Alan and Markus, Thank you for your feedback on ray-hop characteristics including Loran references, and many other effects, in the context of LF phase reinforcement. It will be enjoyable to consider new data with these insights. Also just received my first copy of “LF Today”; thanks for the reference in an earlier message. 73, Jim AA5BW Hi Markus....Ah used the wrong thumb :-)) yes I though it did seem odd..... I had taken the low level instead of the difference. A recalc gives hop 3 at -7dB wrt hop 2 that ties in nicely with your Loran figure. The sea hop estimate was based on 136kHz I just assumed that the sea was smoother than land in the wavelength dimension so less scattering. Overland signals to here seemed relatively weaker than Joe's signal, but there is all the problem with antenna efficiency to consider. and there were not many signals at 4000km at the time. Your Loran transmitters area more reliable source. Of course if you take maximum range, the "downcomming" wave comes in tangentialy so grazes rather than "reflects". Fascinating stuff....thanks for your correction....that thumb erased :-)) ----- Original Message ----- Sent: Monday, November 04, 2013 12:00 AM Subject: Re: LF: Re: RE: Hartmut's Grabber On Last Night... based on Loran-C measurements, I would agree on the ballpark figure of -6 dB per hop. Not sure though if land reflections are really so much worse than seawater, as for low angle incidence the reflection coefficient may still be close to unity. > A 16dB fade suggests 20dB or so difference in amplitude. No. -20 dB = 0.1 x amplitude for the weaker signal. But 1+-0.1 = 1.1...0.9, which would be a very flat fade (1.74 dB max to min). To produce deep fades, the signals would need to be rather similar in magnitude. Sent: Monday, November 04, 2013 12:40 AM Subject: LF: Re: RE: Hartmut's Grabber On Last Night... Hi Jim yes I think it is a phase effect, the reinforcement/cancellation is probably between 2 hop and 3 hop paths. A 16dB fade suggests 20dB or so difference in amplitude. I used to reckon on about a 12dB per ionospheric hop and less, maybe 6dB, for a sea "bounce" based on signal from Joe VO1NA and the Naval station at Newport Corner near Halifax NS. Each hop has a maximum range of about 2000km. There are several effects around the sunset shadow area that could add to the effect too. Higher angles at the ionosphere may penetrate futher before returning giving higher attenuation to the higher hop modes. A thumbnail calculation suggests hop3 ampltude of about -23dB below hop 2 would give a 17dB fade as it goes in and out of phase. Very much a "hand waving" estimate but a useful empirical guide. ----- Original Message ----- Sent: Sunday, November 03, 2013 8:56 PM Subject: LF: RE: Hartmut's Grabber On Last Night... Bob, This is the third time that I’ve noticed Dex’s signal peaking substantially (>15dB?) starting around 0500, at approximately the same time that your signal begins a substantial fade. I’ve seen this twice on captures from Hartmut’s grabber, and once on a capture from Mike’s grabber. An interesting example is 21 October 2013, when captures by Henny, Mike and Hartmut all showed a fade of roughly 10-15dB in your signal from 0500-0530, with Mike’s grabber (which showed Dex’s signal clearly around that time) showing a rise in Dex’s signal of roughly 10-15dB. I wonder if this could be attributable to modal phase reinforcement/cancellation near the terminator? The relative change between your signal and Dex’s seems rather high for that mechanism, but I suppose not out of the question. AGARD CP 305 includes a paper (Morfitt) with some interesting plots in this context, and a number of other papers with interesting propagation studies at frequencies near 74kHz; is there a good place for me to post info of this type? 73, Jim AA5BW Good shots of Me and Dex-Bob WG2XRS/4 NY [XRS4] and [XRS5]...
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