De-mystifying HF Radio Propagation and Modeling :: HFRadio.org

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Introductory note:

All illustrations for this article are derived from the propagation prediction program, "ACE-HF Pro", available here at HFRadio.org. ACE stands for "Animated Communications Effectiveness", a coverage display technique originally developed for U.S. Navy submarine communications.

ACE-HF's advantage is that the effects of solar phenomenon and the day's passage may be easily understood. ACE-HF shows when the HF bands will be open in different world areas. More accurately, the program is known as system simulation and visualization software, a powerful tool for an amateur radio operator that allows you to simulate a radio signal path between two points. The simulation includes the most current propagation modeling, and visually provides the results of your analysis.

I have used the ACE-HF PRO System Simulation & Visualization program to illustrate how useful propagation predictions can be to you, as you begin this journey. The new Version 2.05 of ACE-HF reviewed here, has been called the "Cadillac of propagation programs." That name isn't surprising since the design derives from the professional ACE-HF NETWORK software for government and commercial HF network operators, in use by the military and by commercial groups. The new edition of ACE-HF has many new features for the radio amateur, and has even been expanded for use by shortwave listeners. (See my review and application notes for ACE-HF Pro, version 2.05 for shortwave radio listening, here).

How does HF propagation work?

To demonstrate how HF propagation works--and how HF circuits may be simulated--I used the NW7US to Chicago radio circuit shown in the following figures. ACE-HF is really a full-scale system simulation model, so I had to select some system parameters first. Specifically, I selected the transmitter's power to be 200 watts, the maximum power permitted by the new rules for Technician-class HF operation. I selected Isotropic antennas at each end of the circuit, with an assumed gain of +6 dBi (more about that later). I chose the month of April, and selected the CW mode of operation since that's the majority of what's allowed on most of the newly-available HF bands. This will work to illustrate how easy it is for you to get a handle on using the HF radio spectrum.

From within ACE-HF, I queried the Internet to learn what the predicted smoothed sunspot number is for April 2007. Next, I created my first prediction, the "Maximum Usable Frequency" (MUF) on the radio path between my home in Western Washington, and Chicago (Figure 1). The Maximum Usable Frequency is the highest radio frequency that can be refracted back to earth by the ionosphere. This MUF changes through-out the day, and is tied to the state of the ionosphere's energy level.

Fig. 1 - The MUF chart for a CW radio circuit between Western Washington and Chicago with a smoothed sunspot number of 12:

Fig. 1 - The MUF chart for a CW radio circuit between Western Washington and Chicago with a smoothed sunspot number of 12

The MUF curves give Maximum Usable Frequency predictions vs. time-of-day. The blue curve is the median of the daily MOFs (Maximum Observed Frequencies) over all days of the month at a given hour. The HPF (Highest Possible Frequency) red curve gives values expected only 10% of the time. The FOT (from the French: Frequence Optimum de Travail) green values are defined as the frequencies where the MOFs will be higher on at least 90% of the days of the month at that hour. FOT is sometimes called OWF (Optimum Working Frequency).

The MUF chart also has a blue flashing line to indicate current time, and horizontal lines showing the frequencies of each band. The band lines change automatically if a frequency change is made.

Note that the curves in Figure 1 dip down during the nighttime hours, suggesting that the lower frequencies will be favored at night. But each circuit has a different MUF prediction--you are beginning to see why a prediction program is so valuable! This MUF chart is for SSN 12. Later on we will see how the MUF changes as SSN varies--another reason to use software to predict your operation and assure successful HF contacts.

The MUF curves show how ionospheric propagation changes with time-of-day and frequency, but they do not show how well your signals may be received. For that, we need a full-scale system performance prediction, and for that we must consider both predicted signal strength and noise, because it is the signal-to-noise ratio (SNR) that determines our ability to hear the signal. It doesn't matter how strong the signal level might be if it is overwhelmed by noise. It's the same phenomenon that one encounters in a room full of people. If everyone else is silent, you can hear your neighbor whispering to you from across the room. But if all the people are talking and laughing, you might not hear him even if he is shouting. Again it is SNR that matters, just as in radio communications.

ACE-HF PRO is a system performance model with noise predictions that include interference from atmospheric noise (caused by lightning flashes), man-made noise and galactic noise. All this is computed automatically, but it is different for every circuit, frequency and time-of-day. To illustrate how SNR varies throughout the day, the next three figures show SNR vs. time-of-day for 80 meters, 40 meters and 15 meters, assuming CW transmissions as permitted by the new Technician-class rules.

Fig. 2 - SNR on 80 meters for CW with SSN of 12:
(click here to see full-scale version of the image)

Fig. 2 - SNR on 80 meters for CW with SSN of 12

Fig. 3 - SNR on 40 meters for CW with SSN of 12:
(click here to see full-scale version of the image)

Fig. 3 - SNR on 40 meters for CW with SSN of 12

Fig. 4 - SNR on 15 meters for CW with SSN of 12:
(click here to see full-scale version of the image)

Fig. 4 - SNR on 15 meters for CW with SSN of 12

The areas of these figures change color as the predicted SNR changes. A green area shows that the predicted SNR is above the minimum Required SNR (called RSN). The yellow areas show SNRs within 10 dB of RSN, and the red areas show values that are less than 10 dB below RSN. Obviously, predictions that are in the green show the best times for making your contacts. Note that in the 80-meter predictions of figure 2, there are times during the daylight hours when ionospheric propagation simply doesn't support this circuit to Chicago. So we know right away that a different frequency might be a better choice--or we may have to cool our heels until a time when the ionosphere decides to cooperate for 80-meter operation.

In figure 3, 40-meter connectivity is seen to be better than that on 80 meters. There is at least some value of SNR that suggests a good circuit, although again the SNR is sometimes below the desired RSN threshold. But in figure 4, CW operation at 15 meters doesn't work except at a few very early morning hours and even then the SNR levels are much below what would be desired. And at 10 meters, the chart is blank (so we don't even show it) because of the severe attenuation caused by the ionosphere operating at such very low SSN levels.

My favorite ACE-HF chart is shown in figure 5, where a summary of SNR predictions is given as a function of both frequency and time-of-day.

Fig. 5 - SNR summary chart of CW with SSN of 12:

Fig. 5 - SNR summary chart of CW with SSN of 12

Here, we see that the green areas--those where predicted SNR is above RSN--tend to follow the MUF curves, but this chart is much better to use because it considers all parameters of the system calculation.

Now, we begin to see the likelihood of making contacts in the various HF bands. The lower bands seem to work better and nighttime operation is favored, as was predicted by the MUF chart. But as the Summary Chart shows, the 15- and 10-meter bands aren't supported very well (because of the very low SSN level.) It can be seen that the sunspot level plays a significant role in HF propagation. During this period of the approximately 11-year solar cycle, operation in the higher HF bands doesn't work well over medium to long circuits, but as the years pass things will get better. The next few figures show how the other extreme of the solar cycle, where the SSN could rise to perhaps 130, will affect communications.

First, compare the MUF chart of figure 6 with the earlier one of figure 1.

Fig. 6 - MUF chart for CW with SSN of 130

Fig. 6 - MUF chart for CW with SSN of 130

The higher SSN level is the only thing that has changed. We readily see that the maximum usable frequencies extend to include 10 meters--at least some of the times. This changing SSN value is a powerful influence on HF operation, and it affects us all the same way, whether you are a beginning Technician-class ticket holder, or are a seasoned Extra-class ham.

Let's repeat the SNR vs. time-of-day charts to show the differences caused by using an SSN of 130.

Fig. 7 - SNR on 80 meters CW with SSN of 130:
(click here to see full-scale version of the image)

Fig. 7 - SNR on 80 meters CW with SSN of 130

Fig. 8 - SNR on 40 meters CW with SSN of 130:
(click here to see full-scale version of the image)

Fig. 8 - SNR on 40 meters CW with SSN of 130

Fig. 9 - SNR on 15 meters CW with SSN of 130:
(click here to see full-scale version of the image)

Fig. 9 - SNR on 15 meters CW with SSN of 130

Fig. 10 - SNR on 10 meters CW with SSN of 130:
(click here to see full-scale version of the image)

Fig. 10 - SNR on 10 meters CW with SSN of 130

Figures 7, 8, 9 and 10 show the four ham bands, and this time we see that even 10 meters will support good activity--providing you choose the best times-of-day. And it's interesting to see that at the higher SSN, the 15- and 10-meter bands are better during daytime hours, at least for this circuit. Now, we begin to anticipate the real magic of HF radio: when 10 meters comes open, you can literally work hams around the world! Try that with line-of-sight communications on VHF, without the Internet and repeaters, across a spherical Earth!

Now compare figure 11 with the earlier figure 5, where again all we have changed is the predicted SSN level, changing it from the current value of 12 to the future maximum of 130.

Fig. 11 - SNR summary chart of CW mode with SSN 130:

Fig. 11 - SNR summary chart of CW mode with SSN 130

Now the green areas of good SNR extend to include the 10-meter band, and one can easily see when each band will be open. It's something to look forward to, and as time moves along and the higher bands get better, it's even more important to have a good HF system prediction model on your PC.

By the way, the new FCC rules permit Technician-class hams to use voice circuits in the 10-meter band.

Fig. 12 - SNR summary chart of SSB mode with SSN of 130:

Fig. 12 - SNR summary chart of SSB mode with SSN of 130

The 10-meter band will be open in the same ways just discussed, but the predicted green areas of the SNR Summary chart will be different, as shown in figure 12. This is because the Required SNR threshold for good operator-to-operator communications is higher than is true for CW operation. The ACE-HF default RSN for Single-Sideband (SSB) operation is 48 dB-Hz, compared to 27 dB-Hz for CW operation. This means that is easier to make contacts with CW than with voice, but it's good to have both methods at our disposal.

So far, we have discussed only the circuit from the Seattle area to Chicago. We understand that the predictions--and the likelihood of making a contact--are different for every circuit, but what if we just want to call CQ? What band is best then? And when should we call?

Again, ACE-HF comes to our rescue. The software is famous for its animated area coverage displays, where your station's coverage to entire world areas can be shown. Remember, "ACE" stands for Animated Communications Effectiveness, the copyrighted method that was developed for use by the US military and first used for submarine communications. Area coverage displays are usually made for every hour of the day. They first appear for the current hour, but can then be animated slowly or very quickly--like a movie--so you can understand how coverage will change as the day moves along.

You can construct ACE-HF movies that animate as a function of time-of-day or frequency, and we used the last method in the next figures to show sequential maps at the four Technician-class bands. Figures 13 through 20 show the area that can be covered from our Seattle-area 200-watt CW transmitter. (The open areas show where our signal could be received, and the red-shaded areas show where we are unlikely to make a contact.)

AREA COVERAGE MAPS - SSN = 12

Fig. 13 - 80 meters CW / SSN = 12:

Fig. 13 - Area Coverage 80m CW SSN 12

Fig. 14 - 40 meters CW / SSN = 12:

Fig. 14 - Area Coverage 40m CW SSN 12

Fig. 15 - 15 meters CW / SSN = 12:

Fig. 15 - Area Coverage 15m CW SSN 12

Fig. 16 - 10 meters CW / SSN = 12:

Fig. 16 - Area Coverage 10m CW SSN 12
AREA COVERAGE MAPS - SSN = 130

Fig. 17 - 80 meters CW / SSN = 130:

Fig. 17 - Area Coverage 80m CW SSN 130

Fig. 18 - 40 meters CW / SSN = 130:

Fig. 18 - Area Coverage 40m CW SSN 130

Fig. 19 - 15 meters CW / SSN = 130:

Fig. 19 - Area Coverage 15m CW SSN 130

Fig. 20 - 10 meters CW / SSN = 130:

Fig. 20 - Area Coverage 10m CW SSN 130
The figures in the left-hand column show coverage at the SSN 12 level, while the right-hand column shows the effect of SSN 130. The effect of the solar condition is profound on the higher frequency bands, but even the 80- and 40-meter coverage is different. (In the lower ham bands, low SSN levels actually favor communications.)

These area coverage displays are for just one time-of-day. We selected 01 UTC for our examples after reviewing the SNR curves in figures 11 and 12. But imagine what happens in a real 24-hour movie! The coverage swells and fades, islands of coverage appear and disappear as time moves forward, and the area coverage changes as the day-night terminators move across the globe. These displays show the remarkable variations that occur in HF communications, and the effects are sometimes rather astounding. Small wonder that ACE-HF is often used in teaching the mysteries of HF communications to new operators!

Earlier, we spoke of using generic isotropic antennas for these illustrations. Of course, such theoretical antennas don't really exist. We HF hams use practical constructions like horizontal dipoles, vertical monopoles, and even build elaborate arrays of highly directional antennas that can be pointed at desired countries. ACE-HF Pro includes many software models of HF antennas, but it is impractical to show them all in this article. The radiation patterns of all such antennas are handled automatically by the software, and your coverage will vary somewhat according to the antenna you select. ACE-HF Pro, Version 2.05 includes some new analysis charts that enable you to compare different antennas, show their relative patterns and gains, and then use them in the system calculation to show their effectiveness--an easy "try-before-buy" method when you are in the market for a new HF antenna.

You can see that this is a most exciting time to gain HF operating privileges. Since Solar Cycle 23 is at its end and a new cycle is just beginning, the next few years will see an ever-increasing improvement on the HF spectrum, since the sunspot activity will steadily rise. As discussed in recent editions of this column, Solar Cycle 24 is going to be very exciting, as the predictions call for record-levels of solar activity. That translates into a very strongly-energized ionosphere, and around-the-clock HF propagation on most of the HF amateur bands. Now is the best time for you to raise an HF antenna, install your HF transceiver, and begin using the software tools to assist your on-air adventure.

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