Polarisation on greyline paths

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G1RRR
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Joined: 2012/12/19 - 20:03
Polarisation on greyline paths

I replaced my 132" OCF dipole with a vertical and autotuner recently in an attempt to remove the poor propogation in the nulls of the dipole that exist on some bands. It is well documented that polarisation is not important using ionospheric propagation modes with suggestions of re-polarisation during the reflections. My initial observations with WSPR on 30m support this as did general operating experience. I expect most of us experience this extensively. I see paths open around Europe during daylight and to North America and Canada from just before dusk well into the night ( when I fell asleep! ).

However, I was particularly interested in observing the propagation during the greyline period over the last few weeks. My observation is that on 30m around dusk, contacts from the UK to Australia are possible during this period, which concides with dawn in Australia at this time of year. I was interested in the time period over which the greyline path is open, which seems to lag dawn/dusk at each end of the path by about an hour or so. This is presumably a lag in the D layer ionization.

But another more interesting thing stands out in the WSPR log:

2017-07-15 22:52 G1RRR 10.140194 -21 0 IO90cu 2 VK2KRR QF34mr 16983 67
2017-07-15 22:40 G1RRR 10.140194 -18 0 IO90cu 2 VK2KRR QF34mr 16983 67
2017-07-15 22:28 G1RRR 10.140195 -14 0 IO90cu 2 VK2KRR QF34mr 16983 67
2017-07-15 22:20 G1RRR 10.140195 -12 0 IO90cu 2 VK2KRR QF34mr 16983 67
2017-07-15 22:08 G1RRR 10.140195 -13 0 IO90cu 2 VK2KRR QF34mr 16983 67
2017-07-15 21:52 G1RRR 10.140196 -9 0 IO90cu 2 VK2KRR QF34mr 16983 67
2017-07-15 21:40 G1RRR 10.140196 -5 1 IO90cu 2 VK2KRR QF34mr 16983 67

While these spots were taking place, there were many other spots around Europe and towards the west, which also took place before and after the greyline period. But the interesting thing is that while VK2KRR reported my signal, with a very good SNR at times, no other Australian stations reported anything from me. I found this odd that I could see that other stations in Australia were active on the band at the same time, but could not hear my signal, even though I was heard by VK2KRR with good SNR over a period in excess of an hour. It seems unlikely that the path remained so focused that the path to other stations only a few hundred kilometers away was closed for the whole of the hour period. So why could no one else in the area hear my signal?

This also contradicted some tests I had tried using the OCF dipole previously, where I was received by a number of stations although at a poor SNR.

Using the vertical, I have now got similar results on two separate occasions - only heard by one station (VK2KRR), but with a good SNR.

VK2KRR has kindly posted a description of his station which includes the fact that he is using a Beverage antenna for recieving on 30m. This was interesting, since a look at the operation of that antenna shows that it is vertically polarised even though it is constructed using horizontal elements - https://en.wikipedia.org/wiki/Beverage_antenna .

So my thoughs are that when using the greyline path, polarisation might be important. This is contradictory to general experience with other propagation modes. Maybe the other stations active were using horizontally polarised antenna's which many of us would expect on mid and lower frequency HF bands. The cross polarisation loss could have taken my signal well below the readability level for other stations, as it is commonly suggested that a 30dB loss could be expected. Clearly there is not enough data here to strongly support this, so it would be interesting to see whether others have similar experiences. Comments ?? Or am I way off here??

Keith
G1RRR