Technical Topics > Propagation
foF2 Map Discrepancy
KaySeeks:
--- Quote ---The maps are based on a melding of ionosonde data from various geographically separated ionosonde sources.
The map algorithms average the differential between the ionosonde curves and that forms contours.
--- End quote ---
Yes, of course.
--- Quote ---Also, there's the "Spread F index, or fxI, upon which the MUFs are calculated, and may be utilized for map source data.
--- End quote ---
Of course, this is your speculation as to whether that is what the the spacew.com map is showing. I wrote to spacew.com to ask them why there is this discrepancy. I am waiting for an answer.
For entertainment I just checked the spacew.com map versus the FxI and foF2 for the three or four soundings over the past hour at three digisonde stations far apart from each other with somewhat different propagation conditions overhead (Hermanus, South Africa; Fortaleza, Brazil; Point Arguello, California, USA) and there is no agreement. The spacew.com map is showing something that is well above the reported FxI and foF2 by these digisondes. There isn't even any sort of systematic offset or correlation that I can tell. So I am not seeing any validation of your speculation.
--- Quote ---The best NVIS propagation frequency usually happens between the foF2 and the fxI.
--- End quote ---
That's not what I am reading from Idaho ARES, who seemingly have a vested interest in getting this right:
http://www.idahoares.info/tutorial_hf_nvis_band_selection.shtml (scroll down to the very bottom)
They (and everybody else) is saying stay below foF2. (FxI tends to run slightly above foF2.)
Exo:
--- Quote from: KaySeeks on November 28, 2018, 2039 UTC ---
--- Quote ---The best NVIS propagation frequency usually happens between the foF2 and the fxI.
--- End quote ---
That's not what I am reading from Idaho ARES, who seemingly have a vested interest in getting this right:
http://www.idahoares.info/tutorial_hf_nvis_band_selection.shtml (scroll down to the very bottom)
They (and everybody else) is saying stay below foF2. (FxI tends to run slightly above foF2.)
--- End quote ---
It is a common misconception that foF2 is the top or optimum NVIS frequency, so it isn't surprising that the hams at Idaho ARES have that interpretation.
Prior to around 1978, that was the conventional wisdom.
Also, foF2 is a widely available from many sources.
Usually, you can't go wrong with picking foF2 for NVIS.
But, as a general rule, foF2 isn't optimum.
You can see the effect of fxI in the suggested MUFs for 100, 200, 400 km distances (typical of NVIS), displayed in text at the bottom of most of the Lowell Digisonde ionograms.
Propagation is usually optimum just below or at the MUF.
fxI is utilized in the formula to derive the calculated MUF.
You will notice that the furthest right hand green fish hook scattering (fxI frequency) often tends to correspond with the 100 km or 200 km MUF in the Lowell Digisonde ionogram text.
KaySeeks:
--- Quote from: Exo on November 28, 2018, 2226 UTC ---You can see the effect of fxI in the suggested MUFs for 100, 200, 400 km distances (typical of NVIS), displayed in text at the bottom of most of the Lowell Digisonde ionograms.
You will notice that the furthest right hand green fish hook scattering (fxI frequency) often tends to correspond with the 100 km or 200 km MUF in the Lowell Digisonde ionogram text.
--- End quote ---
This is something that I could never figure out when looking at those reports. The MUFs listed at the bottom for 100, 200, 400 km are usually > foF2. At those distances the angle of incidence is pretty high, i.e., "near vertical", and I could not rationalize the two competing thoughts that:
* anything above ~foF2 should just pass right through the F2 and out into space
* the maximum usable frequency at high angles/short distances (corresponding to what is generally considered NVIS) is generally > foF2.OK. I get it now. Thanks for that.
In any case, we're still back to my original complaint about the spacew.com maps on the propagation page. I don't know what they are showing on those maps but it doesn't seem to actually be foF2 and they should not be labelled that way.
KaySeeks:
And not to be too stupid when I say this:
--- Quote ---This is something that I could never figure out when looking at those reports. The MUFs listed at the bottom for 100, 200, 400 km are usually > foF2.
--- End quote ---
Of course, this is something that we already know. In daytime, skip is longer at higher frequencies, for example, and clearly signals that are at frequencies > foF2 propagate. They just may not be reflected straight back down because they are > foF2.
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