Technical Articles

Propagation Paths–The Long and Short of It

Getting Around on the Bands

This article focuses on one of my favorite interests: How HF signals get around. When asked the question, “Why are you into ham radio?” I answer with, “Because I can hear the world turning!”

Propagation is how radio signals get from Point A to Point B. It’s one of the most fascinating aspects of amateur radio and also makes us somewhat unique. Don’t other radio services depend on propagation? Yes, of course, but while other services try to minimize the effects of propagation variations, hams positively celebrate them! In fact, learning about radio propagation has been a big part of our history since Hertz and Marconi. If you are just getting started in ham radio, take time to learn about propagation and it will serve you well.

There is a huge amount of information about radio propagation spanning the range of VLF (Very Low Frequency) signals through microwaves. Just thinking about it all can be intimidating, so it’s best to tackle things in bite-sized basics. An overview will acquaint you with the basic terms and ideas.  You’ll find some topics more interesting than others, so feel free to pick and choose. In this article, I’ve chosen a few topics to help you understand HF propagation a little better and maybe make you want to know more.

There are plenty of basic introductions to propagation out there. A good start would be to read the Propagation chapter from any recent edition of the ARRL Handbook. It hits the high points of propagation and space weather. You can move on to more complete coverage of propagation in the ARRL Antenna Book.

What’s Up There?

I’m going to assume that you know a little bit about the ionosphere, the uppermost part of the atmosphere that extends from a little over 30 miles to 500 miles above the Earth’s surface. In fact, the International Space Station’s orbit at 254 miles up puts it inside the ionosphere!

  • The ionosphere is made of gas molecules that are exposed to solar ultraviolet (UV) radiation.
  • Solar UV ionizes some of the molecules by freeing one or more of their electrons.
  • The positive gas ions and free electrons make the ionosphere weakly conductive.
  • Conductivity makes the ionosphere able to refract (bend) radio waves.
  • The higher the frequency of the wave, the less it is bent and vice versa.
  • Increasing amounts of solar UV increase ionization and the amount of bending.
  • The ionosphere has several layers (low to high): D, E, and F.
  • Most refraction of HF signals occurs in the F layer and sometimes the E layer.

In addition to these basics, how signals travel and why we are able to communicate involve time, season, frequency, location, the state of our planet’s geomagnetic field, and what’s happening on the surface of the sun. All of those things make it more or less likely that a radio wave takes a particular path on its journey.

Here are some other ideas to help you understand why signals are able to support communications on some paths and not others.

The 3D Ionosphere

As we learn about propagation, usually to pass our license exams, we imagine the ionosphere as a stack of layers that are turned on and off by the sunlight. (See the graphics below–courtesy of the ARRL and The Ham Radio License Manual).

We imagine signals reflecting off of them like a pinball, then off the ground and back into the sky, finally taking enough hops to get to the receiving station. (This is why skip propagation is composed of hops, if you were wondering.) When it gets dark, some of the layers go away, and when it gets light, they come back. The stronger the sunlight, the better higher frequency signals are reflected and the longer those bands are open. That’s good enough to get started, but it’s really more complicated (and fun).

For starters, the sharp-angled reflections we imagine are really refraction, which is a gradual bending of the wave’s direction. Any time a radio wave goes through a conductive media like the ionosphere, its path is altered by the interaction. Luckily for hams, the effect is to bend the wave back toward the ground. It wouldn’t be much fun if the waves were bent the other way.

The smooth, well-behaved layers in our mind’s eye aren’t like that either.  What we think of as a kind of radio mirror is really more like a big radio-refracting cloud. The signals enter the cloud with one direction and come out with another. And just like clouds, the ionosphere’s layers have edges and sides and dense spots and thin spots. That means the signals may be bent sideways, as well as vertically.

The layers are thicker and reach higher at different latitudes, too. The closer the cloud gets to the geomagnetic equator (halfway between the geomagnetic north and south poles, approximately), the more heavily ionized it becomes.  Signals that may be bent only a little if they encounter the cloud above, say, Minneapolis, can be bent a lot over Mexico City or Honolulu.

Don’t MUF It

You probably also learned the abbreviation MUF for Maximum Useable Frequency as part of your license studies. We tend to think of it as a sharp cutoff with signals below the MUF bouncing back to Earth and above the MUF as lost to interplanetary space. But as with just about everything ionospheric, it’s more complicated than that.

The most important thing to understand about MUF is that each possible path has its own MUF. Since any signal that gets from my location to somewhere more than about 2,000 miles away has to make more than one hop, the ionosphere has to be able to bend the signal back to Earth at each hop. A 10,000-mile path might take five hops and each one of them has to be successful. It doesn’t matter if four of the five hops are successful–all five have to “work.” The MUF applies to the whole path, and it is the lowest frequency at which all of the hops work. When all of the hops work, the band is “open” on that path. It only takes one hop to stop working and the band is then “closed” on that path.

Everybody’s Got an Angle

MUF actually depends on one more important thing–takeoff angle. As we find out in trying to skip a rock across water, the angle at which the rock hits the surface is really important–and so it is with HF propagation. 

  • The lower the angle at which your signal encounters the bending region, the less bending is required to send the signal back to Earth at a similar angle.  
  • The less bending that is required, the higher the frequency will be of signals the ionosphere can bend back to Earth. 
  • If your signal takes off from (or is received by) your antenna at a low vertical angle, paths will be open on higher bands. If the takeoff angle is higher than the critical angle for that frequency, the ionosphere won’t be able to bend it back to Earth and the path is closed above that frequency.
  • A low-angle hop is longer than one at high angles, so the path will require fewer hops. Each hop takes a big bite out of your signal strength, so a low-angle signal is probably going to be stronger at the “other end.”
  • All of these benefits mean that, all things being equal, MUF will be higher over a particular path for low-angle signals.

Low angles result in long hops, but sometimes we want to communicate with closer stations. Those paths require high-angle hops. The highest-angle hop is one where the signal goes straight up and is then scattered over a wide region around the transmitting station. This is NVIS (Near-Vertical Incidence Skywave) propagation, common on the lower-frequency HF bands. But at higher frequencies above the critical frequency, the ionosphere just can’t get ‘er done and the signal escapes to space. The angle has to be lowered until signals of that frequency can just barely be bent back to Earth. Most antennas radiate over a wide range of vertical angles, so you can use both low- and high-angle paths.

The distance covered by the first hop determines the skip zone around the transmitting station where you can’t hear other stations–they are too close! For example, if 15 meters is just barely open to the west from my station, I might be able to work stations in California but not Colorado. You will sometimes hear hams talk about short skip. This means the skip zone was small because the ionosphere was strongly ionized and I might have contacts with both Denver and Los Angeles. As a path opens, at first only the low-angle long-distance stations will be heard, but higher and higher angle hops will be possible so those signals will become stronger. Finally, as the path closes, low-angle stations will again be the only ones on the band until they fade away, as well.

Propagation Predictions

Because the ionosphere varies around the world, from my location in the middle of the continent the MUF varies in every direction. There is one MUF between me and Japan, between me and Texas, between me and South Africa, between me and Norway, and so on. Each path has its own MUF. If you use a propagation prediction program like VOACAP (www.voacap.com), that’s what it will show you after you tell it about current solar and geomagnetic conditions. The coverage prediction maps from VOACAP (below)show the predicted coverage on 20 meters from my station in Missouri (the red dot in the middle of North America) at three different times of a mid-April day in conditions like what we are experiencing now. The top shows propagation at 7 AM, 8 PM, and midnight. You can clearly see the skip zone around my station changing in size with time, as well.

How does a propagation model know what the ionosphere is doing right now?  Short answer–it doesn’t! MUF is a statistical estimate that comes from a whole lot of measurements and models developed over a long history of HF operating. Based on the time of day and the season, there is a model for where the clouds of ionization are likely to be thick and thin, high and low. Then the current solar flux (SFI) is figured in. Finally, the information you provided about the stations and noise level are compared with the prediction to see if you could actually communicate. Remember, it’s all based on statistics, so at any particular time the quality of the path could vary quite a bit.

Signals Take All Paths

There also may be more than one combination of hops at different angles and from different ionospheric layers creating different possible paths. Your antenna is radiating signals in a broad pattern spread of horizontal and vertical angles like a flashlight beam. Your signals will take direct paths along the great circle route between stations, called short path, as well as skew paths that are to one side or the other. Remember the 3D clouds? Sometimes, the direct path may not be open but a skew path might get through. For example, if a direct path from the U.S. to Europe begins to close, a skew path over northern Africa might take advantage of stronger ionization at southern latitudes and scatter the signal back to the north.

The many paths a signal can take give it a “DX sound” that the experienced operator learns to recognize. The signals arrive at your antenna at slightly different times (it takes about 1/7th of a second for a signal to go all the way around the world) and with different phases and polarizations. Rapid fading, common if the path goes near the geomagnetic poles, called the auroral zone, adds polar flutter that is quite distinctive.

There are two special paths that skilled operators learn to use whenever they can: grey line and long path. You may think that it takes a big station for these contacts, but even modest stations can get through. You just have to know when and where!

The grey line path is a general term referring to enhanced propagation when one or both stations are at their local sunrise or sunset. These regions straddle the terminator between day and night. Good and interesting things happen at these times! Recalling our 3D clouds again, as the sun rises, for example, those first UV rays hit the highest reaches of the ionosphere first. If you can launch your signal at a low angle into the top of the clouds as they are building up, you may find a path opening. Similarly, at sunset, the highest layers are the last to fade. It’s all about that vertical structure for us to take advantage of.  Paths may open directly along or just near the terminator. If you are a little pistol trying to work a DXpedition, watch a terminator map like the one at dx.qsl.net/propagation/greyline.html or there may be one built into your logging software.

Another special path goes the “wrong way around.” Called long path (for some reason), signals travel in the opposite direction to the direct short path bearing. It may take more hops, but if the short path isn’t open to a distant station, remember to check long path bearings. Long path contacts are often possible when the path is mostly in daylight (on the high bands) or in darkness (on the low bands). Sometimes the long path and short path are open at the same time, creating an echo-like effect. At these times, the strongest path can quickly flip back and forth between short and long path. You might even hear a signal from the antipode to your location coming in from several paths since all paths are both short and long to the antipode!

Would you like to dive in to learn more about propagation? There are many great resources for you. CQ Communications has just released a new edition of the classic Shortwave Propagation Handbook, updated by Carl Luetzelschwab, K9LA. Speaking of K9LA, Carl has also made all of the late Bob Brown, NM7M’s propagation books and papers available on his website: k9la.us. You can also find quite a few of Carl’s tutorials and long-running propagation columns.  

What Does It All Mean

What all this means to you, the ham trying to work DX or bag that last state, is there may be more paths and frequencies available than you think. The computer may say that the Islets of Langerhan are at a bearing of 243 degrees from your location, but there may be a better path off to one side or the other.  You may want to schedule your operating around sunrise or sunset to try for a long-path QSO. If you have a beam, don’t forget to move it around and see what you can hear. The most important thing is to just get on the air and try different things. When everything works and you’re there to hear it, those can be some of your best ham radio memories. I know they make up some of mine!

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