Low-band DX'ing: The high-angle mode hypothesis

Following the publication on this blog, of the VOACAP articles and the comments received from AB7E and OH6BG, my friend Seb F8CMF pointed me an old K3LR post on the "top-band reflector". Here is the post (reproduced with Tim's authorization).
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Topband: Low RX Dipole
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*To*: >
*Subject*: Topband: Low RX Dipole
*From*: k3lr at k3lr.com (Tim Duffy K3LR)
*Date*: Mon May 5 20:52:53 2003
Hi Gale!
It all depends on what other antennas you have. I have had up a 14 ft
high 1/2 wave dipole on 160 for many years. It has always impressed me
as a high angle antenna which limits its prime effectiveness to gray
line use. I also use it in tandem with other antennas and the MFJ
receive antenna noise canceling W8JI magic box (also an effective
antenna phasing unit). My low dipole's claimed fame was a sunrise QSO
with KH2. No other antenna (1100' beverage, vertical array, etc.) could
hear the KH2, but he was 569 on the low dipole.
73!
Tim K3LR
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Remembering that I received ZL1RS only on the magn-loop, this post interested me very much. Taking the opportunity of the "WSPR 80m Special Activity Day" I quickly setup a diversity receive test, using 2 receivers, one connected to the K9AY loop, oriented NW and one to the magn-loop (roughly E/W).


The compared patterns of the K9AY and the magnetic loop. Considering RX antennas, it should be made abstraction of the respective Gains and focus on the RDF. According to K7TJR tables, the RDF of the K9AY considered at 20degrees elevation is 7.2dB, vs 4dB for the magnetic loop, thus a 3.2dB advantage for the K9AY. Of course considering a higher angle, the difference will turn to the magn-loop advantage, as it exhibits a pattern more favorable to NVIS.
The plot shows the SNR's measured on 80m from 9 NA-stations during 2 nights on 2 RX-antennas. Over 500 spots, were used. By distance order VE1VDM (5205kms), W1BW, W1XP, K1JT, WD4KPD, W8LIW, KI4MTI, vE5MU, K4MF(7572 kms). In the middle of the night, the K9AY provides some 5dB average better SNR than the Magnetic-loop wich suggests an elevation angle lower than 20degrees.

The same as above, but focusing on the morning. The fact that the magn-loop takes the advantage near sunrise seems to be a confirmation that some "high angle" mode occurs at this particular moment and not only on paths following the grey-line. The respective antenna patterns suggest an vertical angle >45 degrees. A more "radical" NVIS antenna (like the "cizirf-special") has to be tried to confirm this hypothesis.
Last but not least: for the path F6IRF to W8LIW, VOACAP suggest a vertical angle of 8 degrees, with a peak at 15 degrees in the middle of the night. Once again the VOACAP-model is obviously far away from the above observations. As Dave AB7E mentionned "It (voacap) is especially weak in predicting the effects of takeoff angles /// I'm convinced that much of gray line propagation is chordal hop, which is supported by the comments from K3LR and others (as well as my own observations) that high angle antennas often work better for such openings. When you draw a picture of the earth showing an F2 gradient at sunrise/sunset it is easy to see why that would be true."
What seems to show the above, is that "the high angle mode" does not only concern long DX-paths along the grey-line (such as F<>ZL) but also shorter distances DX paths.