This is Shortwave Tutorial part #3...a much delayed Newsletter this time around! I'm afraid that's my fault. I ran away for a vacation last week and didn't leave anything for the folks at The Grundig Store to send to you! Anyway, a few of my "radio buddies" and I rented a cottage up in the wilds of Central Ontario and spent the week in front of our receivers listening to shortwave, mediumwave (the regular AM band) and longwave, which is down below the mediumwave band, from 153 to 279 kHz.

One of the little specialty areas of the radio hobby is trying to hear mediumwave and longwave stations from overseas. However, this usually requires larger antennas than what is required for good shortwave reception. We have practically unlimited room to put up large antennas at this site, and we typically put up one or more wire antennas that are up to 1500 feet long! Don't get excited now. I don't want anyone thinking "I can't possible put up a 1500 foot antenna in my back yard." You don't need one for shortwave reception as you'll see when we get into discussions about antennas later on. We're using these antennas because we like listening to local AM stations in Europe, the Middle East, north Africa, and Scandinavia!

If you'd like to see what we get up to, and the kinds of things we hear, you can check out my website devoted to our DXpeditions (as we call them) at: http://www.interlog.com/~auroradx/dxpedition.html

But enough about my recreational activities. Let's get back to work!

Please be forewarned that the subject matter we are going to deal with in this issue is considered by many avid shortwave listeners to be downright BORING! We are presenting this somewhat technical article now in an effort to give you a good overall understanding of how radio waves travel. I hope you will take the time to read the following paragraphs as you will find the theory invaluable as you learn more about shortwave listening in future issues.

I've mentioned in the first two Newsletters that you should spend a little time listening to the different shortwave bands at different times of the day and take note of when you get your best results. By now you should have discovered that some of the bands are most active during daylight hours and others are most active after dark. Generally speaking, the lower frequency, longer wavelength bands are most active after dark and the higher frequency, shorter wavelength bands are most active during the day.

The way in which radio waves travel, or propagate, is a very complex subject. We'll only scratch the surface here, but it will be enough to give you a basic understanding of where and when to look for shortwave stations.

SW radio signals actually travel in several ways at the same time. First of all, there's groundwave. Groundwaves are signals that travel along the Earth's surface the entire distance from the transmitter to the receiver. Signals that you listen to on the regular mediumwave (AM broadcast) band during the daytime are reaching you via groundwave. In this mode, they can travel only 100 miles or so, which is about as far as you can expect to hear on the AM band during the day. The other mode of radio wave propagation is called skywave. This is the mode that allows SW radio signals to travel all the way around the world, and we can thank the Sun for providing us with skywave propagation!

As you know, the Sun releases an immense amount of energy every second. As this energy reaches the Earth's atmosphere it causes the electrons to separate from the gas atoms they once were attached to, ionizing them. This thick layer of gas in Earth's upper atmosphere is called the Ionosphere. Shortwave radio signals travel the great distances they do by traveling up to the ionosphere where they are "bent" back down to Earth. In some cases they even bounce back off the Earth's surface, travel back up to the ionosphere and bend back to Earth again, a "double hop" which allows them to travel twice as far!

So, I can hear you ask, why are some SW bands better during the day and others at night? Well, there are a few more things at work that cause this to happen. First of all, the ionosphere actually consists of several layers, and each of them behave differently; some exist only during the day and others only at night.

The layers that affect radio wave propagation are called the D, E, and F layers. The D layer is the lowest layer, at approximately 60 to 90 km above the Earth's surface. It exists only when the ionosphere is exposed to the Sun's radiation (during the day) and disappears when the sun goes down. The characteristics of the D layer cause it to absorb low frequency (long wavelength) radio waves and bend high frequency (short wavelength) radio waves. The maximum frequency, where signals are absorbed by the D layer in the day is approximately 7000 kHz. Below 7000 kHz, signals are absorbed and above that they are bent back to Earth. That's the reason why MW AM broadcast station signals can only be heard for relatively short distances during the day, and hopefully why signals were more plentiful on the higher bands during the day.

The E layer is found 100 to 115 km above the Earth's surface, and it acts much like the D layer. It exists when the sun is shining and absorbs low frequency signals and bend high frequency signals.

The highest ionospheric layer is the F layer which exists above from 160 to almost 500 km above the Earth. The F layer has the greatest effect on shortwave reception because it is very efficient at bending lower frequency radio signals back down to Earth, and it can be found at nighttime.

Just to complicate things further, the Sun doesn't by any means give off the radiation that effects our atmosphere in a consistent fashion at all. The Sun goes through regular periods of low and high activity over a cycle that take approximately 11 years to complete. We know this as the sunspot cycle, because the number of sunspots observed on the Sun's surface is directly related to the Sun's activity. High sunspot numbers indicate greater solar activity, and when solar activity is high the increased solar radiation ionizes the ionosphere to a greater extent and results in a more efficient F layer, which means better shortwave reception for us!

As luck would have it, we are on the upward slope of the current sunspot cycle, which means that for the next 3 or 4 years shortwave reception will be getting better and better!

Well, that's it in a nutshell; a textbook on radio wave propagation condensed down to a handful of paragraphs! As I mentioned before, I've just barely scratched the surface on the topic of radio wave propagation, but it should be enough to help you decide which band to listen to at certain times of the day, and improve your chances of hearing the stations you're after.

Ken Alexander

Click HERE to continue to part #4