Monday, March 3, 2014

SWL Propagation Predictions for April 2014


By Tomas Hood, NW7US / KPC7USA

High-frequency Propagation This Month

As we move into spring in the Northern Hemisphere we experience great DX openings from around the world on HF. This is because the sun is mostly overhead over the equator, creating equal day and night periods in both hemispheres. The Vernal Equinox on March 20, 2014 marks the day when the hours of daylight and darkness are about equal around the world. This creates an ionosphere of similar characteristics throughout more of the world than is possible during other times when it is summer in one hemisphere and winter in the other, and there are extreme differences in the ionosphere. This equalization of the ionospheric which takes place during the equinoctial periods (autumn and spring) is responsible for optimum DX conditions, and starts late in February and lasts through late April. The improvement in propagation is most noticeable on long circuits between the northern and southern hemispheres. During this season conditions are also optimal for long-path as well as short-path openings, and during gray line twilight periods associated with sunrise and sunset.

Spring is also the season of Aurora. Geomagnetic storms that ignite auroras occur more often during the months around the equinoxes during early autumn and spring. This seasonal effect has been observed for more than 100 years.
Look for Aurora-mode propagation when the geomagnetic indices of the planetary K (Kp) rises above 4, and look for visual Aurora after dark when the Kp rises above 5. The higher the Kp, the more likely you may see the visual lights. But, you don't have to see them to hear their influence on propagation. Listen for stations from over the poles that sound raspy or fluttery. Look for VHF DX. Sometimes it will enhance a path at certain frequencies, other times it will degrade the signals. Sometimes signals will fade quickly, and then come back with great strength. The reason for this is that the radio signal is being refracted off of the more highly ionized areas that are lit up. These ionized areas ebb and flow, so the ability to refract changes, sometimes quickly. I've observed the effect of Aurora and associated geomagnetic storminess even on lower HF frequencies. To monitor the live aurora predictions and geomagnetic activity, visit

Because of these various conditions occurring in the month of April, hot DX is possible across the HF and low-VHF spectrum. The seasonal change plays out on HF with activity moving up from 41 meters and down from 11 meters. Propagation on the higher HF frequencies (19 through 11 meters) begins to suffer late in April and into the summer months due to lower MUFs (Maximum Usable Frequencies) in the Northern Hemisphere. MUFs peak very late in the day during summer. Summertime MUFs are lower due to solar heating which cause the ionosphere to expand. An expanded ionosphere produces lower ion density, which results in lower MUFs. Short-path propagation between countries in the Northern Hemisphere will drop out entirely. Higher frequency propagation peaks in the fall. April and May are fall months in the Southern Hemisphere making long-path DX possible. Short-path propagation from South America, South Pacific, and other areas south of the equator will be strong and reliable when open. However, these do not happen every day on the higher frequencies. 

From April to June, excellent propagation occurs on both daytime and nighttime paths. The strongest propagation occurs on paths that span areas of both day and night, following the MUF. During April, peaking in May and still in June, 16 meters may offer 24-hour DX to all parts of the world, with both short- and long-path openings occurring, sometimes at the same time. If you hear a lot of echo on a signal, you might be beamed in the wrong direction. Try the opposite azimuth. Thirty-one through 19 meters are more stable as nighttime bands, with propagation following gray line and nighttime paths.

Low-band propagation is still hot on 41 meters, with Europe in the evening, and Asia in the mornings. Occasional DX openings will occur on 90 and 75 meters around sunrise.

VHF Ionospheric Openings

The possibilities for ionospheric openings on the VHF bands usually improve after March and the spring months. Many of the solar-ionospheric relationships that can produce ionospheric openings on the VHF bands tend to peak during equinoctial periods. On VHF, many different types of propagation modes can appear once or twice during this month. Combination propagation modes may be possible, as well. Aurora is highly likely, as is an increase in Trans-equatorial (TE) propagation. On days of high solar flux, there might be F2-mode VHF openings. Sporadic-E will play out just as we hit May, but until then, there is opportunity for those caring to stay on low VHF spectrum, hunting for signals. There are times when sporadic-E, TE, and F2-layer propagation modes will link, providing strong DX openings on VHF between North America and New Zealand, Australia, or other areas.

If you use – you can follow @hfradiospacewx for hourly updates that include the K index numbers (and, follow this columnist – @nw7us). You can also check the numbers at


The Royal Observatory of Belgium, the world’s official keeper of sunspot records, reports a monthly mean sunspot number of 82.0 for January 2014. The mean value for January results in a 12-month running smoothed sunspot number of 65.5 centered on July 2013. Following the curve of the 13-month running smoothed values, a smoothed sunspot level of 82 is expected for April 2014, plus or minus 14 points.

Canada's Dominion Radio Astrophysical Observatory at Penticton, British Columbia reports a 10.7-cm observed monthly mean solar flux of 158.6 in January 2014, continuing an upward trend (a second peak?). The 12-month smoothed 10.7-cm flux centered on July 2013 is 123.9, up from June’s 120.9.  A smoothed 10.7-cm solar flux of about 138 is predicted for April 2014.

The geomagnetic activity as measured by the planetary-A index (Ap) for January 2014 is 6. The 12-month smoothed Ap index centered on July 2013 is a steady 7.3. Geomagnetic activity should be much the same as we have had during March. Refer to the Last Minute Forecast for the outlook on what days that this might occur (remember that you can get an up-to-the-day Last Minute Forecast at on the main page).

I'd like to hear from you

I welcome your thoughts, questions, and experiences regarding this fascinating science of propagation. You may e-mail me, write me a letter, or catch me on the HF Amateur bands. On Twitter, please follow @NW7US (and if you wish to have an hourly automated update on space weather conditions and other radio propagation-related updates, follow @hfradiospacewx). I invite you to visit my online propagation resource at, where you can get the latest space data, forecasts, and more, all in an organized manner. If you are on Facebook, check out and 

Until next month,

73, Tomas, NW7US
P.O. Box 27654
Omaha, NE 68127

Tuesday, February 4, 2014

February 2014 CQ Plus - Practical Propagation

The Need for a Smoothed Solar Index – Characterizing a Solar Cycle

This is Part I of K9LA's series on the value of determining a smoothed solar index. It appeared in the final edition of WorldRadio Online magazine, January 2013. - KI6SN


            Sunspots have been observed for more than 2000 years. The invention of the telescope in the early 1600s allowed permanent records of sunspot activity to be made. Around the middle of the 18th century European astronomers began keeping records on a regular basis. From these records scientists put together the familiar sunspot cycle plot that started with Cycle 1 in 1755, and continues with the present Cycle 24.
            The raw data is a daily sunspot number. The data in the early cycles is sometimes sparse but by 1850, around the peak of Cycle 9, the data was very reliable.
            (BACKGROUND:  My “Propagation” column in the November 2011 edition of WorldRadio Online, Photo A, discusses the possibility of missing an early solar cycle due to insufficient data. The gaps were shown in Figure 1 of that column. – K9LA.)
            After World War II – 1947, to be exact – we began measuring 10.7 cm solar flux. The 10.7 cm solar flux is objective: it’s a measurement, assuming a calibrated set-up. Visually counting sunspots, on the other hand, is subjective. In other words, human interpretation is required. Just like sunspots, the raw data is a daily 10.7 cm solar flux value.

Modern Methodology
            So why do we need anything more than the daily sunspot number or daily 10.7 cm solar flux? There are two reasons.
  • The first has to do with characterizing a solar cycle, which I’ll address in this month’s column.
  • The second has to do with propagation predictions, which I’ll address in next month’s column.
            Accompanying this column, Figure 1 shows Cycle 23 in terms of the daily sunspot number. The data is very spiky, which simply says the daily sunspot number – and daily 10.7 cm solar flux – are very dynamic. With respect to the figure, three questions to ask are:
  • When did Cycle 23 start?
  • When and how big was the maximum?
  • When did Cycle 23 end?

Figure 1 – Cycle 23 daily sunspot numbers

When, What and Why?
            As for the start of Cycle 23, it was likely sometime in 1996. The spiky data precludes pinning this down to a specific month.
            As for the maximum of Cycle 23, it kind of looks like there might have been two peaks – one around the middle of 2000 when the sunspot number was almost 250 and another in early 2001 when the sunspot number was around 240.
            The dip between these high sunspot numbers also hints at two peaks. But what about the sunspot number around 210 in late 1999? Could this be considered a peak? If so, couldn’t the other daily maximums in the sunspot number be considered peaks? So could Cycle 23 have had many peaks?
            As for the end of Cycle 23, it’s somewhere in the 2008 time frame. Again the spiky data precludes pinning it down to a specific month.
            Since we’re having trouble pinning down the start and end and the peak – orpeaks – let’s average the daily values to give us monthly means. (REMEMBER: The “mean” is the same as the “average.” – K9LA.)
            Figure 2 is the same data as in Figure 1, but with the monthly means added in dark blue.

Figure 2 – Cycle 23 daily and monthly mean sunspot numbers

            The monthly mean data is still kind of spiky, and does not allow us to pin down the start and end of Cycle 23 to a specific month/year. But at least we can now better see that Cycle 23 appears to have had two broad peaks.
            With respect to the peaks, should we take the two highest monthly means – about 170 in mid-2000 and about 150 in late-2001 – as the peaks? But what about all the other peaks in the monthly mean data? Should they figure in somehow?

Making It All Work ‘Smoothly’
            Enter: the use of the smoothed value.
            Now, hang on! The smoothed value for a desired month uses:
  • The monthly mean from the desired month
  • The monthly means from the five months before the desired month
  • The monthly means from the five months after the desired month
  • One-half the monthly mean from the sixth month prior to the desired month
  • One-half the monthly mean from the sixth month after the desired month
            Whew! Thus the smoothed value is heavily averaged. Also note that the smoothed value is six months behind the current month.
            The calculation of the smoothed value requires 13 months of data, but using one-half the monthly means at both ends results in 12 full-month data points. This can cause some confusion, as I’ve seen the smoothed value called a 13-month running average (since 13 months of data is required) and a 12-month running average (since it ends up with 12 full-month data points).       Regardless, the smoothed value is heavily averaged – in other words, it is smoothed.
            Figure 3 is the same data as in Figure 2, but now with the smoothed values added in green.

Figure 3 – Cycle 23 daily, monthly and smoothed sunspot number

            Now it’s easy to see that Cycle 23 indeed had two broad peaks – one in April 2000 at a smoothed sunspot number of 120.8 and the other in November 2011 at a smoothed sunspot number of 115.5.
            These months and smoothed values easily come from the raw smoothed data that is used to produce Figure 3.

That’s More Like It
            Now we can also easily determine a likely start and end for Cycle 23. We can simply use the lowest smoothed sunspot number for both.
            For the start of Cycle 23, May 1996 has the lowest smoothed value (8).
            For the end of Cycle 23, November 2008 and December 2008 have the lowest smoothed value (1.7).
            Be advised that these numerical minimums for the start and end times are usually the starting points – the official start and end times of a solar cycle can be revised a bit based on the comparison of old versus new sunspots and other factors.

Another Option
            Although I went through this exercise using sunspot numbers, I could have used 10.7 cm solar flux with the same results. That is, the smoothed value allows us to better determine the start time, the end time and the peak(s) of a solar cycle. This is why the official measurement of a solar cycle is the smoothed value.

Coming Up . . .
            February CQ Plus' Practical Propagation presents Part II of K9LA's series on why we need a smoothed solar index. This month he explains its value for making accurate propagation predictions. Visit <– KI6SN.