TopBand: IONCAP, et al

[Carl K9LA]

About a week ago, a fellow DXer said that, in his opinion, IONCAP 
underestimated MUFs.  He cited that 12m, and even 10m, was open to EU 
when IONCAP predicted the MUF to be just above 21 MHz.

Although this has little to do with 160m, I think a comment or two is 
appropriate about this subject.  IONCAP, along with all the other 
propagation prediction programs, does not give absolute results.  The 
results are statistical in nature.  The MUF prediction, as well as signal 
strength or S/N, is a monthly median value.  The key words here are 
“monthly median”, and that means on half the days of the month the MUF is 
above this value and on the other half of the days the MUF is below this 
value.  In other words, the predicted value has a 50% probability.

How much above and below the predicted monthly median value can the 
actual MUF vary?  This comes from the tables of daily variation (ESSA 
Technical Report 1-ITSA 1 by Lucas and Haydon, for example).  The tables 
are upper and lower decile values based on SSN, season, latitude, and 
time.  There is a similar table for loss to estimate signal strength (or 
S/N) probabilities.

Applying typical values to a predicted MUF of 21 MHz says the actual MUF, 
on any given day during the month, could be as low as about 16 MHz (90% 
probability) to as high as about 27 MHz (10% probability).  So it’s not 
surprising that 12m, or even 10m, would be open at times during the 
month.  If 10m was open during the whole month, then indeed something is 
wrong – perhaps the erroneous use of daily solar flux instead of a 
smoothed value, or Sporadic E (somewhat tough to predict).  Or something 
else.

IMHO, the predictions are pretty good when used with an understanding of 
what they give – probabilities.  I also agree that most are useless on 
160m because, in addition to not addressing sunrise and sunset effects as 
this fellow DXer noted, they do not address the fact that the Earth’s 
magnetic field plays an important role in 160m propagation (the Proplab 
Pro propagation software that was used in the recent two part 160m 
propagation article in CQ magazine is the exception here – it does 
address magneto-ionic theory).

Carl K9LA

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