Bent Radio

Radio would be a lot simpler if the Earth was flat. Actually, a lot of things would be simpler if the Earth was flat. In fact, if you look back in history, to when the Earth actually was flat, it's pretty clear that life was a lot simpler for all involved.

To be perfectly accurate, radio waves aren't really flat, but they do travel in straight lines unless they find a truly compelling reason to do otherwise. The fact that there are radio amateurs on the wrong side of a round Earth is not generally a compelling enough reason, in and of itself, for radio waves to make the effort.

Lacking additional incentive, the farthest a radio wave will travel is to the horizon, another unfortunate side-effect of Mr. Columbus' "new and improved" round Earth. For a neck-height wire antenna, the horizon is about nine miles. In reality, the "radio horizon" is about 30% farther than the visible horizon. We usually use the "4/3 Earth" rule-of-thumb for radio horizon distance. Calculate the geometrical horizon using an Earth that's 4/3 as big as its actual diameter, and you come pretty close. Now, if you have two radio amateurs on opposite sides of the radio horizon with neck-height antennas, you can actually communicate a little under 18 miles; not quite as grim a picture, but certainly nothing to write home about.

Now, it's no big mystery that the higher you go the farther the horizon is. If you want to talk farther you just raise your antennas above neck height. That's the good news. The really bad news is how quickly you reach the point of diminishing returns by making your antennas higher. The radio horizon only increases as the square root of the antenna height. Look at it like this. Going from moose-neck height to giraffe-neck height gains you a lot. Going from giraffe-neck height to Empire State Building height gains you almost nothing!

What, oh what is a round-Earth bound ham to do, other than to have no friends more than 18 miles away?

There are actually two main approaches to this problem. The first is to use the round-Earth itself to bend the radio waves. There is actually sort of an ironic poetic justice in this method. It's really "sticking it to The Man," The Man being, of course, Christopher Columbus.

So far, all our previous discussion of radio wave propagation has involved propagation through a vacuum, or through air, which, as far as radio is concerned, is about the same thing.

Radio waves traveling through, or in contact with, something other than free space, behave differently. When a radio wave travels through a solid material like dirt, it slows down a little bit. (It also loses some energy due to heating loss, but we can ignore that for now). The important thing to know is that a radio wave passing through dirt moves a little more slowly than a radio wave passing through air. This is not too hard to grasp.

But, what happens if part of a radio wave travels through dirt, and part of it travels through air? Things get very interesting. The best way I know of to demonstrate this is with a Slinky. I trust you have a Slinky lying around someplace. Even one of those abominable plastic ones will work.

Now, if you were to lay the Slinky down on a flat surface and stretch it out a bit, the outline of that Slinky will be a perfect sine wave. (This is the geometric projection of a helix onto a plane, for you geometry whizzes). Now, let's pinch the bottom of the coil together, so each lower wave peak is slightly closer together than the upper wave peaks. What does the Slinky do? It bends toward the closer-together peaks. It has no choice, unless you allow the Slinky to break. Radio waves must be continuous"nature will not allow you to have a broken radio wave. The radio wave must bend toward the "slower" direction portion of itself. Voila, a curved radio wave!

This curved radio wave in contact with the Earth's surface is called a "ground wave," for pretty obvious reasons. Now, ground waves are really only practical for lower radio frequencies, such as the A.M. broadcast band and lower. The heat losses we mentioned a few paragraphs back increase with increasing frequency. Above a couple of megahertz, ground waves do little more than heat up the dirt. But at lower frequencies, they are the dominant mode for worldwide radio communications. And they REALLY work great when the Earth is made of sea water.

Take THAT, Mr. Columbus!

In fact, ground wave (or sea-surface) propagation for many decades was the primary means of over-the-horizon radio communications. 500 KC was the international maritime distress frequency until very recently, having served its purpose well for about 100 years. (It was the frequency used on the Titanic. The fact that nobody heeded the radioman's warning has nothing to do with the effectiveness of the radios or the propagation). I've talked to a few old time radio officers, and they say that 500 kc was like an international party line of telegraph operators"nearly 100% reliable communications over most of the earth.

The main problem with communications at low frequencies, at least as far as hams are concerned, is that antennas are necessarily large. The lowest frequency Amateur Band is 160 meters, which is really at the upper edge of where ground wave communications are possible. A good deal of your efforts to make a 160 meter ground wave antenna will serve little but to heat up earthworms.

Fortunately, there's another approach to over-the-horizon communications, which is actually the primary means for most long-distance amateur radio communications. We can bounce signals off the ionosphere. Actually, bouncing isn't a very accurate description; it's a bending process much like our groundwave.

The ionosphere is much like many of the other spheres that circle the Earth, such as the Atmosphere, Troposphere, Mesosphere, and such. They're all"well"spherical. Sort of.

Which makes one wonder what people called the atmosphere back before Columbus, when the Earth was flat. The Atmosflat?

In any case, the Earth is surrounded by varying gases of different densities and pressures. The vast, vast, vast majority of these gas molecules are neutrals. They have no electrical charge, and have no effect on radio signals, whatsoever. Remember our chapter, Electrons: The Tools of the Trade? We said that in all things electronic, it's really the electrons that do all the work. And they really can't do much if they're tied to molecules or atoms, other than keep the molecules or atoms company. Neutral gases are pretty invisible to radio waves; there's really nothing in them to even respond to radio waves.

Starting at an altitude of about 90 kilometers, however, a tiny number of these atoms can get their electrons slapped off their carcasses, which then become free electrons. Or at least, fairly cheap ones. The main electron-slapping ingredient is ultraviolet radiation from the sun, and you don't have to be a rocket scientist to conclude that probably much more electron-slapping happens during the day than at night.

Precisely how many of these electrons get slapped off their host atoms determines how much stuff we have to bounce radio signals off of. Now a whole lot of interesting things happen when electron-slapping happens, all of which falls into the field of plasma physics. Although I worked in the field for many years, I'll avoid the temptation to deliver a course on plasma physics in this chapter. You can get a much more entertaining introduction to that branch of science by reading my novel, Plasma Dreams. (Shameless commercial plug here).

One of the interesting things that happen is that these slap-happy electrons tend to organize in layers of sorts, and begin to exhibit collective behavior, must like your local Teamsters Union. Except they never ask for overtime. That's why they're called free electrons.

Now, the atoms that get their electrons slapped off them are called ions. Ions also exhibit a collective behavior, which is why we call that collection of ions the ionosphere. These ions don't generally affect radio signals directly, but they do give a certain sense of direction to the free electrons. Without the remaining ions, the free electrons would flail off in every direction, because they are, being like charged, mutually repelled.

Perhaps you're asking why the ions don't go wandering off merrily as well, since they also are now mutually repulsive. Well they do, but very slowly. Even though they have the same magnitude of charge as their off-slapped electrons, they have thousands of times the mass! So, though the ions will gradually drift apart (diffuse), in most regards act mechanically like any other gas. In fact, you actually have weather-like phenomena happening in the ionosphere, just like in the atmosphere. Well not just like, but you do have recognizable patterns of movement and such way up there.

So, the end result is, we have this big sluggish ionosphere keeping free electrons on a very long leash. The result of all this couldn't work better for radio propagation if it was intentionally planned. (Actually, I'm one of those folks who sincerely believe the ionosphere was specifically created for bouncing radio signals off of. Just like believing that trees were created as antenna supports. But that's just me).

Now, there's an interesting little item called the electron density profile. It's sort of a perverted bell curve sort of thing lying on its side. You can see this as a black line on many ionograms available online. (I use the HAARP ionosonde, which shows conditions valid for most of Alaska. (www.haarp.alaska.edu) You can find a nearby ionosonde by looking at the Lowell Digisonde site map. )

What the electron density profile shows you is the relative number of free electrons at any altitude from about fifty kilometers to about six hundred kilometers. This is the best indication of how good the sky is going to be at reflecting radio signals at any particular time. Now, why the funny curve, and not just a straight line? Good question. We actually have two conflicting things happening.

Since air pressure is highest at ground level, and decreases as we go up, there are more atoms available to get their electrons slapped off of. So the lower we go, the greater potential for the creation of free electrons. But"and it's a big but"the ultraviolet light has to travel farther though absorptive air to get to those high density atoms. So while there are more atoms to ionize at lower altitudes, it is easier to ionize them at higher altitudes. The breakeven point is usually at around 250 km altitude or so, the normal peak of the "bell curve". It is generally around here where you will find the most number of free electrons milling about. There's also usually a smaller peak at around 100 km or so.

Now, we haven't really explained how an electron reflects a radio signal, though we've described how these electrons congregate. Actually, you CAN reflect a radio signal off a single electron, and there are scientific devices called incoherent scatter radars which do just that, but this is pretty science-geeky stuff. As radio amateurs, we're much more interested in reflecting radio signals off mobs of electrons.

If we have a decent, well-behaved ionosphere, we have more or less a sheet of electrons, which in some ways, acts a bit like a sheet of copper. We have a region of sky that is highly electrically conductive. Unlike a wire, it's conductive in all directions, north-east-west, and south, not just along a line. We also have some conductivity up and down, because our electron sheet can be many kilometers thick. Oh, I must also clarify one point here. Please do not get the impression that we have any significant volume of sky that consists of nothing but electrons, any more than we have any volume containing nothing but ions. Any region of the ionosphere we look at will have all three items, electrons, neutrals, and ions occupying the region in varying concentrations. High free-electron content just means there are a lot more free electrons at that height than at other heights.

Well, back to our conductive sheet analogy. The electrons are free to move in any direction in response to a radio wave impinging on them. They will line up and slosh back and forth in accordance with the electrical part of the wave passing through their midst. But what do sloshing electrons do? Why, they create radio waves! This is why we were emphatic about the reciprocity theorem in the antenna chapter. It doesn't matter whether you're a slosher or a sloshee. One creates the other, and the other creates the one.

This is also why I was reluctant to describe the ionosphere as reflecting radio waves. It actually absorbs and re-radiates them. Looking at what happens from your station on the ground, this may seem to be a minor point of semantics, but it makes a big difference when we look at the more peculiar aspects of ionospheric or "skywave" radio.

Now, if all those nice slap-happy free electrons happened to coagulate in a nice spherical shell at about 250 kilometers, all around the round Earth, life would be wonderful all the time. Worldwide skywave communications would be possible anywhere at any time.

But alas, there are several flies in the ointment.

First, many of our electrons do find their ways back to the ions from which they were slapped. Well, not the exact same ions, but ions from the same ion mob. Recombination takes place, and our ions become neutrals, of no value for radio propagation. This recombination is relatively slow, which is why shortwave radio propagation doesn't suddenly quit the instant the sun goes down. In fact, a good number of electrons never do recombine, which is why you still have skywave propagation at night, at least on the lower H.F. frequencies.

Secondly, we have ionospheric weather. Remember, our ions are basically floating on top of the atmosphere. As tenuous as the connection is, genuine weather effects down here on the surface do eventually transfer to the ionosphere. It's certainly not a one-to-one correspondence, but there are storms and currents and other weather-like disturbances that all do one thing"they upset the nice calm layers of electrons we need for decent "reflection" to take place. Instead of a "mirror" we have a wall of rocks.

Sooner or later most hams will experience a sudden "blackout" of radio communications, also quite common on the lower frequency bands, during the evening when you're basically using those "leftover" un-recombined electrons. This is seldom if ever due to a sudden, spontaneous recombination, where gillions of electrons miraculously find their way back home to their family ions. No, these sudden outages are frequently caused by electron precipitation"all the electrons are quite literally being sucked down a hole in the bottom of the ionosphere, traveling down the Earth's magnetic field lines into the Earth's surface, where they are dissipated.

Electron precipitation events can be triggered by almost anything: a distant lightning strike, a burst of cosmic energy, auroral activity, or just sheer statistics.

The ionosphere is inherently unstable; it's much like trying to float water on top of oil. It can be done if you're really really really really really really really really really really careful. But the slightest disturbance will cause the liquids to suddenly change places, putting the water on the bottom where it belongs.

In a similar fashion, there's only one place an electron wants to be (other than an atom from which it got slapped), and that's sliding down a magnetic field. Magnetic fields are irresistible water slides for electrons. The fact that a stable ionosphere can exist at all with the Earth having a magnetic field is nothing short of astonishing. The fact that radio works at all in Alaska, where the North magnetic field is concentrated, is astonishment on steroids.

Probably the most useful information the radio amateur can get from the electron density profile is the critical height.

Let's illustrated this point with a little test setup. Let's build a transmitter and receiver, and put a super high gain antenna on each, so we can place them right next to each other without interference. We'll aim our transmitter and receiver antennas straight up into the ionosphere. While we transmit on one antenna, we listen for the ionosphere reflected signal with the other antenna. We actually do something similar to this in what is called NVIS (Near Vertical Incident Skywave) communications, a very reliable H.F. military mode.

If you shoot a radio signal straight up into the ionosphere, whilst listening for reflections, constantly increasing the frequency as you do so, there will be some frequency at which you get no signal reflected back at all. The frequency at which this happens is called the critical frequency, and the height at which this all happens is the critical height. Both of these are very useful for calculating radio propagation, but let's look at critical height first.

Critical height, interestingly enough, always corresponds with the maximum electron density! In other words, you can never get any reflections from anything higher than where the maximum electron density point. This is a good thing to know.

Sometimes, critical height is called "the height of the ionosphere." This is not precisely true, but it's a useful enough misapprehension. A little simple geometry will show you that having a high ionosphere is better than a low one, when you're trying to bounce a signal beyond the horizon. Now, it's certainly possible to use multiple bounces when transmitting beyond the horizon, and indeed this is exceedingly common in amateur radio communications. But generally, it's best to get the job done with as few skips as possible, for at least two reasons. Number one, there are losses associated with every ionospheric bounce, so the fewer of them you need, the stronger your signal will be. Secondly, the prediction of multi-bounce skywave radio assumes the ionosphere at his end of the chain is the same as it is where you are, a very "iffy" assumption, indeed.

At any rate, the critical height is a good quick and dirty means of calculating how far you can get on your first bounce.

Now, critical frequency, on the other hand, has a lot more information to offer. And it also requires a much better understanding of the ionosphere. When thinking about critical frequency, we need to toss out our "mirror" analogy of the ionosphere, and replace it with a "prism" analogy. Remember when we said our "electron sheet" is very thick? A reflection from a thin layer, like the silver coating on a mirror, gives us something called a "specular" reflection. Radio reflections from the ionosphere are anything but specular, except for some really notable exceptions, which is why they're called "notable exceptions."

Most of us are familiar with a standard optical prism. You place this wedge of glass in sunlight, and it spits out all the colors of the rainbow, each one "bent" at a different angle. Well the ionosphere is very similar; different frequencies are bent at different angles. The net result is similar to a prism, but the internal workings are quite different.

The ionosphere's prism-like character results from the fact that the critical electron layer (or layers) are not only very thick relative to the wavelength of radio signals penetrating them, but they are of continuously varying density. We say that the ionosphere has a continuously variable velocity factor.

Remember our bent Slinky experiment? In that case, we were only dealing with two "velocity factors," that of air, and that of dirt. In the ionosphere, we have a continuously varying velocity factor with height, but also one that varies with frequency! The physics is exceedingly complex, but the intermediate results are fairly easy to visualize. The crux of the matter is that higher frequencies penetrate the ionosphere farther before being "bent" than lower frequencies. What this means is that the "height of the ionosphere" is dependent on frequency.

Now, things just begin to get interesting. In the next chapter we will discuss X and O mode propagation, a topic almost totally ignored in amateur literature, and yet of utmost importance for understanding unusual propagation conditions. We will show how EVERY H.F. signal that passes through the ionosphere becomes circularly polarized, and how we can take advantage of it.

Member Comments: