This section discusses two popular propagation-prediction programs, W6ELProp and VOACAP. You can download them from these links. Both are, unfortunately, only usable on Windows® 95 or later Windows® operating systems.
Both programs offer many features that may be confusing to new users. Fortunately, others have written detailed instructions for using them, that you can obtain online.
I suggest you download and install either or both programs, and spend some time with the tutorials before continuing.
W6ELProp
W6ELProp was developed by Shel, W6EL, for the DOS operating system in the 1990s. Back then it was called MINIPROP. As noted in K9EL's tutorial, W6ELProp assumes your transmitter output power is 100 W. That's the first thing we have to change. Deduct 13 dB from the Additive Signal Constants in Options | Frequencies and Constants. Yes, you'll be entering -13 dB, the price we pay for conserving electricity! If you're using a gain antenna on some bands, such as a 20-15-10 triband beam, adjust the Additive Signal Constants accordingly.
Notice in Prediction Parameters that there is a field to enter Minimum Radiation Angle. We've seen that already. The default setting is 1 degree. While this is well below what we can achieve with HF antennas near the ground, using a higher value may result in less accurate results. Still, it's interesting to run a path from your location to another at 1 degree, then run it again with the Minimum Radiation Angle set to, say, 20 degrees, to better represent a low dipole antenna.
For our discussion, we're going to use the following Prediction Parameters.
We'll also assume your location is Chicago, so run predictions with Terminal A set to W9. I'm using April 28, 2008 for the date, and entering a Smoothed Sunspot Number of 5, K index of 1. Set Terminal B to W8, which happens to be Cleveland, Ohio. Run the prediction.
If you scroll through the results, you'll see there are no listings for 20 M. There is good predicted propagation on 80 and 40 M, though. On 80, the two stations should be able to work each other at almost any time, day or night, while 40 is only available well after sunrise on both ends. Now let's get a little mor information. At the top of your screen, click the Advanced tab.
The second-to-last and last columns are the ones we're interested in. The last column tells us Chicago and Cleveland can work through only one hop off the F layer; the column before that one tells us the wave angle: 46 degrees. Note that angle, because we're going to refer to it again. Now go back to the start screen and change Terminal B to W6, California. Run the prediction again.
Now, even though California is much farther away, we have many more band options. You can see why by again clicking the Advanced tab. There are two predominant F-layer modes available, one-hop and two-hop. There's also an occasional two-hop E-layer path, but the signal levels are very low. So let's just concentrate on the F-layer paths, specifically for 20 M. Most of them show a wave angle of 5 degrees. Only for a short time midday is there a two-hop path, at 18 degrees. On 40 M the same wave angles appear, but best signal levels are at least half what they are on 20 M. Note those wave angles before we move on.
Now we're going to run one more path from Chicago, and see what we come up with. Let's use VK3, which is Melbourne, in eastern Australia. Even though our path is now almost 10,000 miles away, note the improved signal strength predictions on 80 and 40 M. They're much better than on 20 M. Why aren't more QRPers chasing DX on the low bands? That's a good question! Bear in mind that W6ELProp is assuming the same antenna performance on all bands (unless you entered a higher gain figure because you have a beam antenna). It's hard to get a 40-M dipole high enough, and nearly impossible to get an 80-M dipole high enough, to match the performance of a high (in terms of wavelength) dipole on 20 M. Is there a solution? You bet! But first, let's see what the wave angles look like. Click the Advanced tab.
Wow! Those are some low numbers! Mostly 3 degrees, with a few paths at 1 degree. It's very difficult to get much radiation at such low angles. The times offering reasonable signal strengths all involve 3-degree wave angles, and five F-layer hops. Now let's go back to Options | Prediction Parameters, and change the Minimum Radiation Angle to 10 degrees. Run the prediction for VK3 again. What happened to our signal?! Click the Advanced tab and you'll see.
We've increased the number of F-layer hops from 5 to 8, and knocked down predicted signal strengths by some 30 dB. That's like reducing our power from 5 W to 5 mW (five-thousandths of a watt)! And that's at a modest wave angle of only 11 degrees, still difficult to achieve in practice.
Let's assume you could make an antenna with a strong radiation lobe at 11 degrees. Now, though, instead of running 5 W, let's say you're running 1500 W. That's a 25-dB increase in power, which almost makes up the difference. Now you know why DXers run high power. You've also discovered the reason I wrote this article. While it's harder to set up a good low-angle radiator than to hang a dipole or G5RV at 30 feet, doing so will greatly expand your DX horizons. We see that even working California from the Midwest is much easier with low angles of radiation. High angles are okay for short distances, and most antennas radiate some power at lower angles. Getting more power into those lower angles is like running more transmitter power, with a bonus: You'll also hear those distant stations better, and that can't be bad, right?
W6ELProp and its predecessor MINIPROP, were my only propagation-prediction programs for many years. If you spend some time getting familiar with W6ELProp, looking at the maps, and running predictions for different months and sunspot numbers, you'll learn a great deal about ionospheric propagation. Remember, this program, and VOACAP, provide statistical probabilities, not absolute guarantees. Now let's see what else we can gain from using VOACAP.