This was posted today on the GQRP Yahoo group. TravisK5HTB ----------------------------------------------
Hello Group ... The propagation figures can be confusing and so here is a quick explanation: HF Propagation Terms Explained If you are looking for overly technical explanations, this might not be the best read for you, I like to keep things simple. It’s possible that the information presented may not be completely accurate, it’s only my interpretation of lots of things that I have read, or heard. I can say that I have used this understanding of HF Propagation, balanced against real monitoring over a number of years and found that what I think should be happening looking at the numbers is usually about right. I do however urge you to delve a bit deeper than this so you might better understand HF Propagation. SOLAR FLUX INDEX or SFI SFI is a figure derived from the number of active sunspots visible on the face of the sun based on observations of thermal and radio noise received at a wavelength of 10.7cm. (2.8GHz). Thermal noise is related directly to the amount of plasma trapped in the magnetic field in active regions of the sun, this is related to the amount of magnetic flux generated by these active regions. The higher the SFI the better for dx'ing on the higher HF bands. SUNSPOTS A SUNSPOT is an area of intense magnetic activity on the surface of the sun which is cooler than the rest of the sun’s surface. Sunspots are vital for long distance radio communications, linked as they are to the ionisation level of the F layer. The more active sunspot groups, the more sunspots the better the dx'ing on the higher HF bands. SMOOTHED SUNSPOT NUMBER or SSN When we plot the daily sunspot numbers, it's goes up and down all over the place. so smart people came up with a way to average out the bumps, called smoothing, which gives us the SMOOTHED SUNSPOT NUMBER or SSN. In the years where there are few sunspots, propagation on the higher bands can be nonexistent. It's not all bad news though as things on the low bands tend to improve with decline in solar activity. Again larger SSN numbers mean better conditions on the higher HF bands. SOLAR CYCLE Every 11 years or so the sun goes through a cycle of events. At the start of this cycle, some new sunspots begin to show up on the blank solar disc. With this first few sunspots things start to improve on the higher HF bands. More and more sunspot groups show up over the years of the cycle, with continuing improvements in HF activity. 11 years for the cycle is just an average, really it can be from 9 to over 15 years. Solar activity refers to a combination of the amount of sunspots, and the solar geomagnetic activity's effect on the earth's magnetic field. For the higher HF bands more sunspots usually means improved conditions. A and K INDEX When talking about the solar cycle the numbers are based on smoothed numbers, this means the monthly average rather than the figures for each day. Using 'smoothed' numbers gives more accurate results, daily solar activity varies dramatically, making accurate results difficult. A and K INDEX are measurements of the magnetic field activity that are used in predictions. The K index is a reading of the magnetic activity as compared to a quiet day made from various locations. This index in the range 0-9. The K index is the average of of these readings. K = 0 Inactive K = 1 Very quiet K = 2 Quiet K = 3 Unsettled K = 4 Active K = 5 Minor storm K = 6 Major storm K = 7 Severe storm K = 8 Very severe storm K = 9 Extremely severe storm The A index relates to geomagnetic stability and is calculated from K indices for the previous day. A = 0 - 7 Quiet A = 8 - 15 Unsettled A = 16 - 29 Active A = 30 - 49 Minor storm A = 50 - 99 Major storm A = 100 - 400 Severe storm A quick example of the A & K numbers If we are waiting for excellent conditions on the HF bands, then we are looking A and K indices of 0 lasting for a few days. Combined with a high SFI. If your interests are the lower HF bands, low A and K indices with very low SFI numbers would be the go. In the real world generally propagation conditions are OK when the A index is 10 or lower, and the K index is 3 or lower and the SFI above 90. However there are countless stories about surprises on supposedly dead bands. Basically: The A index [ LOW is GOOD ] · 1 to 6 best · 7 to 9 is OK · 11 or more not so good K index [ LOW is GOOD ] · 0 or 1 best · 2 is OK · 3 or more not so good · 5 is really terrible SFI index [ HIGH is GOOD ] · 70 not good · 80 good · 90 better · 100+ best Dave, G4AKC http://www.qrz.com/db/g4akc
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