 dB of the stronger polarisation channel signal peak strength, or less than 50% of duration compared to the better polarisation. Signals of the two different polarisation deviating less that were marked as "both polarisations received".

    On some combinations due to unsuitable antenna illumination pattern and poor S/N ratio, waning meteor activity, only one, or a two samples were analysed. This caused small number statistics, which unfortunately makes data less accurate for evaluating the differences. Automatic set-up would have been better for more accurate results.

    The test arrangements included a preparation of equal-gain preamplifiers and driver software switching between the polarisations and A/D converter. Finding the willing stations proved also to be time consuming. Noise from the computer, limited transmitting power and stormy weather during Geminids shower hampered the effort.

    The results suggest, that while vertical polarisation does provide fair amount of reflections, by no means it can be said to be superior on a typical meteor scatter path. In fact horizontal polarisation seems to work a little better in most cases.

    Before setting up a space diversity receiving system, one could try polarisation diversity first. On a few occasions, the signal fading on overdense trails was out of phase; when the signal disappeared to the noise on H polarisation, it came up on vertical and then again the other way around until the trail decayed.


    Results:
    Path H Bursts V Bursts H Pings V Pings
    Forward scatter, TX horizontal polarisation, PA3BIY-OH5IY, 1650 km path: 35 % +-26 % 65 % +-38 % 75 % +-22 % 25 % +-7.5 %
    Forward scatter, TX circular polarisation, DL8YHR-OH5IY, 1487 km path: 55 % +-16% 45 % +-13 % 70 % +-22 % 30 % +-9 %
    Back scatter, TX circular polarisation, OH2TI-OH5IY: 70 % +-22 % 30 % +-9 % nil one
    Back scatter, TX horizontal polarisation, OH2TI-OH5IY: 50 % +-35 % 50 % +-35 % 50 % +-35 % 50 %+-35 %
    * Burst: overdense trail, ping: underdense trail.
    The receiving antenna (9-element F9FT cross-Yagi) was mounted on an non-conductive mast 2.5 wavelengths above ground and all transmitting antennas were more than 10 wavelengths above ground. Both polarisations were fed to separate equal-gain receiving preamplifiers feeding two Icom R-7000 receivers, or via a diode switch, to a single R-7000 receiver. S-meter voltage was fed via an A/D converter to PC for data logging. The PC also controlled polarisation selector diode and AGC time constant capacitor switching during tests run in. Several megabytes of data was stored and later the useful parts were manually extracted and analysed and put to their proper categories.

    I would like to thank F. Hobelmann, P. Hoefsloot and J. Sirviö for taking part in the test.

    References: See the Compact MS-Soft's documentation's reference section.