Propagation for EMCOMM
A lot is written about propagation, but it is usually aimed at DXers. As a result, few people seem to understand propagation as it relates to emergency communication where the path is typically fairly short.
Most of the time, we are interested in VHF. But VHF propagation is pretty uninteresting from the point of view of emcomm. Propagation events that disrupt VHF communications are extremely rare, and events that enhance VHF are unusual enough that they simply can't really be exploited for emergency communications. Where propagation matters is relatively short range HF propagation.
For the most part, HF propagation is caused by reflection off the F layer in the ionosphere. There are a lot of other modes, and it isn't really "reflection", but for emcomm purposes we can look at it that way. We have all heard of the "Maximum Use able Frequency" or MUF. This is the highest frequency at which RF will "bounce off" the ionosphere. This occurs at the shallowest possible angle, that is, RF which leaves the antenna headed for the horizon and hits the F layer at a glancing angle. This results in a "hop" of about 1500 km. (see the figure above) Actually, the single hop distance is dependent on the F layer height, which can vary quite a bit, but 1500 km is a commonly used figure.
What many folks don't realize is that this 1500 km hop represents the shortest possible path at the MUF. Waves approaching the F layer at a sharper angle will simply pass through (dotted line). As we reduce the frequency, we can approach the ionosphere at a sharper angle, until we reach the critical frequency (Fc). At the critical frequency, even emissions aimed straight up will be reflected. Since we are usually interested in short paths, it is the critical frequency that is interesting to us. (Here in Michigan, most paths to the State EOC in Lansing are less than a few hundred kilometers.)
These days, there is a lot of good material available on the web. We have all seen pointers, I am sure, to various MUF predictions, but the most valuable piece of information to predicting success on short hops is the Near Real-Time Critical Frequency Map, shown on the left. As you can see, Fc is not constant across the planet. Like the MUF, it depends on where the sun has been lately, somewhat modified by the solar wind and magnetic disturbances. Like the MUF, the critical frequency isn't a black and white thing, either. If you want to be successful with in-state HF communications, you will need to operate at a frequency somewhat lower than the Fc over Michigan.
The solar wind
A frequency below the critical frequency represents only the basic minimum to achieve communications. Unfortunately, there are other bad things that can happen. The sun may appear to be constant in the sky, but in fact it is in constant turmoil. It is a wild and angry place, full of storms and all nature of disturbances. Even at times of a "quiet sun", it is still a violent place.
The sun streams energy and
particles out into space all the time. This particle stream reaching the earth is called the solar wind. While it is convenient to think of the solar wind as a steady breeze, it is anything but. Storms and disruptions on the sun cause fairly dramatic changes in the velocity and density of the solar wind. A strong solar wind, or a rapidly changing solar wind, causes the earth's magnetosphere to shake and jiggle, almost like a flag flapping in the breeze. When we put our conductive antenna into this changing magnetic field, we create a generator. Currents are induced into the antenna that we see as noise.
The Real-Time Space Weather Plots give us a look at the solar wind (upper left two plots on the figure above). When either the velocity or density is high (indicated by a red background on the graph) or changing rapidly, we can expect a lot of noise. The noise tends to be less at higher frequencies, but the critical frequency limits how high we can go. We can also gain some benefit by switching to CW; the narrower bandwidth allows us to filter much of the noise, and it is the nature of CW that we simply don't have to hear it as well for intelligibility.
Stronger particle streams can cause aurora. The likelihood of an auroral storm is shown on the right hand graphs. Here in Michigan, aurora can badly disrupt HF communications. However, strong aurora can provide HF-like circuits at VHF, although this tends to be a CW-only mode. Unfortunately, the State EOC, our most common HF target, is not equipped for VHF CW.
Absorption
As if noise isn't enough, high flux can push through the E and F layers and ionize the D layer. An ionized D layer absorbs radio waves. Fortunately, D layer ionization dissipates almost immediately after sunset, so even in periods of high flux, communications is often possible after dark. The map on the left, The D-Region Absorption Prediction, shows the highest frequency expected to be absorbed. At the current flux levels, there is typically no absorption, so the map is entirely black. In periods of higher flux, the map will show a (typically) blue or purple area roughly centered on the sun. When the earth is hit by a coronal mass ejection (CME), the map may get very colorful, but the color will fade quite quickly. Like noise, absorption is highest at the lower frequencies, so one can sometimes escape the noise by going up in frequency. Fortunately, times of high noise tend to also be times of high Fc.
Forecasts
Just like terrestrial weathermen,
there are space weather forecasters. There are watches and warnings for anticipated propagation events, just as there are for severe weather events here on earth. The sun is farther away and perhaps a bit less predictable than the earth, so like earth weather forecasters, space weather forecasters aren't always right, but it can be helpful to keep an eye on the forecast. A convenient site is the Space Weather Alerts and Warnings site at NOAA. This site shows a number of things, most interesting tend to be the geomagnetic alerts and warnings. At a K index of 4, 75 meters is getting challenging, much above 4 and you would be well advised to move higher in frequency. Clicking on an alert or warning on this page will produce a popup with a little more information about the event.
Cycles
While the various online data tell us what is happening now, knowledge of the four cycles that affect propagation can help us judge the range of conditions we will encounter. While our predictions might not be perfect, they are a little like the weather - if we plan something in February it will probably be cold, in July, probably not. By understanding the propagation cycles, we can get a general feeling for what to expect at any given time.
The Solar Cycle
We have all heard of the solar cycle, and we all know we are near a solar minimum. The 11 year solar cycle has the greatest, and perhaps most predictable, affect on propagation.
The chart at the left shows the past few solar cycles. The irradiance, flares, sunspot numbers and flux all tend to track quite closely. What the chart does not show, however, is that while the values track closely when properly scaled, the actual ranges are quite different. The irradiance only changes by a fraction of a percent; the 10.7 radio flux changes by over 3 to 1. This 10.7 flux is what is most interesting to us. When the flux is at 70, short range communication may be possible during the day on 80 meters. When the flux is 200, we might manage short hops on 20.
The solar day
The sun rotates just like the earth. The changes we see in solar radiation are the result of storms and other disturbances on the sun's surface. As the sun rotates, these disturbances rotate in and out of view, causing an apparent cycle here on earth. The sun, however, is not solid like the earth. As a result, different latitudes rotate with different speeds. While the apparent solar day is about 28 earth days, this may vary by 3 or 4 days, depending on where on the sun the strongest disturbances are located.
Of all the cycles, this one is the least significant. At times of an active sun, the flux will vary quote a bit over the solar day. At the low point of the cycle, the difference will be barely noticeable. However, if there are large storms on the sun, the rotation can cause these storms to come in and out of view. These storms might be quite disruptive, so a very bad day today might mean an equally bad day in a month.
The seasons here on earth also have a significant influence. In the winter, the sun hits the ionosphere at a shallower angle, and thus is less effective at ionizing the F layer. In addition, the sun will have fewer hours in the day to do its work, so again, the impact won't be as high. In general, we will need to use lower frequencies in the winter, and higher frequencies in the summer. D layer absorption will also be less noticeable in the winter.
Terrestrial Day
Like the seasons, the time of day also plays a large role. Shortly after solar noon, the ionization will be at its greatest. As a result, the critical frequency will rise until just after solar noon, and then fall, slowly until sunset, and then more rapidly until just before sunrise. This is why we often see the band "getting long" in the evening. As the critical frequency falls, short hops are no longer possible, and as it continues to fall, even relatively long paths become "short".
Summary
For communications across the state, we cannot count on a single band. On winter nights near a solar minimum, we will need to use 160 meters for in-state paths. During the solar minimum, 75 can do the job in the daytime for much of the year, and as we move into spring, it becomes useful later in the day. As the solar flux increases, the frequencies we use will need to shift up. Lower frequencies will become noisier and signals will become weaker during the day as the D layer begins to eat our signals. As the flux starts to approach 150, we will find that 75 is too noisy and we will need to go to 40 for daytime paths, and as it crosses about 150 we will find 40 useful later into the evening.
While it is what the DXer craves, flux levels in the 200 neighborhood tend to be problematic for emcomm. Only rarely is 20 useful for in-state paths, but 40 can become too noisy during the day to be useful. CW can give us a little more flexibility, and in times of high flux, also opens up 30 as a possibility. But at the end of the day, you just can't get around the laws of physics.