Take Off Angle and HF Propagation

My 40M A – B tests between the DX Engineering DV-40-P phased vertical array and the Cushcraft D40 rotatable dipole have left me wondering. Why does the phased vertical array work better on the evening Short Path and the dipole on the morning Long Path to ZS.

This “oldie but goodie” paper seems to describe at least some of what is going on:

http://nvlpubs.nist.gov/nistpubs/jres/65D/jresv65Dn2p167_A1b.pdf

My guess is that the transmitting antenna that is more in the darkness needs a lower angle than when in light. 40M is interesting because it opens for long haul DX during the fading sun for a SP path, and stays open after sunrise for up to two hours depending on time of the year.

My rotatable dipole is up only about 40′, which means it has a significantly higher take off angle than the phased array. But because it is already light out – the lower layers in the ionosphere are charging up on my end or the circuit and fading on the ZS side via LP. On the SP – it is the reverse, but when I have QSO’s with ZS on the SP – my side is several hours into the darkness meaning I am probably using the combined F layers or even the E layer. There would be fewer hops on the SP than the LP – that’s for sure.

Because the LP is so much farther than the SP, I am going to guess that there could be chordal ducting going on. The signals are many times as strong as on SP – if they are weaker, its not by much. Something interesting is going on. The waves are traveling through a long and very dark path, but both sides of the circuit are not far from light – either fading or gaining.

I can’t find a definitive explanation – not in the ON4UN book, the Ian Poole book – or this paper. ON4UN does call out the LP path that I am discussing – because during deep Winter, my ZS morning LP extends as far as QSO’s that I have had in Northern Ireland!

I’ll keep looking for an answer – but this proves that doing A – B antenna testing is a very interesting and valuable thing to do.

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