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