Monday, October 6, 2008


October 8, 2008 – Educational Radio Net, Session 20
Jim Hadlock K7WA

One of the things that got me interested in radio was hearing stations from far away places. At first I used my clock-radio in the AM broadcast band. I discovered that at night I could hear stations from Los Angeles, Salt Lake City, and even Mexico! Later I built a Knight Kit shortwave radio and began listening to broadcasts from South America, Russia and Japan. As a ham I've enjoyed the DX (long distance communication) aspect of our hobby for nearly fifty years, but the idea of a small radio signal propagating to and from far away places still intrigues me. Last year when I was in the Caribbean I made a 2-way contact with Paul, NG7Z, in Bothell on 40 meter CW – we were both running 5 watts of power. That, to me, is an example of the miracle of radio propagation – a very small signal covering a great distance.

The subject for tonight is High Frequency Propagation. We will discuss some of the factors that determine how a radio signal travels to far away places and resources for analyzing and predicting propagation conditions. If you have spent much time listening or operating in the high frequency bands between 160 meters and 10 meters you know that propagation is highly variable. How far you can communicate depends on many factors.

Lets begin with frequency. As I discovered with my clock-radio, far away signals on the AM broadcast band come in better at night. This characteristic applies to signals in the 160 meter, 80 meter, and 40 meter amateur bands as well. During daylight these bands may provide local coverage, but at night they can support world-wide communications. The higher amateur bands, 20 meters, 15 meters, and 10 meters are usually open during the daytime and quiet at night. These daily effects are due to the sun's radiation ionizing atoms and molecules in the earth's upper atmosphere, and the different layers of ionized material either absorbing, bending, or passing through radio signals of different frequencies. During daylight ionization in what's called the D-Layer, about 50 miles high, tends to absorb radio signals. This absorption is greater at low frequencies than high frequencies. If the signals pass through the D-Layer they can be refracted, or bent back to earth, by ionization in the F-Layer, 150 to 300 miles high. If the frequency is too high to be bent the radio signal will pass through the F-Layer and continue out into space. A few weeks ago Bob, K9PQ, discussed the terms Maximum Useable Frequency (MUF) and Lowest Useable Frequency (LUF) which are defined by this absorption/refraction process. After the sun sets the D-Layer starts breaking down due to the absence of solar radiation and propagation improves on the lower frequency bands. Although the F-Layer also breaks down in the absence of solar radiation, it often supports some propagation through the night. At sunrise solar radiation begins to build the D-Layer and F-Layer again.

In addition to the daily propagation cycle, seasonal effects vary greatly. Spring and Fall are similar, but Winter and Summer are very different. Due to the tilt of the earth's axis, radiation from the sun is weaker in the winter and stronger in the summer. The long winter nights make for very good low band propagation, while during the short summer nights the higher bands may remain open 24 hours a day.

The solar radiation which ionizes atoms and molecules in the earth's atmosphere is not constant. One of the best indications of strong radio propagation is the presence of sunspots on the surface of the sun. Sunspots are areas on the sun associated with ultraviolet radiation which ionizes the upper atmosphere. Sunspots can appear and disappear quickly or remain for several solar rotations (the sun rotates on its axis every 27.5 earth-days). Sunspots have been observed since Galileo invented the telescope in the early seventeenth century. Sunspots have been counted and recorded as long as they have been observed with records going all the way back to 1610. Currently there are two official sunspot numbers in common use, the daily "Boulder Sunspot Number," computed by the NOAA Space Environment Center, and the "International Sunspot Number" recorded in Europe. Both numbers use a method devised by Rudolph Wolf in 1848, which combines a count of groups of sunspots and a count of individual sunspots:

R=k(10g+s), where "R" is the sunspot number, "g" is the number of sunspot groups, "s" is the total number of individual sunspots in all the groups, and "k" is a variable scaling factor (usually <1) href="">

Understanding Solar Indices –

The Sun, the Earth, the Ionosphere: What the Numbers Mean and Propagation
Predictions –

W1AW Propagation Bulletin –


NCDXF Beacons –

Contributed by Wr5J, Curt Black
Big Bear Solar Observatory -

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