The science of radio wave propagation has been developing ever since it was discovered that radio waves somehow went around the earth without simply flying off into space. Everyone who reads the Digital Journal already knows of the impact of HF propagation on commercial, defense, and amateur radio communications. The study of radio wave propagation can constitute a professional career path or a lifelong avocation. Or, as one more example of the great diversity in amateur radio, it can be a topic of the moment for some period of time. But for a ham interested in any aspect of HF operation, digital or otherwise, a basic knowledge of propagation is a most useful tool in the shack.

A great deal of technology has converged in the last few years to make the tool of propagation analysis much more readily available to the radio amateur. Years ago one could only rely on charts in the monthly ham magazines and the updates on WWV at 18 minutes past the hour to make fairly vague analyses of present or future conditions. The professionals over the years have, however, collected much more and better data, and have created better quantitative and analytical models. The decreased cost and increased power of personal computers have made it possible for individual hams to realistically access these better models. And the Internet has made distribution of detailed, nearly real-time data easily available to much larger numbers of hams.

This is the first of a series of articles on HF propagation and, in particular, propagation software. The purpose of this series of articles is not to teach the science of radio wave propagation. That would take a very large number of articles and the topic is already adequately covered in a number of fine popular books, textbooks, and university courses on the subject. Our goal here is rather to address radio wave propagation analysis as a tool for the amateur HF operator. We'll take the practical viewpoint whenever possible and try to get to the nuts and bolts of effectively, and correctly, using software for propagation analysis. We will try to take the point of view of a DX chaser or contester but ask enough of the science to make sure we understand what to put into the software so that what we get out is reliable and useful. Thus, we may not discuss upper atmosphere electron concentrations in any great detail, but we most certainly will discuss signal-to-noise ratios.

Perhaps the most sensible way to introduce the use and capabilities of propagation software is to first frame the question of what each user needs from the analysis and then relate those needs to the science of propagation in a practical way. And since practicality is the theme of the exercise, we must also define how to compare one software package with another. This, of course, has many different aspects ranging from scientific (i.e. how geomagnetic activity is treated) to practical (i.e. DOS, Windows, or both) to the esthetic (i.e. good graphics or poor). Next, we'll review most of the propagation software packages available and try to give you a balanced and objective appraisal of them. I can say from the start that I have learned at least something, and often a lot, from every package I have analyzed. But I can also say that, when a new expedition is starting, I know which package I choose first. I will try to avoid betraying biases, and I recognize that many of you may have very good reasons to make a different first choice. Hopefully by the end of this series of reviews, you will have enough information to choose thoughtfully.

So let's get started. For the amateur who is chasing new DX countries, the primary question is pretty straightforward. On what amateur band and at what time can effective signals be exchanged between the two stations. we'll come back to this most important concept of 'effective soon.' For the contester, the primary question is the same except that it isn't defined in terms of a single DX location - it's all locations world-wide! For practical reasons it can be at least narrowed to a smaller number of regional locations - say Europe and Asia - and it simply becomes the same exercise multiplied. After that it becomes a matter of contesting strategy.

If the main general question is "at what time and at what frequency", then the first answer that comes to mind is usually MUF, the maximum useable frequency. We are introduced to the concept of MUF, to a greater or lesser extent, quite early in our ham careers. The MUF is indeed a useful concept and can give qualitative insight into propagation. For simplicity, it isn't all bad to consider the MUF as exactly the answer to our question, provided we are generous with the definition of "exact". Frequencies well above the MUF are unlikely to be reflected back to earth and those well below the MUF undergo strong absorption or E-layer reflections that make them unusable.

This is all correct, as far as it goes, but calculations of the MUF only tell a part of the story. Actually, the basis for MUF calculations is determined from statistical data on the ionosphere. There are fairly well known relationships between ionospheric parameters and key solar parameters, such as solar flux (sunspots), time of day, season of the year, etc. But, Mother Nature being what she is, the statistical variation is quite large. The MUF is defined as the median value, which means that on any given day there is a 50-50 chance that higher frequency paths will be open.

The MUF has two lesser known cousins - the HPF (highest possible frequency) and FOT (from a French term which is roughly the optimum working frequency). The HPF is the upper limit and can only be exceeded 10% of the time - which is still 3 days a month or 1 day out of a typical 10-day DXpedition. The FOT is defined as 0.85 times the MUF and can be exceeded roughly 90% of the time. The frequency spread between the FOT and the HPF is often 4 MHz or more, which is enough to include two different ham bands. So the "required reliability" (i.e. the odds) is something for us to consider.

Even with statistical considerations aside, MUF calculations don't give enough information to do the job correctly. Let's return to the word "effective" for the moment. Effective communications requires that the path be open and that there be distinguishable signals at both ends of the path. Distinguishable means different things under different circumstances. If you are the only station trying to make the contact, then distinguishable implies adequate signal-to-noise ratio, which depends on bandwidth (mode) and things that don't depend on the ionosphere - like transmitter input power, feedline losses, receiver and transmitter antenna gain profiles, receiver sensitivity and location, and the distance between the receiver and transmitter. It should be easy to see why this problem requires a software solution. In competition with other stations, distinguishable may mean signal-to-interference ratio. For example, if your path to Kermadec from the Midwest US crosses southern California, then noise may matter a lot less than the power output of your California competitors. I'm not aware of any program that calculates signal-to-interference ratio directly but you may need to calculate signal-to-noise ratios for both your path and theirs to determine when your chances are best.

Now we can define the task. The target location must be defined, mode and hardware specified, required probability determined, solar information obtained, and competition identified. Then the software model can be run to determine optimum and backup frequencies and times.

Our goal is to compare various propagation analysis software packages so we should start by defining the criteria for comparison. Based on our discussion so far, some of the following questions should be asked. Are MUF, HPF, and FOT included? What model is used? How is signal strength or signal-to-noise ratio quantified? How is required reliability addressed? Is single band information readily available?

There are other important questions. How is geomagnetic activity treated? For example, many paths from the central US into Asia or the mideast cross over or through the auroral zone. These signals can be dramatically affected by geomagnetic activity. Are real antenna parameters included? We think of our Yagi antennas as having a certain gain but, in fact, that gain is the peak gain at a certain elevation angle. If the path includes a different arrival angle, and the odds are very good that it will, then a different gain value should be used.

Is the locations database in the software useful, or can it at least be easily modified for amateur use? There's nothing more frustrating than having to look up longitude and latitude in some other reference. Are the graphics clear? Can printing and screen dumping be accomplished easily? Does the program run under DOS or Windows or both? What are the speed and memory (or other hardware) requirements? How much does the program cost? Are manuals included, available, or useful? Is there on-line help? Is it useful? What extras are offered that make the program different from its competitors?

In the next installment of this series, we'll introduce the basis for many propagation analysis programs - IONCAP (Ionospheric Communications Analysis and Prediction Program) - a program developed originally for mainframes and now ported to higher power personal computers. Then we'll begin our software review with two generations of a program from the Voice of America - VOACAP and VOAWIN. In subsequent installments, we'll review most of the software packages available and, in one installment, we will review popular and professional books on propagation. What happens after that depends on you. We would like to hear from you about what you would like to see on these pages. Get in touch with me, or AA5AU or N2HOS, and tell us what you think.